Python syntax and semantics


The syntax of the Python programming language is the set of rules that defines how a Python program will be written and interpreted. The Python language has many similarities to Perl, C, and Java. However, there are some definite differences between the languages.

Design philosophy

Python was designed to be a highly readable language. It has a relatively uncluttered visual layout and uses English keywords frequently where other languages use punctuation. Python aims to be simple and consistent in the design of its syntax, encapsulated in the mantra "There should be one—and preferably only one—obvious way to do it", from "The Zen of Python".
This mantra is deliberately opposed to the Perl and Ruby mantra, "there's more than one way to do it".

Keywords

Python has the following keywords or reserved words; they cannot be used as identifiers.
;Notes

Indentation

Python uses whitespace to delimit control flow blocks. Python borrows this feature from its predecessor ABC: instead of punctuation or keywords, it uses indentation to indicate the run of a block.
In so-called "free-format" languages — that use the block structure derived from ALGOL — blocks of code are set off with braces or keywords. In most coding conventions for these languages, programmers conventionally indent the code within a block, to visually set it apart from the surrounding code.
Consider a function, foo, which is passed a single parameter, x, and if the parameter is 0 will call bar and baz, otherwise it will call qux, passing x, and also call itself recursively, passing x-1 as the parameter. Here are implementations of this function in both C and Python:
foo function in C with K&R indent style:

void foo

foo function in Python:

def foo:
if x 0:
bar
baz
else:
qux
foo

Python mandates a convention that programmers in ALGOL-style languages often follow. Incorrectly indented code can be understood by human reader differently than does a compiler or interpreter.
This example illustrates an error introduced by incorrect indentation:

def foo:
if x 0:
bar
baz
else:
qux
foo

Here, in contrast to the above Python foo example, the function call foo always gets executed, resulting in an endless recursion. Such an indentation error is only possible in programming languages that do not mark blocks with distinct markers, like curly brackets in C. In this particular case, not even an editor with automatic indentation could prevent the erroneous behaviour of this Python code. This unintended error can easily pass into the code base without prior noticing by the programmer. In most other programming languages, this would not be possible, and this makes the Python syntax less robust than most other languages.
Both space characters and tab characters are currently accepted as forms of indentation in Python. Since many tools do not visually distinguish them, mixing spaces and tabs can create bugs that take specific efforts to find. Moreover, formatting routines which remove whitespace—for instance, many Internet forums—can destroy the syntax of a Python program, whereas a program in a bracketed language would merely become more difficult to read.
Many popular code editors handle Python's indentation conventions seamlessly, sometimes after a configuration option is enabled.

Data 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 variables in Python hold references to objects, and these references are passed to functions; a function cannot change the value of variable references in its calling function. Some people have called this parameter-passing scheme "Call by object reference." An object reference means a name, and the passed reference is an "alias", i.e. a copy of the reference to the same object, just as in C/C++. The object's value may be changed in the called function with the "alias", for example:

>>> alist =
>>> def my_func:
... al.append
... print
...
>>> my_func
>>> alist

Function my_func changed the value of alist with the formal argument al, which is an alias of alist. However, any attempt to operate on the alias itself will have no effect on the original object. In Python, non-innermost-local and not-declared-global accessible names are all aliases.
Among dynamically typed languages, Python is moderately type-checked. Implicit conversion is defined for numeric types, so one may validly multiply a complex number by an integer without explicit casting. However, there is no implicit conversion between, for example, numbers and strings; 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 arbitrary-precision arithmetic, complex numbers, and decimal floating point numbers.
Python supports a wide variety of string operations. Strings in Python are immutable, so a string operation such as a substitution of characters, that in other programming languages might alter a string in place, returns a new string in Python. Performance considerations sometimes push for using special techniques in programs that modify strings intensively, such as joining character arrays into strings only as needed.

Collection types

One of the very useful aspects of Python is the concept of collection 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, tuples, and strings. All sequences are indexed positionally 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. Lists, on the other hand, are mutable; elements can be inserted, deleted, modified, appended, or sorted in-place.
Mappings, on the other hand, are unordered types implemented in the form of dictionaries which "map" a set of immutable keys to corresponding elements. For example, one could define a dictionary having a string "toast" mapped to the integer 42 or vice versa. The keys in a dictionary must be of an immutable Python type, such as an integer or a string, because under the hood they are implemented via a hash function. This makes for much faster lookup times, but requires keys not change.
Dictionaries are also central to the internals of the language as they reside at the core of all Python objects and classes: the mappings between variable names and the values which the names reference are stored as dictionaries. Since these dictionaries are directly accessible, metaprogramming is a 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, and implements set theoretic operations such as union, intersection, difference, symmetric difference, and subset testing. 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. 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.
The language supports extensive introspection of types and classes. Types can be read and compared—types are instances of 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.

