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Python is an interpreted
high-level High-level and low-level, as technical terms, are used to classify, describe and point to specific Objective (goal), goals of a systematic operation; and are applied in a wide range of contexts, such as, for instance, in domains as widely varied as ...
general-purpose programming language In computer software Software is a collection of Instruction (computer science), instructions and data (computing), data that tell a computer how to work. This is in contrast to Computer hardware, physical hardware, from which the system is bu ...
. Its design philosophy emphasizes
code readability Computer programming is the process of designing and building an executable In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform in ...
with its use of significant indentation. Its
language construct A language construct is a syntactically In linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods for studying and modeling them. The tradit ...
s as well as its
object-oriented Object-oriented programming (OOP) is a programming paradigm Program, programme, programmer, or programming may refer to: Business and management * Program management, the process of managing several related projects * Time management * P ...
approach aim to help programmers write clear, logical code for small and large-scale projects. Python is dynamically-typed and garbage-collected. It supports multiple
programming paradigm Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mainly with implications for the execution model of the language, suc ...
s, including structured (particularly, procedural), object-oriented and
functional programming In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , ...
. It is often described as a "batteries included" language due to its comprehensive
standard library A standard library in computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a specific task. Programming involves tasks such as: ...
.
Guido van Rossum Guido van Rossum (; born 31 January 1956) is a Dutch programmer A computer programmer, sometimes called a software developer, a programmer or more recently a coder (especially in more informal contexts), is a person who creates computer s ...

Guido van Rossum
began working on Python in the late 1980s, as a successor to the ABC programming language, and first released it in 1991 as Python 0.9.0. Python 2.0 was released in 2000 and introduced new features, such as
list comprehensionA list comprehension is a syntactic In linguistics, syntax () is the set of rules, principles, and processes that govern the structure of Sentence (linguistics), sentences (sentence structure) in a given Natural language, language, usually includ ...
s and a cycle-detecting garbage collection system (in addition to
reference counting In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algo ...
). Python 3.0 was released in 2008 and was a major revision of the language that is not completely backward-compatible. Python 2 was discontinued with version 2.7.18 in 2020. Python consistently ranks as one of the most popular programming languages.


History

Python was conceived in the late 1980s by
Guido van Rossum Guido van Rossum (; born 31 January 1956) is a Dutch programmer A computer programmer, sometimes called a software developer, a programmer or more recently a coder (especially in more informal contexts), is a person who creates computer s ...

Guido van Rossum
at
Centrum Wiskunde & Informatica The (abbr. CWI; English: "National Research Institute for Mathematics and Computer Science") is a research center in the field of mathematics and theoretical computer science. It is part of the Netherlands Organisation for Scientific Research ( ...
(CWI) in the
Netherlands ) , national_anthem = ( en, "William of Nassau") , image_map = EU-Netherlands.svg , map_caption = , image_map2 = BES islands location map.svg , map_caption2 = , image_map3 ...

Netherlands
as a successor to the ABC programming language, which was inspired by
SETL SETL (SET Language) is a very high-level programming language based on the mathematical theory of sets. It was originally developed by (Jack) Jacob T. Schwartz at the New York University New York University (NYU) is a private resear ...
, capable of
exception handling In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest commo ...
and interfacing with the
Amoeba An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Cl ...
operating system. Its implementation began in December 1989. Van Rossum shouldered sole responsibility for the project, as the lead developer, until 12 July 2018, when he announced his "permanent vacation" from his responsibilities as Python's " benevolent dictator for life", a title the Python community bestowed upon him to reflect his long-term commitment as the project's chief decision-maker. In January 2019, active Python core developers elected a five-member "Steering Council" to lead the project. Python 2.0 was released on 16 October 2000, with many major new features, including a cycle-detecting garbage collector (in addition to
reference counting In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algo ...
) for
memory management Memory management is a form of resource management In organizational studies Organizational studies is "the examination of how individuals construct organizational structures, processes, and practices and how these, in turn, shape social relat ...
and support for
Unicode Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's wri ...

Unicode
. Python 3.0 was released on 3 December 2008. It was a major revision of the language that is not completely backward-compatible. Many of its major features were backported to Python 2.6.x and 2.7.x version series. Releases of Python 3 include the 2to3 utility, which automates the translation of Python 2 code to Python 3. Python 2.7's end-of-life date was initially set at 2015 then postponed to 2020 out of concern that a large body of existing code could not easily be forward-ported to Python 3. No more security patches or other improvements will be released for it. With Python 2's end-of-life, only Python 3.6.x and later are supported. Python 3.9.2 and 3.8.8 were expedited as all versions of Python (including 2.7) had security issues, leading to possible remote code execution and web cache poisoning.


