Python programming language

Python is an interpreted, interactive programming language created by Guido van Rossum in 1990. Python is fully dynamically typed and uses automatic memory management; it is thus similar to Tcl, Perl, Scheme, and Ruby. Python is developed as an open source project, managed by the non-profit Python Software Foundation. Python 2.4.1 was released on March 30, 2005.

History

Python was created in the early 1990s by Guido van Rossum at Stichting Mathematisch Centrum (CWI) in the Netherlands as a successor of the ABC programming language. Guido is Python's principal author, although it includes many contributions from others. Guido's continuing central role in deciding the direction of Python is jokingly acknowledged by referring to him as its Benevolent Dictator for Life (BDFL).

The last version released from CWI was Python 1.2. In 1995, Guido continued his work on Python at the Corporation for National Research initiatives (CNRI) in Reston, Virginia where he released several versions of the software. Python 1.6 was the last of the versions released by CNRI. In 2000, Guido and the Python core development team moved to BeOpen.com to form the BeOpen PythonLabs team. Python 2.0 was the first and only release from BeOpen.com.

Following the release of Python 1.6, and after Guido van Rossum left CNRI to work with commercial software developers, it became clear that the ability to use Python with software available under the GNU General Public License (GPL) was very desirable. CNRI and the Free Software Foundation (FSF) interacted to develop enabling wording changes to the Python license. Python 1.6.1 is essentially the same as Python 1.6, with a few minor bug fixes, and with a different license that enables later versions to be GPL-compatible. Python 2.1 is a derivative work of Python 1.6.1, as well as of Python 2.0.

After Python 2.0 was released by BeOpen.com, Guido van Rossum and the other PythonLabs developers joined Digital Creations. All intellectual property added from this point on, starting with Python 2.1 and its alpha and beta releases, is owned by the Python Software Foundation (PSF), a non-profit modeled after the Apache Software Foundation. See http://www.python.org/psf/ for more information about the PSF.

Philosophy

Python is a multi-paradigm language, like Perl, Oz or C++ and unlike Smalltalk or Haskell. This means that, rather than forcing coders to adopt one particular style of coding, it permits several. Object orientation, structured programming, functional programming, aspect-oriented programming, and more recently, design by contract are all supported. Python is dynamically type-checked and uses garbage collection for memory management. An important feature of Python is dynamic name resolution, which allows method and variable names to be bound during program execution.

While offering choice in coding methodology, Python's designers reject exuberant syntax, such as in Perl, in favor of a more sparse, less cluttered one. As with Perl, Python's developers expressly promote a particular "culture" or ideology (http://python.org/dev/culture.html) based on what they want the language to be, favoring language forms they see as "beautiful", "explicit" and "simple". For the most part, Perl and Python users differ in their interpretation of these terms and how they are best implemented (see TIMTOWTDI (http://c2.com/cgi/wiki?ThereIsMoreThanOneWayToDoIt)).

Another important goal of the Python developers is making Python fun to use. This is reflected in the origin of the name (after the television series Monty Python's Flying Circus); in the common practice of using Monty Python references in example code; and in an occasionally playful approach to tutorials and reference materials.

Although Python is sometimes classified as a "scripting programming language", it has been used to develop many large software projects such as the Zope application server and the Mnet and BitTorrent file sharing systems. It is also extensively used by Google. Python proponents prefer to call it a high level dynamic programming language, on the grounds that "scripting language" implies a language that is only used for simple shell scripts or that refers to a language like JavaScript: much simpler and, for most purposes, less capable than "real" programming languages such as Python.

Another important goal of the language is ease of extensibility. New built-in modules are easily written in C or C++. Python can also be used as an extension language for existing modules and applications that need a programmable interface.

Though the designer of Python is somewhat hostile to functional programming and the Lisp tradition, there are significant parallels between the philosophy of Python and that of minimalist Lisp-family languages such as Scheme. Many past Lisp programmers have found Python appealing for this reason.

Data types and structures

Since Python is a dynamically typed language, Python values, not variables, carry type. This has implications for many aspects of the way the language functions.

All values in Python are references to objects, and these references are passed to functions by value; a function cannot change the value a variable references in its calling function. Some people (including Guido van Rossum himself) have called this parameter-passing scheme "Call by object reference."

