Python (programming language)
Python is an interpreted, high-level, general-purpose programming language. Created by Guido van Rossum and first released in 1991, Python's design philosophy emphasizes code readability with its notable use of significant whitespace. Its language constructs and object-oriented 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 paradigms, including structured, object-oriented, and functional programming. Python is often described as a "batteries included" language due to its comprehensive standard library.
Python was conceived in the late 1980s as a successor to the ABC language. Python 2.0, released in 2000, introduced features like list comprehensions and a garbage collection system with reference counting.
Python 3.0, released in 2008, was a major revision of the language that is not completely backward-compatible, and much Python 2 code does not run unmodified on Python 3.
The Python 2 language was officially discontinued in 2020, and "Python 2.7.18 is the last Python 2.7 release and therefore the last Python 2 release." No more security patches or other improvements will be released for it. With Python 2's end-of-life, only Python 3.5.x and later are supported.
Python interpreters are available for many operating systems. A global community of programmers develops and maintains CPython, a free and open-source reference implementation. A non-profit organization, the Python Software Foundation, manages and directs resources for Python and CPython development.
HistoryPython was conceived in the late 1980s by Guido van Rossum at Centrum Wiskunde & Informatica in the Netherlands as a successor to the ABC language, capable of exception handling and interfacing with the Amoeba 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. He now shares his leadership as a member of a five-person steering council. In January 2019, active Python core developers elected Brett Cannon, Nick Coghlan, Barry Warsaw, Carol Willing and Van Rossum to 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 and support for 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
2to3utility, 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.
Features and philosophyPython is a multi-paradigm programming language. Object-oriented programming and structured programming are fully supported, and many of its features support functional programming and aspect-oriented programming. Many other paradigms are supported via extensions, including design by contract and logic programming.
Python uses dynamic typing and a combination of reference counting and a cycle-detecting garbage collector for memory management. It also features dynamic name resolution, which binds method and variable names during program execution.
Python's design offers some support for functional programming in the Lisp tradition. It has
reducefunctions; list comprehensions, dictionaries, sets, and generator expressions. The standard library has two modules that implement functional tools borrowed from Haskell and Standard ML.
The language's core philosophy is summarized in the document The Zen of Python, which includes aphorisms such as:
- Beautiful is better than ugly.
- Explicit is better than implicit.
- Simple is better than complex.
- Complex is better than complicated.
- Readability counts.
Python strives for a simpler, less-cluttered syntax and grammar while giving developers a choice in their coding methodology. In contrast to Perl'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 at the Python Software Foundation 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 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, a just-in-time compiler. Cython is also available, which translates a Python script into C and makes direct C-level API calls into the Python interpreter.
An important goal of Python's developers is keeping it fun to use. This is reflected in the language's name—a tribute to the British comedy group Monty Python—and in occasionally playful approaches to tutorials and reference materials, such as examples that refer to spam and eggs instead of the standard foo and bar.
A common neologism 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 semanticsPython 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 to delimit blocks, and semicolons after statements are optional. It has fewer syntactic exceptions and special cases than C or Pascal.
IndentationPython uses whitespace indentation, rather than curly brackets 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, which some other languages share, but in most languages indentation doesn't have any semantic meaning.
Statements and control flowPython's statements include :
- The assignment statement. This operates differently than in traditional imperative programming languages, and this fundamental mechanism illuminates many other features of the language. Assignment in C, e.g.,
x = 2, translates to "typed variable name receives a copy of numeric value 2". The value is copied into an allocated storage location for which the variable name is the symbolic address. The memory allocated to the variable is large enough for the declared type. In the simplest case of Python assignment, using the same example,
x = 2, translates to " name x receives a reference to a separate, dynamically allocated object of numeric type of value 2." This is termed binding the name to the object. Since the name's storage location doesn't contain the indicated value, it is improper to call it a variable. Names may be subsequently rebound at any time to objects of greatly varying types, including strings, procedures, complex objects with data and methods, etc. Successive assignments of a common value to multiple names, e.g.,
x = 2;
y = 2;
z = 2result in allocating storage to three names and one numeric object, to which all three names are bound. Since a name is a generic reference holder it is unreasonable to associate a fixed data type with it. However at a given time a name will be bound to some object, which will have a type; thus there is dynamic typing.
ifstatement, which conditionally executes a block of code, along with
forstatement, which iterates over an iterable object, capturing each element to a local variable for use by the attached block.
whilestatement, which executes a block of code as long as its condition is true.
trystatement, which allows exceptions raised in its attached code block to be caught and handled by
exceptclauses; it also ensures that clean-up code in a
finallyblock will always be run regardless of how the block exits.
raisestatement, used to raise a specified exception or re-raise a caught exception.
classstatement, which executes a block of code and attaches its local namespace to a class, for use in object-oriented programming.
defstatement, which defines a function or method.
withstatement, from Python 2.5 released in September 2006, which encloses a code block within a context manager, allowing Resource Acquisition Is Initialization -like behavior and replaces a common try/finally idiom.
breakstatement, exits from the loop.
continuestatement, skips this iteration and continues with the next item.
passstatement, which serves as a NOP. It is syntactically needed to create an empty code block.
assertstatement, used during debugging to check for conditions that ought to apply.
yieldstatement, which returns a value from a generator function. From Python 2.5,
yieldis also an operator. This form is used to implement coroutines.
importstatement, which is used to import modules whose functions or variables can be used in the current program. There are three ways of using import:
import , ,...