Design philosophy and features

Python is a
multi-paradigm programming language Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mainly with implications for the execution model of the language, suc ...
.
Object-oriented programming Object-oriented programming (OOP) is a programming paradigm Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mai ...
and
structured programming Structured programming is a programming paradigm Programming paradigms are a way to classify programming languages based on their features. Languages can be classified into multiple paradigms. Some paradigms are concerned mainly with impl ...
are fully supported, and many of its features support functional programming and
aspect-oriented programming In computing, aspect-oriented programming (AOP) is a programming paradigm that aims to increase Modularity (programming), modularity by allowing the separation of concerns, separation of cross-cutting concerns. It does so by adding additional be ...
(including by
metaprogramming Metaprogramming is a programming technique in which computer program In imperative programming In computer science, imperative programming is a programming paradigm that uses Statement (computer science), statements that change a program's sta ...
and
metaobjectIn computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algorith ...
s (magic methods)). Many other paradigms are supported via extensions, including
design by contract Design by contract (DbC), also known as contract programming, programming by contract and design-by-contract programming, is an approach for designing software. It prescribes that software designers should define formal, precise and verifiable in ...

design by contract
and
logic programming Logic programming is a programming paradigm which is largely based on formal logic. Any program written in a logic programming language is a set of sentences in logical form, expressing facts and rules about some problem domain. Major logic prog ...
. Python uses
dynamic typing In programming language A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programming languages are used in computer programming to i ...
and a combination of
reference counting In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algo ...
and a cycle-detecting garbage collector for
memory management Memory management is a form of resource management In organizational studies Organizational studies is "the examination of how individuals construct organizational structures, processes, and practices and how these, in turn, shape social relat ...
. It also features dynamic name resolution (
late binding Late binding, dynamic binding, or dynamic linkage—though not an identical process to dynamically linking imported code libraries—is a computer programming mechanism in which the method being called upon an object, or the function being called ...
), which binds method and variable names during program execution. Python's design offers some support for functional programming in the
Lisp Lisp (historically LISP) is a family of programming language A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbo ...
tradition. It has functions;
list comprehensionA list comprehension is a syntactic In linguistics, syntax () is the set of rules, principles, and processes that govern the structure of Sentence (linguistics), sentences (sentence structure) in a given Natural language, language, usually includ ...
s,
dictionaries A dictionary is a listing of lexeme A lexeme () is a unit of lexical meaning that underlies a set of words that are related through inflection In linguistic morphology Morphology, from the Greek and meaning "study of shape", may refe ...
, sets, and generator expressions. The standard library has two modules ( and ) that implement functional tools borrowed from Haskell and
Standard ML Standard ML (SML) is a general-purpose modular functional programming language with compile-time type checking and type inference. It is popular among compiler In computing, a compiler is a computer program that Translator (computing), trans ...
. The language's core philosophy is summarized in the document ''The Zen of Python'' (''PEP 20''), which includes
aphorism An aphorism (from Ancient Greek, Greek ἀφορισμός: ''aphorismos'', denoting 'delimitation', 'distinction', and 'definition') is a concise, terse, laconic, or memorable expression of a general truth or principle. They are often handed do ...
s such as: * Beautiful is better than ugly. * Explicit is better than implicit. * Simple is better than complex. * Complex is better than complicated. * Readability counts. Rather than having all of its functionality built into its core, Python was designed to be highly
extensible Extensibility is a software engineering and systems design Systems design is the process of defining the architecture File:Plan d'exécution du second étage de l'hôtel de Brionne (dessin) De Cotte 2503c – Gallica 2011 (adjusted).jpg, up ...

extensible
(with modules). This compact modularity has made it particularly popular as a means of adding programmable interfaces to existing applications. Van Rossum's vision of a small core language with a large standard library and easily extensible interpreter stemmed from his frustrations with
ABC ABC are the first three letters of the Latin script known as the alphabet. ABC or abc may also refer to: Arts, entertainment, and media Broadcasting * American Broadcasting Company, a commercial U.S. TV broadcaster ** Disney–ABC Television ...
, which espoused the opposite approach. Python strives for a simpler, less-cluttered syntax and grammar while giving developers a choice in their coding methodology. In contrast to
Perl Perl is a family of two high-level High-level and low-level, as technical terms, are used to classify, describe and point to specific Objective (goal), goals of a systematic operation; and are applied in a wide range of contexts, such as, for ...
's " there is more than one way to do it" motto, Python embraces a "there should be one— and preferably only one —obvious way to do it" design philosophy. Alex Martelli, a
Fellow A fellow is a broad concept whose exact meaning depends on context. In learned Learning is the process of acquiring new understanding, knowledge, behaviors, skills, value (personal and cultural), values, attitudes, and preferences. The abil ...
at the
Python Software Foundation The Python Software Foundation (PSF) is an American nonprofit organization A nonprofit organization (NPO), also known as a non-business entity, not-for-profit organization, or nonprofit institution, is a legal entity organized and operated for ...
and Python book author, writes that "To describe something as 'clever' is ''not'' considered a compliment in the Python culture." Python's developers strive to avoid premature optimization, and reject patches to non-critical parts of the
CPython CPython is the reference implementation In the software development process In software engineering, a software development process is the process of dividing software development work into smaller, parallel or sequential steps or subpro ...
reference implementation that would offer marginal increases in speed at the cost of clarity. When speed is important, a Python programmer can move time-critical functions to extension modules written in languages such as C, or use
PyPy PyPy () is an alternative implementation of the Python PYTHON was a Cold War contingency plan of the Government of the United Kingdom, British Government for the continuity of government in the event of Nuclear warfare, nuclear war. Background ...
, a just-in-time compiler.
Cython Cython is a programming language that aims to be a superset of the Python programming language Python is an interpreted high-level general-purpose programming language. Python's design philosophy emphasizes code readability with its not ...
is also available, which translates a Python script into C and makes direct C-level API calls into the Python interpreter. Python's developers aim for the language to be fun to use. This is reflected in its name—a tribute to the British comedy group
Monty Python Monty Python (also collectively known as the Pythons) were a British surreal comedy troupe A comedy troupe is a group of comedians and associated personnel who work together to perform comedy as entertainment. The term is often used inte ...
—and in occasionally playful approaches to tutorials and reference materials, such as examples that refer to spam and eggs (a reference to a Monty Python sketch) instead of the standard . A common
neologism A neologism (; from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...
in the Python community is ''pythonic'', which can have a wide range of meanings related to program style. To say that code is pythonic is to say that it uses Python idioms well, that it is natural or shows fluency in the language, that it conforms with Python's minimalist philosophy and emphasis on readability. In contrast, code that is difficult to understand or reads like a rough transcription from another programming language is called ''unpythonic''. Users and admirers of Python, especially those considered knowledgeable or experienced, are often referred to as ''Pythonistas''.