Among dynamically typed languages, Python is moderately type-checked — it is more strictly typed than Perl, for example. Implicit conversion is defined for numeric types, so one may validly multiply a complex number by a long integer (for instance) without explicit casting. However, there is no implicit conversion between (e.g.) numbers and strings; unlike in Perl, a string is an invalid argument to a mathematical function expecting a number.

Base types

Python has a broad range of basic data types. Alongside conventional integer and floating point arithmetic, it transparently supports arbitrarily large integers, arbitrarily precise decimals (http://www.python.org/peps/pep-0327.html)(as of 2.4) and complex numbers.

It supports the usual panoply of string operations, with one caveat: strings in Python are immutable objects, so any string operation that might (in other programming languages) alter a string (such as a substitution of characters) will instead return a new string.

Collection types

One of the very useful aspects of Python is the concept of collection (or container) types. In general a collection is an object that contains other objects in a way that is easily referenced or indexed. Collections come in two basic forms: sequences and mappings.

The ordered sequential types are lists (dynamic arrays), tuples, and strings. All sequences are indexed positionally (0 through length − 1) and all but strings can contain any type of object, including multiple types in the same sequence. Both strings and tuples are immutable, making them perfect candidates for dictionary keys (see below). Lists, on the other hand, are mutable; elements can be inserted, deleted, modified, appended, or sorted in place.

On the other side of the collections coin are mappings, which are unordered types implemented in the form of dictionaries which "map" a set of immutable keys, to corresponding elements much like a mathematical function. The keys in a dictionary must be of an immutable Python type such as an integer or a string. For example, one could define a dictionary having a string "foo" mapped to the integer 42 or vice versa. This is done under the covers via a hash function which makes for faster lookup times, but is also the culprit for a dictionary's lack of order and is the reason mutable objects (i.e. other dictionaries or lists) cannot be used as keys. Dictionaries are also central to the internals of the language as they reside at the core of all Python objects and classes: the mapping between variable names (strings) and the values which the names reference is stored as a dictionary (see Object system). Since these dictionaries are directly accessible (via an object's __dict__ attribute), meta-programming is a surprisingly straightforward and natural process in Python.

A set collection type was added to the core language in version 2.4. A set is an unindexed, unordered collection that contains no duplicates. This container type has many applications where only membership information is required and acts essentially like a dictionary without values. There are two types of sets: set and frozenset, the only difference being that set is mutable and frozenset is immutable. Elements in a set must be hashable and immutable. Thus, for example, a frozenset can be an element of a regular set whereas the opposite is not true.

Python also provides extensive collection manipulating abilities such as built in containment checking and a generic iteration protocol.

Object system

In Python, everything is an object, even classes. Classes, as objects, have a class, which is known as their metaclass. Python also supports multiple inheritance and mixins (see also MixinsForPython).

The language supports extensive introspection of types and classes. Types can be read and compared—indeed, as in Smalltalk, types are instances of a type. The attributes of an object can be extracted as a dictionary.

Operators can be overloaded in Python by defining special member functions—for instance, defining __add__ on a class permits one to use the + operator on members of that class. (Compare C++'s operator+ and similar method names.)

Syntax

Python was designed to be highly readable. It has a simple visual layout, uses English keywords frequently where other languages use punctuation, and has notably fewer syntactic constructions than many structured languages such as C, Perl, or Pascal.

For instance, Python has only two structured loop forms:

for, which loops over elements of a list or iterator (like Perl foreach)
while, which loops as long as a boolean expression is true.

It thus lacks C-style complex for, a do...while, and Perl's until, though of course equivalents can be expressed. Likewise, it has only if...elif...else for branching—no switch or labeled goto (goto was implemented (http://www.entrian.com/goto/) as a joke for April 1st 2004, in an add-on module).

Syntactical significance of indentation

One unusual aspect of Python's syntax is the method used to delimit program blocks. Sometimes termed "the whitespace thing", it is one aspect of Python syntax that many programmers otherwise unfamiliar with Python have heard of, since it is nearly unique among currently widespread languages (another language sharing this feature is Haskell).