Literals

Strings

Python has various kinds of string literals.

Normal string literals

Either single or double quotes can be used to quote strings. Unlike in Unix shell languages, Perl or Perl-influenced languages such as Ruby or Groovy, single quotes and double quotes function identically, i.e. there is no string interpolation of $foo expressions. However, interpolation can be done in various ways: with "f-strings", using the format method or the old % string-format operator.
For instance, the Perl statement:

print "I just printed $num pages to the printer $printer\n"

is equivalent to any of these Python statements:

print
print
print
print
print
prints pages to the printer %

Multi-line string literals

There are also multi-line strings, which begin and end with a series of three single or double quotes and function like here documents in Perl and Ruby.
A simple example with variable interpolation is:

print

Raw strings

Finally, all of the previously mentioned string types come in "raw" varieties, which do no backslash-interpolation and hence are very useful for regular expressions; compare "@-quoting" in C#. Raw strings were originally included specifically for regular expressions. Due to limitations of the tokenizer, raw strings may not have a trailing backslash. Creating a raw string holding a Windows path ending with a backslash requires some variety of workaround.
Examples include:

>>> # A Windows path, even raw strings cannot end in a backslash
>>> r"C:\Foo\Bar\Baz\"
File "", line 1
r"C:\Foo\Bar\Baz\"
^
SyntaxError: EOL while scanning string literal
>>> dos_path = r"C:\Foo\Bar\Baz\ " # avoids the error by adding
>>> dos_path.rstrip # and removing trailing space
'C:\\Foo\\Bar\\Baz\\'
>>> quoted_dos_path = r'""'.format
>>> quoted_dos_path
'"C:\\Foo\\Bar\\Baz\\ "'
>>> # A regular expression matching a quoted string with possible backslash quoting
>>> re.match"', quoted_dos_path).group.rstrip
'C:\\Foo\\Bar\\Baz\\'
>>> code = 'foo'
>>> # Reverse the arguments in a two-arg function call
>>> re.sub,', r'
'foo'
>>> # Note that this won't work if either argument has parens or commas in it.

Concatenation of adjacent string literals

String literals appearing contiguously and only separated by whitespace, are allowed and are aggregated into a single longer string.
Thus

title = "One Good Turn: " \
'A Natural History of the Screwdriver and the Screw'

is equivalent to

title = "One Good Turn: A Natural History of the Screwdriver and the Screw"

Numbers

Numeric literals in Python are of the normal sort, e.g. 0, -1, 3.4, 3.5e-8.
Python has arbitrary-length integers and automatically increases the storage size as necessary. Prior to Python version 3, there were two kinds of integral numbers: traditional fixed size integers and "long" integers of arbitrary range. The conversion to "long" integers was performed automatically when required, and thus the programmer usually didn't have to be aware of the two integral types. In newer language versions the fixed-size integers are completely gone.
Python supports normal floating point numbers, which are created when a dot is used in a literal, when an integer and a floating point number are used in an expression, or as a result of some mathematical operations.
Python also supports complex numbers natively. Complex numbers are indicated with the J or j suffix, e.g. 3 + 4j.

Lists, tuples, sets, dictionaries

Python has syntactic support for the creation of container types.
Lists are mutable sequences of items of arbitrary types, and can be created either with the special syntax

a_list =

or using normal object creation

a_second_list = list
a_second_list.append
a_second_list.append

Tuples are immutable sequences of items of arbitrary types. There is also a special syntax to create tuples

a_tuple = 1, 2, 3, "four"

Although tuples are created by separating items with commas, the whole construct is usually wrapped in parentheses to increase readability. An empty tuple is denoted by .
Sets are mutable containers of hashable items of arbitrary types, with no duplicates. The items are not ordered, but sets support iteration over the items. A syntax for set creation appeared in Python 2.7/3.0

some_set =

In earlier Python versions, sets would be created by initializing the set class with a list argument. Python sets are very much like mathematical sets, and support operations like set intersection and union.
Python also features a frozenset class for immutable sets.
Dictionaries are mutable mappings tying keys and corresponding values.
Python has special syntax to create dictionaries

a_dictionary =

The dictionary syntax is similar to the set syntax, the difference is the presence of colons. The empty literal results in an empty dictionary rather than an empty set, which is instead created using the non-literal constructor: set.