Syntax and semantics

Python is meant to be an easily readable language. Its formatting is visually uncluttered, and it often uses English keywords where other languages use punctuation. Unlike many other languages, it does not use
curly brackets A bracket is either of two tall fore- or back-facing punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding ...
to delimit blocks, and semicolons after statements are allowed but are rarely, if ever, used. It has fewer syntactic exceptions and special cases than C or
Pascal Pascal, Pascal's or PASCAL may refer to: People and fictional characters * Pascal (given name), including a list of people with the name * Pascal (surname), including a list of people and fictional characters with the name ** Blaise Pascal, French ...
.


Indentation

Python uses
whitespace White space or whitespace may refer to: Technology * Whitespace character, Whitespace characters, characters in computing that represent horizontal or vertical space * White spaces (radio), allocated but locally unused radio frequencies * TV White ...
indentation, rather than
curly brackets A bracket is either of two tall fore- or back-facing punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding ...
or keywords, to delimit blocks. An increase in indentation comes after certain statements; a decrease in indentation signifies the end of the current block. Thus, the program's visual structure accurately represents the program's semantic structure. This feature is sometimes termed the
off-side rule A computer programming language is said to adhere to the off-side rule if block (programming), blocks in that language are expressed by their indent style, indentation. The term was coined by Peter J. Landin, possibly as a pun on the offside (assoc ...
, which some other languages share, but in most languages indentation does not have any semantic meaning. The recommended indent size is four spaces.


Statements and control flow

Python's
statements Statement or statements may refer to: Common uses *Statement (computer science)In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to ...
include (among others): * The assignment statement, using a single equals sign =. * The if statement, which conditionally executes a block of code, along with else and elif (a contraction of else-if). * The statement, which iterates over an iterable object, capturing each element to a local variable for use by the attached block. * The
while ''While'' is a word in the English language that functions both as a noun and as a Grammatical conjunction, subordinating conjunction. Its meaning varies largely based on its intended function, position in the phrase and even the writer or speake ...

while
statement, which executes a block of code as long as its condition is true. * The
try Try or TRY may refer to: Music Albums * ''Try!'', an album by the John Mayer Trio * Try (Bebo Norman album), ''Try'' (Bebo Norman album) (2014) Songs * Try (Blue Rodeo song), "Try" (Blue Rodeo song) (1987) * Try (Colbie Caillat song), "Try" (Colbi ...

try
statement, which allows exceptions raised in its attached code block to be caught and handled by except clauses; it also ensures that clean-up code in a finally block will always be run regardless of how the block exits. * The raise statement, used to raise a specified exception or re-raise a caught exception. * The class statement, which executes a block of code and attaches its local namespace to a
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently f ...
, for use in object-oriented programming. * The def statement, which defines a
function Function or functionality may refer to: Computing * Function key A function key is a key on a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern comp ...
or
method Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to: *Scien ...
. * The
with With or WITH may refer to: * With, a preposition in English * Carl Johannes With (1877–1923), Danish doctor and arachnologist * With (character), a character in ''D. N. Angel'' * With (novel), ''With'' (novel), a novel by Donald Harrington * With ...
statement, which encloses a code block within a context manager (for example, acquiring a
lock Lock(s) may refer to: Common meanings *, a mechanical device used to secure items of importance *, a device for boats to transit between different levels of water, as in a canal Arts and entertainment *, a 2016 Punjabi film *, a sentient machin ...
before the block of code is run and releasing the lock afterwards, or opening a
file File or filing may refer to: Mechanical tools and processes * File (tool) A file is a tool used to remove fine amounts of material from a workpiece. It is common in woodworking, metalworking, and other similar trade and hobby tasks. Most are ...
and then closing it), allowing resource-acquisition-is-initialization (RAII)-like behavior and replaces a common try/finally idiom. * The break statement, exits from a loop. * The continue statement, skips this iteration and continues with the next item. * The del statement, removes a variable, which means the reference from the name to the value is deleted and trying to use that variable will cause an error. A deleted variable can be reassigned. * The pass statement, which serves as a NOP. It is syntactically needed to create an empty code block. * The
assert Assertion or assert may refer to: Computing * Assertion (software development) Assertion or assert may refer to: Computing * Assertion (software development) Assertion or assert may refer to: Computing * Assertion (software development), a com ...
statement, used during debugging to check for conditions that should apply. * The yield statement, which returns a value from a function and yield is also an operator. This form is used to implement
coroutine Coroutines are computer program components that generalize subroutines for non-preemptive multitasking, by allowing execution to be suspended and resumed. Coroutines are well-suited for implementing familiar program components such as cooperative mu ...