In so-called "free-format" languages that use the block structure ultimately derived from Algol—including Pascal, C, Perl, and many others—blocks of code are set off with braces ({ }) or keywords such as Pascal's begin and end. In all these languages, however, programmers conventionally indent the code within a block, to set it off visually from the surrounding code.

Python, instead, borrows a feature from the lesser-known language ABC—instead of punctuation or keywords, it uses this indentation itself to indicate the run of a block. A brief example will make this clear. Here are C and Python recursive functions which do the same thing—computing the factorial of an integer:

Factorial function in C:

int factorial(int x) {      
    if (x == 0) {                      
        return 1;                   
    } else {
        return x * factorial(x-1);
    }
}

Factorial function in Python:

def factorial(x):
    if x == 0:
        return 1
    else:
        return x * factorial(x-1)

Some programmers used to Algol-style languages, in which whitespace is semantically empty, at first find this confusing or even offensive. A few have drawn unflattering comparison to the column-oriented style used on punched-card Fortran systems. When Algol was new, it was a major development to have "free-form" languages in which only symbols mattered and not their position on the line.

To Python programmers, however, "the whitespace thing" is simply the enforcement of a convention that programmers in Algol-style languages already follow anyway. They also point out that the free-form syntax has the disadvantage that, since indentation is ignored, good indentation cannot be enforced. Thus, incorrectly indented code may be misleading, since a human reader and a compiler could interpret it differently.

The whitespace thing has minor disadvantages. Both space characters and tab characters are currently accepted as forms of indentation. Since they are not visually distinguishable (in many tools), mixing spaces and tabs can create bugs that are particularly difficult to find (a perennial suggestion among Python users has been removing tabs as block markers—except, of course, among those Pythonistas who propound removing spaces instead).

Because whitespace is syntactically significant, it is not always possible for a program to automatically correct the indentation on Python code as can be done with C or Lisp code. Moreover, formatting routines which remove whitespace—for instance, many Internet forums—can completely destroy the syntax of a Python program, whereas a program in a bracketed language would merely become more difficult to read.

Comments and docstrings

Python has two ways to annotate Python code. One is by using comments to indicate what some part of the code does.

 def getline():
     return sys.stdin.readline()       # Get one line and return it

Comments begin with the hash character ("#") and are terminated by the end of line. Python does not support comments that span more than one line. The other way is to use docstrings (documentation string), that is strings delimited with either """ or '''.

 def getline():
     """Get one line from stdin and return it."""
     return sys.stdin.readline()

Docstrings can be as large as the programmer wants and contain line breaks. They must always begin in the first line of the body of a function, class or other Python entity. They can also be written at the top of a module. In contrast with comments, docstrings are themselves Python objects and are part of the interpreted code that Python runs. That means that a running program can retrieve its own docstrings and do manipulations with that info. But the normal usage is to give other programmers information about how to invoke the object being documented in the docstring.

There are tools available that can extract the docstrings to generate an API documentation from the code. Docstring documentation can also be accessed from the interpreter with the help() function, or from the shell with the pydoc command.

A hack is to use docstrings to comment out blocks of code. However, if the block that is being out-commented contains docstrings, it won't work and the code won't run. A better approach is to use an editor that has a multi-line comment function, which most Python-aware editors have.

Functional programming

As mentioned above, another strength of Python is the availability of a functional programming style. As may be expected, this makes working with lists and other collections much more straightforward. One such construction is the list comprehension, introduced from the functional language Haskell, as seen here in calculating the first five powers of two:

numbers = [1, 2, 3, 4, 5]
powers_of_two = [2**n for n in numbers]

The Quicksort algorithm can be expressed elegantly using list comprehensions:

def qsort(L):
  if L == []: return []
  return qsort([x for x in L[1:] if x< L[0] ]) + L[0:1] + \
         qsort([x for x in L[1:] if x>=L[0] ])

Although naive execution of this form of Quicksort is less space-efficient than forms which alter the sequence in-place, it is often cited as an example of the expressive power of list comprehensions.

Because Python permits functions as arguments, it is also possible to partially simulate more subtle functional constructs, such as the continuation.