ExpressionsSome Python expressions are similar to languages such as C and 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
//and floating point
/division. Python also added 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. Python's
isoperator may be used to compare object identities. In Python, comparisons may be chained, for example
a <= b <= c.
- Python uses the words
notfor its boolean operators rather than the symbolic
!used in Java and C.
- Python has a type of expression termed a list comprehension. Python 2.4 extended list comprehensions into a more general expression termed a generator expression.
- Anonymous functions are implemented using lambda expressions; however, these are limited in that the body can only be one expression.
- Conditional expressions in Python are written as
x if c else y.
- Python makes a distinction between lists and tuples. Lists are written as
, are mutable, and cannot be used as the keys of dictionaries. 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
tinitially equal to
t = t +first evaluates
t +, 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, are associated in the 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 analogous to
printfformat strings in C, e.g.
"spam=%s eggs=%d" %evaluates to
"spam=blah eggs=2". In Python 3 and 2.6+, this was supplemented by the
formatmethod of the
"spam= eggs=".format. Python 3.6 added "f-strings":
blah = "blah"; eggs = 2; f'spam= eggs='.
- Python has various kinds of string literals:
- * Strings delimited by single or double quote marks. Unlike in Unix shells, Perl 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.
- * 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 Windows-style paths. Compare "
@-quoting" in C#.
- Python has array index and array slicing expressions on lists, denoted as
a. Indexes are 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
areturns a copy of the entire list. Each element of a slice is a shallow copy.
- List comprehensions vs.
- Conditional expressions vs.
execbuilt-in functions ; the former is for expressions, the latter is for statements.
a = 1cannot 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
ifis syntactically valid C code but
if c = 1:...causes a syntax error in Python.
Methodson objects are functions attached to the object's class; the syntax
instance.methodis, for normal methods and functions, syntactic sugar for
Class.method. Python methods have an explicit
selfparameter to access instance data, in contrast to the implicit
selfin some other object-oriented programming languages.
TypingPython 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 dynamically typed, Python is strongly typed, forbidding operations that are not well-defined rather than silently attempting to make sense of them.
Python allows programmers to define their own types using classes, which are most often used for object-oriented programming. New instances of classes are constructed by calling the class or
EggsClass), and the classes are instances of the metaclass
type, allowing metaprogramming and 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
objectis inherited from, directly or indirectly. 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. An experimental optional static type checker named mypy supports compile-time type checking.
|mutable||Sequence of bytes|
|immutable||Sequence of bytes|
|immutable||Complex number with real and imaginary parts|
|mutable||Associative array of key and value pairs; can contain mixed types, keys must be a hashable type|
|immutable||An ellipsis placeholder to be used as an index in NumPy arrays|
|immutable||Double precision floating point number. The precision is machine dependent but in practice is generally implemented as a 64-bit IEEE 754 number with 53 bits of precision|
|immutable||Unordered set, contains no duplicates; can contain mixed types, if hashable|
|immutable||Integer of unlimited magnitude|
|mutable||List, can contain mixed types|
|immutable||An object representing the absence of a value, often called null in other languages|
|immutable||A placeholder that can be returned from overloaded operators to indicate unsupported operand types.|
|immutable||A Sequence of numbers commonly used for looping specific number of times in |
|mutable||Unordered set, contains no duplicates; can contain mixed types, if hashable|
|immutable||A character string: sequence of Unicode codepoints|
|immutable||Can contain mixed types|
MathematicsPython has the usual symbols for arithmetic operators, the floor division operator
//and the remainder operator
%. It also has
**for exponentiation, e.g.
9**0.5 3.0, and a matrix multiply operator
@. These operators work like in traditional math; with the same precedence rules, the operators infix.
Division between integers produces floating point results. The behavior of division has changed significantly over time:
- 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
- Python 2.2 changed integer division to round towards negative infinity, e.g.
-7/3 -3. The floor division
//operator was introduced. So
-7.5//3 -3.0. Adding
from __future__ import divisioncauses a module to use Python 3.0 rules for division.
- Python 3.0 changed
/to always be floating-point division, e.g..
/is true 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
//b a//b + 1is always true. It also means that the equation
bis a positive integer, it has to lie in the interval
2. Versions before 3 used round-away-from-zero:
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 < ctests whether
ais less than
bis 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
Python uses arbitrary-precision arithmetic for all integer operations. The
Decimaltype/class in the
decimalmodule provides decimal floating point numbers to a pre-defined arbitrary precision and several rounding modes. The
Fractionclass in the
fractionsmodule 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.