coroutine
s. * The return statement, used to return a value from a function. * The
import An import is the receiving country in an export An export in international trade International trade is the exchange of capital Capital most commonly refers to: * Capital letter Letter case (or just case) is the distincti ...
statement, which is used to import modules whose functions or variables can be used in the current program. The assignment statement (=) operates by binding a name as a
reference Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''name ...
to a separate, dynamically-allocated
object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Entity, something that is tangible and within the grasp of the senses ** Object (abstract), an object which does not exist at any particular time or pl ...
. Variables may subsequently be rebound at any time to any object. In Python, a variable name is a generic reference holder and does not have a fixed
data type In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of , ...
associated with it. However, at a given time, a variable will refer to ''some'' object, which will have a type. This is referred to as
dynamic typing In programming language A programming language is a formal language comprising a Instruction set architecture, set of instructions that produce various kinds of Input/output, output. Programming languages are used in computer programming to i ...
and is contrasted with statically-typed programming languages, where each variable may only contain values of a certain type. Python does not support
tail call In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algorit ...
optimization or first-class continuations, and, according to Guido van Rossum, it never will. However, better support for
coroutine Coroutines are computer program components that generalize subroutines for non-preemptive multitasking, by allowing execution to be suspended and resumed. Coroutines are well-suited for implementing familiar program components such as cooperative mu ...

coroutine
-like functionality is provided, by extending Python's generator (computer programming), generators. Before 2.5, generators were lazy evaluation, lazy iterators; information was passed unidirectionally out of the generator. From Python 2.5, it is possible to pass information back into a generator function, and from Python 3.3, the information can be passed through multiple stack levels.


Expressions

Some Python Expression (computer science), expressions are similar to those found in languages such as C and Java (programming language), Java, while some are not: * Addition, subtraction, and multiplication are the same, but the behavior of division differs. There are two types of divisions in Python. They are floor division (or integer division) // and floating-point/division. Python also uses the ** operator for exponentiation. * From Python 3.5, the new @ infix operator was introduced. It is intended to be used by libraries such as NumPy for matrix multiplication. * From Python 3.8, the syntax :=, called the 'walrus operator' was introduced. It assigns values to variables as part of a larger expression. * In Python,

compares by value, versus Java, which compares numerics by value and objects by reference. (Value comparisons in Java on objects can be performed with the equals() method.) Python's is operator may be used to compare object identities (comparison by reference). In Python, comparisons may be chained, for example . * Python uses the words and, or, not for its boolean operators rather than the symbolic &&, , , , ! used in Java and C. * Python has a type of expression termed a ''list comprehension#Python, list comprehension'' as well as a more general expression termed a '' generator expression''. * Anonymous functions are implemented using Lambda (programming), lambda expressions; however, these are limited in that the body can only be one expression. * Conditional expressions in Python are written as (different in order of operands from the ?:, c ? x : y operator common to many other languages). * Python makes a distinction between list (computer science), lists and tuples. Lists are written as , are mutable, and cannot be used as the keys of dictionaries (dictionary keys must be immutable in Python). Tuples are written as , are immutable and thus can be used as the keys of dictionaries, provided all elements of the tuple are immutable. The + operator can be used to concatenate two tuples, which does not directly modify their contents, but rather produces a new tuple containing the elements of both provided tuples. Thus, given the variable t initially equal to , executing first evaluates , which yields , which is then assigned back to t, thereby effectively "modifying the contents" of t, while conforming to the immutable nature of tuple objects. Parentheses are optional for tuples in unambiguous contexts. * Python features ''sequence unpacking'' wherein multiple expressions, each evaluating to anything that can be assigned to (a variable, a writable property, etc.), are associated in an identical manner to that forming tuple literals and, as a whole, are put on the left-hand side of the equal sign in an assignment statement. The statement expects an ''iterable'' object on the right-hand side of the equal sign that produces the same number of values as the provided writable expressions when iterated through and will iterate through it, assigning each of the produced values to the corresponding expression on the left. * Python has a "string format" operator %. This functions analogously to printf format string, printf format strings in C, e.g. evaluates to "spam=blah eggs=2". In Python 3 and 2.6+, this was supplemented by the format() method of the str class, e.g. . Python 3.6 added "f-strings": . * Strings in Python can be concatenation, concatenated, by "adding" them (same operator as for adding integers and floats). E.g. returns "spameggs". Even if your strings contain numbers, they are still added as strings rather than integers. E.g. returns "22". * Python has various kinds of string literals: ** Strings delimited by single or double quote marks. Unlike in Unix shells,
Perl Perl is a family of two high-level High-level and low-level, as technical terms, are used to classify, describe and point to specific Objective (goal), goals of a systematic operation; and are applied in a wide range of contexts, such as, for ...
and Perl-influenced languages, single quote marks and double quote marks function identically. Both kinds of string use the backslash (\) as an escape character. String interpolation became available in Python 3.6 as "formatted string literals". ** Triple-quoted strings, which begin and end with a series of three single or double quote marks. They may span multiple lines and function like here documents in shells, Perl and Ruby (programming language), Ruby. ** Raw string varieties, denoted by prefixing the string literal with an r. Escape sequences are not interpreted; hence raw strings are useful where literal backslashes are common, such as regular expressions and Microsoft Windows, Windows-style paths. Compare "@-quoting" in C Sharp (programming language), C#. * Python has array index and array slicing expressions on lists, denoted as a[key], or . Indexes are zero-based numbering, zero-based, and negative indexes are relative to the end. Slices take elements from the ''start'' index up to, but not including, the ''stop'' index. The third slice parameter, called ''step'' or ''stride'', allows elements to be skipped and reversed. Slice indexes may be omitted, for example returns a copy of the entire list. Each element of a slice is a shallow copy. In Python, a distinction between expressions and statements is rigidly enforced, in contrast to languages such as Common Lisp, Scheme (programming language), Scheme, or Ruby (programming language), Ruby. This leads to duplicating some functionality. For example: * List comprehensions vs. for-loops * Conditional (programming), Conditional expressions vs. if blocks * The eval() vs. exec() built-in functions (in Python 2, exec is a statement); the former is for expressions, the latter is for statements. Statements cannot be a part of an expression, so list and other comprehensions or Lambda (programming), lambda expressions, all being expressions, cannot contain statements. A particular case of this is that an assignment statement such as cannot form part of the conditional expression of a conditional statement. This has the advantage of avoiding a classic C error of mistaking an assignment operator = for an equality operator