Lambda

Python's lambda keyword may misdirect some functional-programming fans. Python lambda blocks may contain only expressions, not statements. Thus, they are not the most general way to return a function for use in higher-order functions. Instead, the usual practice is to define and return a function using a locally-scoped name, as in the following example of a simple curried function:

def add_and_print_maker(x):
    def temp(y):
        print "%d + %d = %d" % (x, y, x+y)
    return temp

The function can also be implemented with nested lambdas, as would be done in Scheme. To do this requires working around the Python lambda's limitation, by defining a function to encapsulate the print statement:

def print_func(obj):
   print obj

add_and_print_maker = \
   lambda(x): lambda(y): \
      print_func("%d + %d = %d" % (x, y, x+y))

The resulting add_and_print_maker functions perform identically: given a number x they return a function which, when given a number y, will print a sentence of arithmetic. Although the first style may be more common, the second can be more clear to programmers with a functional-programming background.

Python's short-circuit semantics of the binary boolean operators and and or creates another useful functional feature. Using those two operators, any type of control flow can be implemented within lambda expressions [1] (http://www-106.ibm.com/developerworks/linux/library/l-prog.html). They are usually used for simpler purposes, however. See the heading logical operators below.

Closures

Python has had support for lexical closures since version 2.2. See the examples in the above section on lambda for sample closure use.

Python's syntax, though, sometimes leads programmers of other languages to think that closures are not supported. They might try the following, for example:

def foo(initial_value=0):
    var = initial_value
    def var_setter(newval):
        var = newval
    def var_getter():
        return var
   return var_setter, var_getter

setter, getter = foo()
setter(19)
getter()      # <- would return 0

..and they then assume closures do not work in Python. The problem, however, is a misunderstanding of Python's naming and binding rules. When a name-binding operation occurs anywhere in a function body (e.g., an assignment), that name is made local to the function and can not be a free variable. However, the same functionality can be had by using part of a mutable object as the store, for example:

def foo(initial_value=0):
    varholder = [initial_value]   # put the value into a list
    def var_setter(newval):
        varholder[0] = newval
    def var_getter():
        return varholder[0]
    return var_setter, var_getter

setter, getter = foo()
setter(21)
getter()      # <- would return 21

Since the name of the variable from the enclosing scope (varholder) is not rebound in var_setter, varholder becomes a free variable in var_setter and var_setter becomes a closure.

Generators

Introduced in Python 2.2 as optional feature and finalized in version 2.3, generators are Python's mechanism for lazy evaluation of a function that would otherwise return a space-prohibitive or computationally intensive list. The uses of generators are similar to the uses of Scheme streams.

One example from the python.org website:

def generate_ints(N):
   for i in xrange(N):
       yield i

You can now use the generator generate_ints:

for i in generate_ints(N):
   print i

Note that the variable N should be defined before executing the second piece of code.

The definition of a generator appears identical to that of a function, except the keyword yield is used in place of return. However, a generator is an object with persistent state, which can repeatedly enter and leave the same dynamic extent. A generator call can then be used in place of a list, or other structure whose elements will be iterated over. Whenever the for-loop in the example requires the next item, the generator is called, and yields the next item.

Logical operators

In Python 2.2 and earlier, the expressions "", 0, 0.0, 0e0, 0j, None, (), [], {}, etc. are false, and everything else is true. When using binary Boolean operators in Python, the syntax is to have the operator be in between the two statements in question. So to see if the statements x==5 and 3 are true, one would write "x==5 and 3". To evaluate this, the interpreter would first check if x==5 returned true. If it did not, it would return 0, but since it did, it goes on to the next statement. Next, it checks if 3 is true. Since 3 is true, 3 is returned. If three were not true, 0 would be returned. If the order of all of this were reversed to 3 and x==5, 1 would be returned because that is what x==5 evaluates to (because 1 is the default truth value). The or function works similarly. To find out if "2/3 or 5" is true, the interpreter first finds the truth value of 2/3. Since 2/3 evaluates to 0, as described above, it would return false. If it had returned true, then its value would be returned. Next, the interpreter looks at the second expression. Since, in this case, it returns true, 5 would be returned. It is common in Python to write statements such as print p or q to take advantage of this feature.