Operators

Arithmetic

Python includes the +, -, *, /, %, and ** operators, with their usual mathematical precedence.
Traditionally, x / y performed integer division if both x and y were integers, and returned a float if either was a float. However, because Python is a dynamically typed language, it was not always possible to tell which operation was being performed, which often led to subtle bugs. For example, with
def mean:
return sum / len
A call to mean would return 3.5, but mean would return 3. If this was not the intended behavior, it was necessary to use a workaround such as
def mean:
return float / len
To avoid this issue, was made to change the behavior of the Python division operator. In Python 2.2, a new operator // was introduced for floor division, both for integer and floating-point arguments. The / operator was changed so that the quotient of two integers returned a float, but for backwards compatibility, this behavior had to be explicitly requested until Python 3.0.

Comparison operators

The basic comparison operators such as , <, >=, and so forth are used on all manner of values. Numbers, strings, sequences, and mappings can all be compared. Although disparate types are defined to have a consistent relative ordering, this is considered a historical design quirk and will no longer be allowed in Python 3.0.
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 has short-circuit semantics, meaning that evaluation is guaranteed to stop as soon as a verdict is clear: if a < b is false, c is never evaluated as the expression cannot possibly be true anymore.
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 < b will evaluate f exactly once, whereas a < f and f < b will evaluate it twice if the value of a is less than f and once otherwise.

Logical operators

Python 2.2 and earlier does not have an explicit boolean type. In all versions of Python, boolean operators treat zero values or empty values such as "", 0, None, 0.0, , and as false, while in general treating non-empty, non-zero values as true. In Python 2.2.1 the boolean constants True and False were added to the language. The binary comparison operators such as and > return either True or False.
The boolean operators and and or use minimal evaluation. For example, y 0 or x/y > 100 will never raise a divide-by-zero exception. These operators return the value of the last operand evaluated, rather than True or False. Thus the expression evaluates to 5, and evaluates to 4.

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.

Comprehensions

One such construction is the list comprehension, which can be expressed with the following format:

L =

Using list comprehension to calculate the first five powers of two:

powers_of_two =

The Quicksort algorithm can be expressed elegantly using list comprehensions:

def qsort:
if L :
return
pivot = L
return +
+
qsort)

Python 2.7+ also supports set comprehensions and dictionary comprehensions.

First-class functions

In Python, functions are first-class objects that can be created and passed around dynamically.
Python's limited support for anonymous functions is the lambda construct. An example is the anonymous function which squares its input, called with the argument of 5:

f = lambda x: x**2
f

Lambdas are limited to containing an expression rather than statements, although control flow can still be implemented less elegantly within lambda by using short-circuiting, and more idiomatically with conditional expressions.

Closures

Python has had support for lexical closures since version 2.2. Here's an example:

def derivative:
"""Return a function that approximates the derivative of f
using an interval of dx, which should be appropriately small.
"""
def function:
return - f) / dx
return function

Python's syntax, though, sometimes leads programmers of other languages to think that closures are not supported. Variable scope in Python is implicitly determined by the scope in which one assigns a value to the variable, unless scope is explicitly declared with global or nonlocal.
Note that the closure's binding of a name to some value is not mutable from within the function. Given:

>>> def foo:
... print
... print
... def bar:
... b = c
... print
... bar
... print
>>> foo
a: 1
b: 2
b*: 1
b: 2

and you can see that b, as visible from the closure's scope, retains the value it had; the changed binding of b inside the inner function did not propagate out. The way around this is to use a nonlocal b statement in bar. In Python 2, the usual workaround is to use mutable value and change that value, not the binding. E.g., a list with one element.