in conditions: is syntactically valid (but probably unintended) C code but causes a syntax error in Python.


Methods

Method (programming), Methods on objects are function (programming), functions attached to the object's class; the syntax is, for normal methods and functions, syntactic sugar for . Python methods have an explicit this (computer programming), self parameter to access instance data, in contrast to the implicit self (or this) in some other object-oriented programming languages (e.g., C++, Java, Objective-C, or Ruby (programming language), Ruby). Apart from this, Python also provides methods, often called ''dunder methods'' (due to their names beginning and ending with double-underscores), to allow user-defined classes to modify how they are handled by native operations such as length, comparison, in arithmetic operations, type conversion, and many more.


Typing

Python uses duck typing and has typed objects but untyped variable names. Type constraints are not checked at compile time; rather, operations on an object may fail, signifying that the given object is not of a suitable type. Despite being Type system#Dynamic type checking and runtime type information, dynamically-typed, Python is strong and weak typing, strongly-typed, forbidding operations that are not well-defined (for example, adding a number to a string) rather than silently attempting to make sense of them. Python allows programmers to define their own types using class (computer science), classes, which are most often used for object-oriented programming. New object (computer science), instances of classes are constructed by calling the class (for example, or ), and the classes are instances of the metaclass type (itself an instance of itself), allowing metaprogramming and reflection (computer science), reflection. Before version 3.0, Python had two kinds of classes: ''old-style'' and ''new-style''. The syntax of both styles is the same, the difference being whether the class object is inherited from, directly or indirectly (all new-style classes inherit from object and are instances of type). In versions of Python 2 from Python 2.2 onwards, both kinds of classes can be used. Old-style classes were eliminated in Python 3.0. The long-term plan is to support gradual typing and from Python 3.5, the syntax of the language allows specifying static types but they are not checked in the default implementation,
CPython CPython is the reference implementation In the software development process In software engineering, a software development process is the process of dividing software development work into smaller, parallel or sequential steps or subpro ...
. An experimental optional static type checker named ''mypy'' supports compile-time type checking.


Arithmetic operations

Python has the usual symbols for arithmetic operators (+, -, *, /), the floor division operator // and the modulo operation % (where the remainder can be negative, e.g. 4 % -3

-2
). It also has ** for exponentiation, e.g. 5**3

125
and 9**0.5

3.0
, and a matrix‑multiplication operator @ . These operators work like in traditional math; with the same order of operations, precedence rules, the operators Infix notation, infix (+ and - can also be Unary operation, unary to represent positive and negative numbers respectively). The division between integers produces floating-point results. The behavior of division has changed significantly over time: * Current Python (i.e. since 3.0) changed / to always be floating-point division, e.g. . * Python 2.2 changed integer division to round towards negative infinity, e.g. 7/3

2
and -7/3

-3
. The floor division // operator was introduced. So 7//3

2
, -7//3

-3
, 7.5//3

2.0
and -7.5//3

-3.0
. Adding causes a module to use Python 3.0 rules for division (see next). * Python 2.1 and earlier used C's division behavior. The / operator is integer division if both operands are integers, and floating-point division otherwise. Integer division rounds towards 0, e.g. and . In Python terms, / is ''true division'' (or simply ''division''), and // is ''floor division.'' / before version 3.0 is ''classic division''. Rounding towards negative infinity, though different from most languages, adds consistency. For instance, it means that the equation is always true. It also means that the equation is valid for both positive and negative values of a. However, maintaining the validity of this equation means that while the result of a%b is, as expected, in the half-open interval [0, ''b''), where b is a positive integer, it has to lie in the interval (''b'', 0] when b is negative. Python provides a round function for rounding a float to the nearest integer. For Rounding#Tie-breaking, tie-breaking, Python 3 uses round to even: round(1.5) and round(2.5) both produce 2. Versions before 3 used Rounding#Rounding away from zero, round-away-from-zero: round(0.5) is 1.0, round(-0.5) is −1.0. Python allows boolean expressions with multiple equality relations in a manner that is consistent with general use in mathematics. For example, the expression a < b < c tests whether a is less than b and b is less than c. C-derived languages interpret this expression differently: in C, the expression would first evaluate a < b, resulting in 0 or 1, and that result would then be compared with c. Python uses arbitrary-precision arithmetic for all integer operations. The Decimal type/class in the decimal module provides decimal floating point, decimal floating-point numbers to a pre-defined arbitrary precision and several rounding modes. The Fraction class in the fractions module provides arbitrary precision for rational numbers. Due to Python's extensive mathematics library, and the third-party library NumPy that further extends the native capabilities, it is frequently used as a scientific scripting language to aid in problems such as numerical data processing and manipulation.