Later in Python 2.2.1 the constants True and False were added and, as a result, all of the binary comparison operators (==, >, etc) return either True or False, while the rest of the aforementioned boolean operations (and, etc) still return the value that the last expression evaluated to. Thus the expression "2 == 2" will return the value True and "2 == 2 and 5" still returns the integer 5.

(Under the hood, in Python 2.3 and 2.4, True and False are builtin objects of type bool, a subclass of int. To keep backwards-compatibility, True and False only differ from 0 and 1 in type and string representation. Thus, statements treating the result of comparisons as ints still work. Ex. print intAsStr+(" "*(fieldWidth-len(intAsStr))) In 2.2.1–2.2.3, they are names for the int objects 1 and 0, respectively.)

Comparison operators

The basic comparison operators such as ==<code>, <code><, >=, and so forth, are used on all manner of values. Numbers, strings, sequences, and mappings can all be compared. Objects of dissimilar type (such as a string and a number) can be compared; the result is arbitrary, but consistent.

Chained comparison expressions such as a < b < c have roughly the meaning that they have in mathematics, rather than the unusual meaning found in C and similar languages. The terms are evaluated and compared in order. The operation is short circuit, meaning that evaluation stops as soon as the expression is proven false: if a < b is false, c is never evaluated.

For expressions without side effects, a < b < c is equivalent to a < b and b < c. However, there is a substantial difference when the expressions have side effects. a < f(x) < b will evaluate f(x) exactly once, whereas a < f(x) and f(x) < b may evaluate it once or twice.

Object-oriented programming

Python's support for object oriented programming paradigm is vast. It supports polymorphism, not only under the statically typed "polymorphism of classes inherited from the same base class", but fully in the Liskov Substitution Principle-sense for all objects. And everything in Python is an object, including classes, functions, numbers and modules. Python also has support for metaclasses, an advanced tool for enhancing classes' functionality. Naturally, inheritance, including multiple inheritance, is supported. It has limited support for private variables using name mangling. See the "Classes" section of the tutorial (http://www.python.org/doc/current/tut/node11.html#SECTION0011600000000000000000) for details. Many Python users don't feel the need for private variables, though. The slogan "We're all consenting adults here" is used to describe this attitude. Some consider information hiding to be unpythonic, in that it suggests that the class in question contains unaesthetic or ill-planned internals.

From the tutorial: As is true for modules, classes in Python do not put an absolute barrier between definition and user, but rather rely on the politeness of the user not to "break into the definition."

OOP doctrines such as the use of accessor methods to read data members are not enforced in Python. Just as Python offers functional-programming constructs but does not attempt to demand referential transparency (in contrast with Haskell), it offers (and extensively uses!) its object system but does not demand OOP behavior (in contrast with Java or Smalltalk). Moreover, it is always possible to redefine the class using properties so that when a certain variable is set or retrieved in calling code, it really invokes a function call, so that foo.x = y might really invoke foo.set_x(y). This nullifies the practical advantage of accessor functions, and it remains OOP because the property 'x' becomes a legitimate part of the object's interface: it need not reflect an implementation detail.

In version 2.2 of Python, "new-style" classes were introduced. With new-style classes, objects and types were unified, allowing the subclassing of types. Even new types entirely can be defined, complete with custom behavior for infix operators. This allows for many radical things to be done syntactically within Python, such as the ability to use C++-style input and output. A new multiple inheritance model was adopted with new-style classes, making a much more logical order of inheritance, adopted from Common Lisp. The new method __getattribute__ was also defined for unconditional handling of attribute access.

Exception handling

Python supports (and extensively uses) exception handling as a means of testing for error conditions and other "exceptional" events in a program. Indeed, it is even possible to trap the exception caused by a syntax error.

Python style calls for the use of exceptions whenever an error condition might arise. Indeed, rather than testing for access to a file or resource before actually using it, it is conventional in Python to just go ahead and try to use it, catching the exception if access is rejected.

Exceptions can also be used as a more general means of non-local transfer of control, even when an error is not at issue. For instance, the Mailman mailing list software, written in Python, uses exceptions to jump out of deeply-nested message-handling logic when a decision has been made to reject a message or hold it for moderator approval.