Generators

Introduced in Python 2.2 as an 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.
This is an example to lazily generate the prime numbers:

from itertools import count
def generate_primes:
primes =
for n in count:
if 0 < stop_at < n:
return # raises the StopIteration exception
composite = False
for p in primes:
if not n % p:
composite = True
break
elif p ** 2 > n:
break
if not composite:
primes.append
yield n

To use this function simply call, e.g.:

for i in generate_primes: # iterate over ALL primes
if i > 100:
break
print

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 scope. 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.
Generators don't have to be infinite like the prime-number example above. When a generator terminates, an internal exception is raised which indicates to any calling context that there are no more values. A for loop or other iteration will then terminate.

Generator expressions

Introduced in Python 2.4, generator expressions are the lazy evaluation equivalent of list comprehensions. Using the prime number generator provided in the above section, we might define a lazy, but not quite infinite collection.

from itertools import islice
primes_under_million =
two_thousandth_prime = islice.next

Most of the memory and time needed to generate this many primes will not be used until the needed element is actually accessed. Unfortunately, you cannot perform simple indexing and slicing of generators, but must use the itertools modules or "roll your own" loops. In contrast, a list comprehension is functionally equivalent, but is greedy in performing all the work:

primes_under_million =
two_thousandth_prime = primes_under_million

The list comprehension will immediately create a large list, even if most elements are never accessed. The generator comprehension is more parsimonious.

Dictionary and set comprehensions

While lists and generators had comprehensions/expressions, in Python versions older than 2.7 the other Python built-in collection types had to be kludged in using lists or generators:
>>> dict for n in range)

Python 2.7 and 3.0 unify all collection types by introducing dict and set comprehensions, similar to list comprehensions:
>>> # regular list comprehension
>>>
>>> # set comprehension
>>>
>>> # dict comprehension

Objects

Python supports most object oriented programming techniques. It allows polymorphism, not only within a class hierarchy but also by duck typing. Any object can be used for any type, and it will work so long as it has the proper methods and attributes. 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 for details.
Many Python users don't feel the need for private variables, though.
The slogan "We're all responsible users 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. However, the strongest argument for name mangling is prevention of unpredictable breakage of programs: introducing a new public variable in a superclass can break subclasses if they don't use "private" variables.
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, it offers an object system but does not demand OOP behavior. 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 spam.eggs = toast might really invoke spam.set_eggs. This nullifies the practical advantage of accessor functions, and it remains OOP because the property eggs 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 entirely new types can be defined, complete with custom behavior for infix operators. This allows for many radical things to be done syntactically within Python. A new for multiple inheritance was also adopted with Python 2.3. It is also possible to run custom code while accessing or setting attributes, though the details of those techniques have evolved between Python versions.

With statements

The "with" statement handles resources. One function is called when entering scope and another when leaving. This prevents forgetting to remove the resource and also handles more complicated situations such as exceptions.

Properties

Properties allow specially defined methods to be invoked on an object instance by using the same syntax as used for attribute access. An example of a class defining some properties is:

class MyClass:
def __init__:
self._a = None
@property
def a:
return self._a
@a.setter # makes the property writable
def a:
self._a = value

Descriptors

A class that defines one or more of the special methods __get__, __set__, __delete__ can be used as a descriptor. Creating an instance of a descriptor as a class member of a second class makes the instance a property of the second class.

Class and static methods

Python allows the creation of class methods and static method via the use of the @classmethod and @staticmethod [|decorators]. The first argument to a class method is the class object instead of the self-reference to the instance. A static method has no special first argument. Neither the instance, nor the class object is passed to a static method.

Exceptions

Python supports 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. 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 used as an alternative to the if-block, especially in threaded situations. A commonly invoked motto is EAFP, or "It is Easier to Ask for Forgiveness than Permission," which is attributed to Grace Hopper. The alternative, known as LBYL, or "Look Before You Leap", explicitly tests for pre-conditions.
In this first code sample, following the LBYL approach, there is an explicit check for the attribute before access:

if hasattr:
ham = spam.eggs
else:
handle_error

This second sample follows the EAFP paradigm:

try:
ham = spam.eggs
except AttributeError:
handle_error

These two code samples have the same effect, although there will be performance differences. When spam has the attribute eggs, the EAFP sample will run faster. When spam does not have the attribute eggs, the EAFP sample will run slower. The Python can be used in specific cases to determine performance characteristics. If exceptional cases are rare, then the EAFP version will have superior average performance than the alternative. In addition, it avoids the whole class of time-of-check-to-time-of-use vulnerabilities, other race conditions, and is compatible with duck typing. A drawback of EAFP is that it can be used only with statements; an exception cannot be caught in a generator expression, list comprehension, or lambda function.