Programming examples

"Hello, World!" program, Hello world program: print('Hello, world!') Program to calculate the factorial of a positive integer: n = int(input('Type a number, and its factorial will be printed: ')) if n < 0: raise ValueError('You must enter a non-negative integer') factorial = 1 for i in range(2, n + 1): factorial *= i print(factorial)


Libraries

Python's large standard library, commonly cited as one of its greatest strengths, provides tools suited to many tasks. For Internet-facing applications, many standard formats and protocols such as MIME and Hypertext Transfer Protocol, HTTP are supported. It includes modules for creating graphical user interfaces, connecting to relational databases, pseudorandom number generator, generating pseudorandom numbers, arithmetic with arbitrary-precision decimals, manipulating regular expressions, and unit testing. Some parts of the standard library are covered by specifications (for example, the Web Server Gateway Interface (WSGI) implementation wsgiref follows PEP 333), but most modules are not. They are specified by their code, internal documentation, and test suites. However, because most of the standard library is cross-platform Python code, only a few modules need altering or rewriting for variant implementations. the Python Package Index (PyPI), the official repository for third-party Python software, contains over 329,000 packages with a wide range of functionality, including:


Development environments

Most Python implementations (including CPython) include a read–eval–print loop (REPL), permitting them to function as a command line interpreter for which the user enters statements sequentially and receives results immediately. Python comes with a Integrated development environment, Integrated development environment (IDE) called IDLE, which is more beginner based. Other shells, including IDLE and IPython, add further abilities such as improved auto-completion, session state retention and syntax highlighting. As well as standard desktop integrated development environments, there are Web browser-based IDEs; SageMath (intended for developing science and math-related Python programs); PythonAnywhere, a browser-based IDE and hosting environment; and Canopy IDE, a commercial Python IDE emphasizing scientific computing.


Implementations


Reference implementation

CPython CPython is the reference implementation In the software development process In software engineering, a software development process is the process of dividing software development work into smaller, parallel or sequential steps or subpro ...
is the reference implementation of Python. It is written in C, meeting the C89 (C version), C89 standard with several select C99 features (with later C versions out, it's considered outdated; CPython includes its own C extensions, but third-party extensions are not limited to older C versions, can e.g. be implemented with C11 (C standard revision), C11 or C++). It compiles Python programs into an intermediate bytecode which is then executed by its virtual machine. CPython is distributed with a large standard library written in a mixture of C and native Python. It is available for many platforms, including Windows (starting with Python 3.9, the Python installer deliberately fails to install on Windows 7 and 8; Windows XP was supported until Python 3.5) and most modern Unix-like systems, including macOS (and Apple M1 Macs, since Python 3.9.1, with experimental installer) and unofficial support for e.g. OpenVMS, VMS. Platform portability was one of its earliest priorities, during the Python 1 and Python 2 time-frame, even OS/2 and Solaris (operating system), Solaris were supported; support has since been dropped for a lot of platforms.


Other implementations

*
PyPy PyPy () is an alternative implementation of the Python PYTHON was a Cold War contingency plan of the Government of the United Kingdom, British Government for the continuity of government in the event of Nuclear warfare, nuclear war. Background ...
is a fast, compliant interpreter of Python 2.7 and 3.6. Its Just-in-time compilation, just-in-time compiler brings a significant speed improvement over CPython but several libraries written in C cannot be used with it. * Stackless Python is a significant fork of CPython that implements microthreads; it does not use the call stack in the same way, thus allowing massively concurrent programs. PyPy also has a stackless version. * MicroPython and CircuitPython are Python 3 variants optimized for microcontrollers, including Lego Mindstorms EV3. * Pyston is a variant of the Python runtime that uses just-in-time compilation to speed up the execution of Python programs. * Cinder is a performance-oriented fork of CPython 3.8 that contains a number of optimizations including bytecode inline caching, eager evaluation of coroutines, a method-at-a-time Just-in-time compilation, JIT and an experimental bytecode compiler.


Unsupported implementations

Other just-in-time Python compilers have been developed, but are now unsupported: * Google began a project named Unladen Swallow in 2009, with the aim of speeding up the Python interpreter fivefold by using the LLVM, and of improving its multithreading ability to scale to thousands of cores, while ordinary implementations suffer from the global interpreter lock. * Psyco is a discontinued Just-in-time compilation, just-in-time run-time algorithm specialization, specializing compiler that integrates with CPython and transforms bytecode to machine code at runtime. The emitted code is specialized for certain data types and is faster than the standard Python code. Psyco does not support Python 2.7 or later. * PyS60 was a Python 2 interpreter for Series 60 mobile phones released by Nokia in 2005. It implemented many of the modules from the standard library and some additional modules for integrating with the Symbian operating system. The Nokia N900 also supports Python with GTK widget libraries, enabling programs to be written and run on the target device.