Exceptions are often, especially in threaded situations, used as an alternative to the if-block. A commonly-invoked motto is EAFP, or "It is Easier to Ask for Forgiveness than to ask for Permission." Consider these two equivalent pieces of code:

try:
  baz = foo.bar
except AttributeError:
  handle_error()

if hasattr(foo, 'bar'):
  baz = foo.bar
else:
  handle_error()

Standard library

Missing image
Python_batteries_included.jpg

Python comes with "batteries included"

Python has a large standard library, which makes it well suited to many tasks. This comes from a so-called "batteries included" philosophy for Python modules. The modules of the standard library can be augmented with custom modules written in either C or Python. The standard library is particularly well tailored to writing Internet-facing applications, with a large number of standard formats and protocols (such as MIME and HTTP) supported. Modules for creating graphical user interfaces, connecting to relational databases, and manipulating regular expressions are also included.

The standard library is one of Python's greatest strengths. The bulk of it is cross-platform compatible, meaning that even heavily leveraged Python programs can often run on Unix, Windows, Macintosh, and other platforms without change.

It is currently being debated whether or not third-party but open source Python modules such as Twisted, NumPy, or wxPython should be included in the standard library, in accordance with the batteries included philosophy.

Other features

Like Lisp, and unlike Perl, the Python interpreter also supports an interactive mode in which expressions can be entered from the terminal and results seen immediately. This is a boon for those learning the language and experienced developers alike: snippets of code can be tested in interactive mode before integrating them into a program proper.

Python also includes a unit testing framework for creating exhaustive test suites. While static typing aficionados see this as a replacement for a static type-checking system, Python programmers largely do not share this view.

Standard Python does not support continuations (and never will, according to Guido van Rossum), but there is a variant known as Stackless Python that does.

Neologisms

A few neologisms have come into common use within the Python community. One of the most common is "pythonic", which can have a wide range of meanings related to program style. To say that a piece of code is pythonic is to say that it uses Python idioms well; that it is natural or shows fluency in the language. Likewise, to say of an interface or language feature that it is pythonic is to say that it works well with Python idioms; that its use meshes well with the rest of the language.

In contrast, a mark of unpythonic code is that it attempts to "write C++ (or Lisp, or Perl) code in Python"—that is, provides a rough transcription rather than an idiomatic translation of forms from another language.

The prefix Py- can be used to show that something is related to Python, much as a prefixed J- denotes Java. Examples of the use of this prefix in names of Python applications or libraries include PyGame, a binding of SDL to Python; PyUI, a GUI encoded entirely in Python; PySol, a series of card games programmed in Python; and PyAlaMode, an IDE for Python created by Orbtech, a company specializing in Python.

Users and admirers of Python—most especially those considered knowledgeable or experienced—are often referred to as Pythonistas.

Supported platforms

The most popular (and therefore best maintained) platforms Python runs on are Microsoft Windows, Linux, BSD, Mac OS X, and Java (the Java version being a completely separate implementation). A Mac GUI on Python is maintained by an external project called MacPython, and was included in Mac OS 10.3 "Panther". Other supported platforms include:

Most of the third-party libraries for Python (and even some first-party ones) are only available on Windows, Linux, BSD, and Mac OS X.

Python was originally developed as a scripting language for the Amoeba operating system capable of making system calls; that version is no longer maintained.

Python usage

Python is actively used by many people, both in industry and academia for a wide variety of purposes.

Major organizations using Python

Google uses Python for many tasks including the backends of web apps such as Gmail and Google Maps (the front-ends are AJAX based), and for many of its search-engine internals.

Software written in Python

BitTorrent, the original implementation and several derivatives.
Chandler is a personal information manager including calendar, email, tasks and notes support.
GNOME, the desktop environment, makes use of python for building graphical interfaces with the GTK toolkit.
Mailman, one of the more popular packages for running email mailing lists.
MoinMoin, a popular wiki engine in Python.
OpenRPG provides a virtual table on which to play Role Playing Games over the internet.
Portage, the heart of Gentoo Linux. An advanced package management system based on the *BSD style ports system.
Solipsis, a system for massively shared virtual world.
ViewCVS, a web-based interface for browsing CVS repositories
Wikipedia has a Python framework for creating webbots (http://meta.wikimedia.org/wiki/Category:Pywikipedia).
Zope, an object-oriented web-application platform. Zope includes an application server with an integrated object-oriented database and a built-in web-based management interface.