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. Single-line comments begin with the hash character and are terminated by the end of line. Comments spanning more than one line are achieved by inserting a multi-line string that is not used in assignment or otherwise evaluated, but sits in between other statements.
Commenting a piece of code:

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

Commenting a piece of code with multiple lines:

def getline:
return sys.stdin.readline """this function
gets one line
and returns it"""

Docstrings, that is, strings that are located alone without assignment as the first indented line within a module, class, method or function, automatically set their contents as an attribute named __doc__, which is intended to store a human-readable description of the object's purpose, behavior, and usage. The built-in help function generates its output based on __doc__ attributes. Such strings can be delimited with " or ' for single line strings, or may span multiple lines if delimited with either """ or ''' which is Python's notation for specifying multi-line strings. However, the style guide for the language specifies that triple double quotes are preferred for both single and multi-line docstrings.
Single line docstring:

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

Multi-line docstring:

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. 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 manipulate that information. 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 pydoc.
The doctest standard module uses interactions copied from Python shell sessions into docstrings, to create tests, whereas the module uses them to define command-line options.

Function annotations

Function annotations are defined in . They allow attaching data to the arguments and return of a function. The behaviour of annotations is not defined by the language, and is left to third party frameworks. For example, a library could be written to handle static typing:

def haul -> Distance

Decorators

A decorator is any callable Python object that is used to modify a function, method or class definition. A decorator is passed the original object being defined and returns a modified object, which is then bound to the name in the definition. Python decorators were inspired in part by Java annotations, and have a similar syntax; the decorator syntax is pure syntactic sugar, using @ as the keyword:

@viking_chorus
def menu_item:
print

is equivalent to

def menu_item:
print
menu_item = viking_chorus

Decorators are a form of metaprogramming; they enhance the action of the function or method they decorate. For example, in the sample below, viking_chorus might cause menu_item to be run 8 times for each time it is called:

def viking_chorus:
def inner_func:
for i in range:
myfunc
return inner_func

Canonical uses of function decorators are for creating class methods or static methods, adding function attributes, tracing, setting pre- and postconditions, and synchronization, but can be used for far more besides, including tail recursion elimination, memoization and even improving the writing of decorators.
Decorators can be chained by placing several on adjacent lines:

@invincible
@favourite_colour
def black_knight:
pass

is equivalent to

def black_knight:
pass
black_knight = invincible)

or, using intermediate variables

def black_knight:
pass
blue_decorator = favourite_colour
decorated_by_blue = blue_decorator
black_knight = invincible

In the example above, the favourite_colour decorator factory takes an argument. Decorator factories must return a decorator, which is then called with the object to be decorated as its argument:

def favourite_colour:
def decorator:
def wrapper:
print
func
return wrapper
return decorator

This would then decorate the black_knight function such that the colour, "Blue", would be printed prior to the black_knight function running. Closure ensures that the colour argument is accessible to the innermost wrapper function even when it is returned and goes out of scope, which is what allows decorators to work.
Despite the name, Python decorators are not an implementation of the decorator pattern. The decorator pattern is a design pattern used in statically typed object-oriented programming languages to allow functionality to be added to objects at run time; Python decorators add functionality to functions and methods at definition time, and thus are a higher-level construct than decorator-pattern classes. The decorator pattern itself is trivially implementable in Python, because the language is duck typed, and so is not usually considered as such.

Easter eggs

Users of curly bracket programming languages, such as C or Java, sometimes expect or wish Python to follow a block-delimiter convention. Brace-delimited block syntax has been repeatedly requested, and consistently rejected by core developers. The Python interpreter contains an easter egg that summarizes its developers' feelings on this issue. The code from __future__ import braces raises the exception SyntaxError: not a chance. The __future__ module is normally used to provide features from future versions of Python.
Another hidden message, The Zen of Python, is displayed when trying to import this.
The message Hello world! is printed when the import statement import __hello__ is used. In Python 2.7, instead of Hello world! it prints Hello world....
An antigravity module was added to Python 2.7 and 3.0. Importing it opens a web browser to xkcd comic that portrays a humorous fictional use for such a module, intended to demonstrate the ease with which Python modules enable additional functionality. In Python 3.0, this module also contains an implementation of the "geohash" algorithm, a reference to xkcd comic .