Cross-compilers to other languages

There are several compilers to high-level object languages, with either unrestricted Python, a restricted subset of Python, or a language similar to Python as the source language: *
Cython Cython is a programming language that aims to be a superset of the Python programming language Python is an interpreted high-level general-purpose programming language. Python's design philosophy emphasizes code readability with its not ...
compiles (a superset of) Python 2.7 to C (while the resulting code is also usable with Python 3 and also e.g. C++). * Nuitka compiles Python into C++. * Pythran compiles a subset of Python 3 to C++. * Pyrex (programming language), Pyrex (latest release in 2010) and Shed Skin (latest release in 2013) compile to C and C++ respectively. * Google's Grumpy (latest release in 2017) transpiles Python 2 to Go (programming language), Go. * IronPython allows running Python 2.7 programs (and an Software release life cycle#Alpha, alpha, released in 2021, is also available for "Python 3.4, although features and behaviors from later versions may be included" on the .NET Common Language Runtime. * Jython compiles Python 2.7 to Java bytecode, allowing the use of the Java libraries from a Python program. * MyHDL is a Python-based hardware description language (HDL), that converts MyHDL code to Verilog or VHDL code. * Numba uses LLVM to compile a subset of Python to machine code. * Brython, Transcrypt and Pyjs (latest release in 2012) compile Python to JavaScript. * RPython can be compiled to C, and is used to build the PyPy interpreter of Python.


Performance

A performance comparison of various Python implementations on a non-numerical (combinatorial) workload was presented at EuroSciPy '13. Python's performance compared to other programming languages is also benchmarked by The Computer Language Benchmarks Game.


Development

Python's development is conducted largely through the ''Python Enhancement Proposal'' (PEP) process, the primary mechanism for proposing major new features, collecting community input on issues and documenting Python design decisions. Python coding style is covered in PEP 8. Outstanding PEPs are reviewed and commented on by the Python community and the steering council. Enhancement of the language corresponds with development of the CPython reference implementation. The mailing list python-dev is the primary forum for the language's development. Specific issues are discussed in the Roundup (issue tracker), Roundup bug tracker hosted a
bugs.python.org
Development originally took place on a Self-hosting (web services), self-hosted source-code repository running Mercurial, until Python moved to GitHub in January 2017. CPython's public releases come in three types, distinguished by which part of the version number is incremented: * Backward-incompatible versions, where code is expected to break and needs to be manually ported. The first part of the version number is incremented. These releases happen infrequently—version 3.0 was released 8 years after 2.0. * Major or "feature" releases are largely compatible with the previous version but introduce new features. The second part of the version number is incremented. Starting with Python 3.9, these releases are expected to happen annually. Each major version is supported by bugfixes for several years after its release. * Bugfix releases, which introduce no new features, occur about every 3 months and are made when a sufficient number of bugs have been fixed upstream since the last release. Security vulnerabilities are also patched in these releases. The third and final part of the version number is incremented. Many beta release, alpha, beta, and release-candidates are also released as previews and for testing before final releases. Although there is a rough schedule for each release, they are often delayed if the code is not ready. Python's development team monitors the state of the code by running the large unit test suite during development. The major academic conference on Python is PyCon. There are also special Python mentoring programmes, such as Pyladies. Python 3.10 deprecated wstr (to be removed in Python 3.12; meaning Python extensions need to be modified by then), and added pattern matching to the language.


API documentation generators

Tools that can generate documentation for Python API include pydoc (available as part of standard library), Sphinx (documentation generator), Sphinx, Pdoc and its forks, Doxygen and Graphviz, among others.


Naming

Python's name is derived from the British comedy group
Monty Python Monty Python (also collectively known as the Pythons) were a British surreal comedy troupe A comedy troupe is a group of comedians and associated personnel who work together to perform comedy as entertainment. The term is often used inte ...
, whom Python creator Guido van Rossum enjoyed while developing the language. Monty Python references appear frequently in Python code and culture; for example, the metasyntactic variables often used in Python literature are Spam (Monty Python), ''spam'' and ''eggs'' instead of the traditional foobar, ''foo'' and ''bar''. The official Python documentation also contains various references to Monty Python routines. The prefix ''Py-'' is used to show that something is related to Python. Examples of the use of this prefix in names of Python applications or libraries include Pygame, a language binding, binding of Simple DirectMedia Layer, SDL to Python (commonly used to create games); PyQt and PyGTK, which bind Qt (software), Qt and GTK to Python respectively; and
PyPy PyPy () is an alternative implementation of the Python PYTHON was a Cold War contingency plan of the Government of the United Kingdom, British Government for the continuity of government in the event of Nuclear warfare, nuclear war. Background ...
, a Python implementation originally written in Python.


Popularity

Since 2003, Python has consistently ranked in the top ten most popular programming languages in the TIOBE Programming Community Index where, , it is the most popular language (ahead of Java (programming language), Java, and C). It was selected Programming Language of the Year (for "the highest rise in ratings in a year") in 2007, 2010, 2018, and 2020 (the only language to do so four times). An empirical study found that scripting languages, such as Python, are more productive than conventional languages, such as C and Java, for programming problems involving string manipulation and search in a dictionary, and determined that memory consumption was often "better than Java and not much worse than C or C++". Large organizations that use Python include Wikipedia, Google, Yahoo!, CERN, NASA, Facebook, Amazon (company), Amazon, Instagram, Spotify and some smaller entities like Industrial Light & Magic, ILM and ITA Software, ITA. The social news networking site Reddit was written mostly in Python.