Packages for Python

mod_python, an Apache module allowing direct integration of Python scripts with the Apache web server.
Numeric Python, a language extension that adds support for large, multi-dimensional arrays and matrices.
Pygame http://www.pygame.org Python game development
PyGTK, http://www.pygtk.org/, a popular cross-platform GUI library based on GTK+
PyQt (http://www.riverbankcomputing.co.uk/pyqt/), another popular cross-platform GUI library based on Qt
SciPy, a library of scientific and numerical routines
Twisted, a networking framework for Python
wxPython (http://www.wxpython.org/), a port of wxWidgets and a popular cross-platform GUI library for Python
ZODB (http://zope.org/Wikis/ZODB/FrontPage/guide/zodb.html) a Python-specific object-oriented database
Quixote (http://www.mems-exchange.org/software/quixote/) a framework for developing Web applications in Python

Python Implementations

CPython (http://www.python.org) - The default/reference Python implementation
Jython - a Java implementation of Python
IronPython - Python for .NET and Mono platforms
Stackless Python (http://www.stackless.com/)
PyPy (http://codespeak.net/pypy/index.cgi?news/) - an implementation of Python in Python
Parrot - A virtual machine being developed primarily as the runtime for Perl 6, but with an intention of also supporting dynamic languages like Python, Ruby, TCL, etc. A subset of Python can currently be executed by the Parrot runtime.
Logix (http://logix.livelogix.com/) - alternate front-end to Python with macros
Boo - based on Python but with static typing, for .NET and Mono

External links and references

Template:Wikibookspar

Python Books

The Python Language Reference Manual (http://www.network-theory.co.uk/python/language/) by Guido van Rossum and Fred L. Drake, Jr. (ISBN 0-9541617-8-5)
A Byte of Python (http://www.byteofpython.info/) is a beginner's book on Python.
How to Think Like a Computer Scientist: Learning with Python (http://www.greenteapress.com/thinkpython/) is an introduction to function-based programming constructs using Python - free download available or hardcopy may be purchased. (ISBN 0971677506)
Text Processing in Python (http://www.gnosis.cx/TPiP/) is an intermediate Python book, available both online for free and for money from Addison-Wesley. (ISBN 0321112547)
Dive into Python (http://www.diveintopython.org/toc.html) demonstrates clever and useful Python paradigms for readers who know how to program already. It is available online, or hardcopy may be purchased. Wikipedia has a page about Dive into Python.

Python Links

The Python Homepage (http://www.python.org/)
Python Documentation (http://www.python.org/doc/versions.html)
Several Python Tutorials and References (http://www.python.org/doc/) are available from Python official website.
Python Wiki (http://www.python.org/cgi-bin/moinmoin)
Python Faqts (http://www.faqts.com/knowledge_base/index.phtml/fid/199)
The Architecture of Python (http://wiki.cs.uiuc.edu/cs427/PYTHON) discusses Python internals.
Py (http://www.pyzine.com/) is The Python Online Technical Journal.
Samples for extending and embedding python with C/C++ (http://www.codesampler.com/python.htm)
PyCode (http://www.pycode.com/) contains Python Resources and Modules collection.
The Vaults of Parnassus (http://www.vex.net/parnassus/) contains links to Python resources.
A-Python-A-Day (http://xahlee.org/perl-python/python.html) is a mailing list with daily examples of using Python. The home page has links to numerous past tips. Examples are given in Python and Perl to help the Perl programmer learn Python.
Python Performance Tips (http://manatee.mojam.com/~skip/python/fastpython.html) contains tips and tricks to help you improve the performance of your python programs.
PythonPhotos.org (http://pythonphotos.org/) is a public photo archive for the Python developer community.
Vaults story (http://www.vex.net/parnassus/parnassus-story.html) contains Python humour.
Video: Introducing Python (featuring "A Python Love Story") (http://www.ibiblio.org/obp/pyBiblio/pythonvideo.php)

Non-English Resources

Python Wiki in Portuguese (http://www.pythonbrasil.com.br/)

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