Python programming examplesprogram:
Program to calculate the factorial of a positive integer:
n = int
if n < 0:
fact = 1
for i in range:
fact *= i
LibrariesPython'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 HTTP are supported. It includes modules for creating graphical user interfaces, connecting to relational databases, generating pseudorandom numbers, arithmetic with arbitrary-precision decimals, manipulating regular expressions, and unit testing.
Some parts of the standard library are covered by specifications, 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, the official repository for third-party Python software, contains over 200,000 packages with a wide range of functionality, including:
- Data analytics
- Graphical user interfaces
- Image processing
- Machine learning
- Mobile App
- Scientific computing
- System administration
- Test frameworks
- Text processing
- Web frameworks
- Web scraping
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 ; PythonAnywhere, a browser-based IDE and hosting environment; and Canopy IDE, a commercial Python IDE emphasizing scientific computing.
Other implementationsis a fast, compliant interpreter of Python 2.7 and 3.6. Its 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 C memory stack, thus allowing massively concurrent programs. PyPy also has a stackless version.
MicroPython and CircuitPython are Python 3 variants optimized for microcontrollers. This includes Lego Mindstorms EV3.
Unsupported implementationsOther 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 five-fold 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 was a just-in-time 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 standard Python code.
Cross-compilers to other languagesThere 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 compiles Python to C and C++.
- Google's Grumpy compiles Python to Go.
- IronPython follows a similar approach in order to run Python programs on the.NET Common Language Runtime.
- Jython enables the use of the Java class library from a Python program.
- MyHDL compiles Python to VHDL.
- Nuitka compiles Python into C++.
- Numba uses LLVM to compile Python to machine code.
- Pyrex and Shed Skin compile to C and C++ respectively.
- Pythran compiles Python to C++.
- RPython can be compiled to C, and is used to build the PyPy interpreter of Python.
DevelopmentPython's development is conducted largely through the Python Enhancement Proposal 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 bug tracker hosted at . Development originally took place on a 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 need to be manually ported. The first part of the version number is incremented. These releases happen infrequently—for example, version 3.0 was released 8 years after 2.0.
- Major or "feature" releases, about every 18 months, are largely compatible but introduce new features. The second part of the version number is incremented. 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 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, and using the BuildBot continuous integration system.
The major academic conference on Python is PyCon. There are also special Python mentoring programmes, such as Pyladies.
NamingPython's name is derived from the British comedy group Monty Python, 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 and eggs instead of the traditional 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 binding of SDL to Python ; PyQt and PyGTK, which bind Qt and GTK to Python respectively; and PyPy, a Python implementation originally written in Python.
API documentation generatorsPython API documentation generators include:
most popular programming languages in the TIOBE Programming Community Index where, as of 2020, it is the third most popular language. It was selected Programming Language of the Year in 2007, 2010, and 2018.
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, Instagram, Spotify and some smaller entities like ILM and ITA. The social news networking site Reddit is written entirely in Python.
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, Pylons, Pyramid, TurboGears, web2py, Tornado, Flask, 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 data mapper to a relational database. Twisted is a framework to program communications between computers, and is used by 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 mathematical software with a notebook interface programmable in Python: its library covers many aspects of mathematics, including algebra, combinatorics, numerical mathematics, number theory, and calculus.
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, Cinema 4D, Lightwave, Houdini, Maya, modo, MotionBuilder, Softimage, the visual effects compositor Nuke, 2D imaging programs like GIMP, Inkscape, Scribus and Paint Shop Pro, and musical notation programs like scorewriter and capella. GNU Debugger uses Python as a 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 and Go.
Python is commonly used in artificial intelligence projects and machine learning projects with the help of libraries like TensorFlow, Keras, Pytorch and Scikit-learn. As a scripting language with modular architecture, simple syntax and rich text processing tools, Python is often used for natural language processing.
Many operating systems include Python as a standard component. It ships with most Linux distributions, AmigaOS 4, FreeBSD, NetBSD, OpenBSD and macOS and can be used from the command line. Many Linux distributions use installers written in Python: Ubuntu uses the Ubiquity installer, while Red Hat Linux and Fedora use the Anaconda installer. Gentoo Linux uses Python in its package management system, Portage.
Python is used extensively in the information security industry, including in exploit development.
Most of the 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 PythonPython's design and philosophy have influenced many other programming languages:
- Boo uses indentation, a similar syntax, and a similar object model.
- Cobra uses indentation and a similar syntax, and its Acknowledgements document lists Python first among languages that influenced it.
- GDScript, a scripting language very similar to Python, built-in to the Godot game engine.
- Go is designed for the "speed of working in a dynamic language like Python" and shares the same syntax for slicing arrays.
- Groovy was motivated by the desire to bring the Python design philosophy to Java.
- Julia was designed to be "as usable for general programming as Python".
- Nim uses indentation and a similar syntax.
- 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, a programming language developed by Apple, has some Python-inspired syntax.