Uses

Python can serve as a scripting language for web applications, e.g., via for the Apache web server. With Web Server Gateway Interface, a standard API has evolved to facilitate these applications. Web frameworks like Django (web framework), Django, Pylons (web framework), Pylons, Pyramid (web framework), Pyramid, TurboGears, web2py, Tornado (web server), Tornado, Flask (web framework), Flask, Bottle (web framework), Bottle and Zope support developers in the design and maintenance of complex applications. Pyjs and IronPython can be used to develop the client-side of Ajax-based applications. SQLAlchemy can be used as a Data mapper pattern, data mapper to a relational database. Twisted (software), Twisted is a framework to program communications between computers, and is used (for example) by Dropbox (service), Dropbox. Libraries such as NumPy, SciPy and Matplotlib allow the effective use of Python in scientific computing, with specialized libraries such as Biopython and Astropy providing domain-specific functionality. SageMath is a computer algebra system with a notebook interface programmable in Python: its library covers many aspects of mathematics, including algebra, combinatorics, numerical mathematics, number theory, and calculus. OpenCV has Python bindings with a rich set of features for computer vision and Digital image processing, image processing. Python is commonly used in artificial intelligence projects and machine learning projects with the help of libraries like TensorFlow, Keras, PyTorch, Pytorch and Scikit-learn. As a scripting language with modular programming, modular architecture, simple syntax and rich text processing tools, Python is often used for natural language processing. Python can also be used to create games, with libraries such as Pygame, which can make 2D games. Python has been successfully embedded in many software products as a scripting language, including in finite element method software such as Abaqus, 3D parametric modeler like FreeCAD, 3D animation packages such as 3ds Max, Blender (software), Blender, Cinema 4D, Lightwave, Houdini (software), Houdini, Maya (software), Maya, modo (software), modo, MotionBuilder, Autodesk Softimage, Softimage, the visual effects compositor Nuke (software), Nuke, 2D imaging programs like GIMP, Inkscape, Scribus and Paint Shop Pro, and musical notation programs like scorewriter and Capella (notation program), capella. GNU Debugger uses Python as a Prettyprint, pretty printer to show complex structures such as C++ containers. Esri promotes Python as the best choice for writing scripts in ArcGIS. It has also been used in several video games, and has been adopted as first of the three available programming languages in Google App Engine, the other two being Java (software platform), Java and Go (programming language), Go. Many operating systems include Python as a standard component. It ships with most Linux distributions, AmigaOS 4 (using Python 2.7), FreeBSD (as a package), NetBSD, OpenBSD (as a package) and macOS and can be used from the command line (terminal). Many Linux distributions use installers written in Python: Ubuntu (operating system), Ubuntu uses the Ubiquity (software), Ubiquity installer, while Red Hat Linux and Fedora Linux use the Anaconda (installer), Anaconda installer. Gentoo Linux uses Python in its package management system, Portage (software), Portage. Python is used extensively in the information security industry, including in exploit development. Most of the Sugar (software), Sugar software for the One Laptop per Child XO, now developed at Sugar Labs, is written in Python. The Raspberry Pi single-board computer project has adopted Python as its main user-programming language. LibreOffice includes Python, and intends to replace Java with Python. Its Python Scripting Provider is a core feature since Version 4.0 from 7 February 2013.


Languages influenced by Python

Python's design and philosophy have influenced many other programming languages: * Boo (programming language), Boo uses indentation, a similar syntax, and a similar object model. * Cobra (programming language), Cobra uses indentation and a similar syntax, and its ''Acknowledgements'' document lists Python first among languages that influenced it. * CoffeeScript, a programming language that cross-compiles to JavaScript, has Python-inspired syntax. * ECMAScript/JavaScript borrowed iterators and generator (computer science), generators from Python. * GDScript, a scripting language very similar to Python, built-in to the Godot (game engine), Godot game engine. * Go (programming language), Go is designed for the "speed of working in a dynamic language like Python" and shares the same syntax for slicing arrays. * Groovy (programming language), Groovy was motivated by the desire to bring the Python design philosophy to Java (programming language), Java. * Julia (programming language), Julia was designed to be "as usable for general programming as Python". * Nim (programming language), Nim uses indentation and similar syntax. * Ruby (programming language), Ruby's creator, Yukihiro Matsumoto, has said: "I wanted a scripting language that was more powerful than Perl, and more object-oriented than Python. That's why I decided to design my own language." * Swift (programming language), Swift, a programming language developed by Apple, has some Python-inspired syntax. Python's development practices have also been emulated by other languages. For example, the practice of requiring a document describing the rationale for, and issues surrounding, a change to the language (in Python, a PEP) is also used in Tcl, Erlang (programming language), Erlang, and Swift.


See also

* Python syntax and semantics * pip (package manager)


References


Sources

* * * *


Further reading

* * * * * *


External links

* Python (programming language), Articles with example Python (programming language) code Class-based programming languages Computational notebook Computer science in the Netherlands Concurrent programming languages Cross-platform free software Cross-platform software Dutch inventions Dynamically typed programming languages Educational programming languages High-level programming languages Information technology in the Netherlands Multi-paradigm programming languages Object-oriented programming languages Programming languages Programming languages created in 1991 Scripting languages Text-oriented programming languages {{Good article