.. _tut-modules:

*******

Modules

*******

If you quit from the Python interpreter and enter it again, the definitions you

have made (functions and variables) are lost. Therefore, if you want to write a

somewhat longer program, you are better off using a text editor to prepare the

input for the interpreter and running it with that file as input instead.  This

is known as creating a *script*.  As your program gets longer, you may want to

split it into several files for easier maintenance.  You may also want to use a

handy function that you've written in several programs without copying its

definition into each program.

To support this, Python has a way to put definitions in a file and use them in a

script or in an interactive instance of the interpreter. Such a file is called a

*module*; definitions from a module can be *imported* into other modules or into

the *main* module (the collection of variables that you have access to in a

script executed at the top level and in calculator mode).

A module is a file containing Python definitions and statements.  The file name

is the module name with the suffix :file:`.py` appended.  Within a module, the

module's name (as a string) is available as the value of the global variable

``__name__``.  For instance, use your favorite text editor to create a file

called :file:`fibo.py` in the current directory with the following contents::

   # Fibonacci numbers module

   def fib(n):    # write Fibonacci series up to n

       a, b = 0, 1

       while b < n:

           print b,

           a, b = b, a+b

   def fib2(n): # return Fibonacci series up to n

       result = []

       a, b = 0, 1

       while b < n:

           result.append(b)

           a, b = b, a+b

       return result

Now enter the Python interpreter and import this module with the following

command::

   >>> import fibo

This does not enter the names of the functions defined in ``fibo``  directly in

the current symbol table; it only enters the module name ``fibo`` there. Using

the module name you can access the functions::

   >>> fibo.fib(1000)

   1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987

   >>> fibo.fib2(100)

   [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

   >>> fibo.__name__

   'fibo'

If you intend to use a function often you can assign it to a local name::

   >>> fib = fibo.fib

   >>> fib(500)

   1 1 2 3 5 8 13 21 34 55 89 144 233 377

.. _tut-moremodules:

More on Modules

===============

A module can contain executable statements as well as function definitions.

These statements are intended to initialize the module. They are executed only

the *first* time the module is imported somewhere. [#]_

Each module has its own private symbol table, which is used as the global symbol

table by all functions defined in the module. Thus, the author of a module can

use global variables in the module without worrying about accidental clashes

with a user's global variables. On the other hand, if you know what you are

doing you can touch a module's global variables with the same notation used to

refer to its functions, ``modname.itemname``.

Modules can import other modules.  It is customary but not required to place all

:keyword:`import` statements at the beginning of a module (or script, for that

matter).  The imported module names are placed in the importing module's global

symbol table.

There is a variant of the :keyword:`import` statement that imports names from a

module directly into the importing module's symbol table.  For example::

   >>> from fibo import fib, fib2

   >>> fib(500)

   1 1 2 3 5 8 13 21 34 55 89 144 233 377

This does not introduce the module name from which the imports are taken in the

local symbol table (so in the example, ``fibo`` is not defined).

There is even a variant to import all names that a module defines::

   >>> from fibo import *

   >>> fib(500)

   1 1 2 3 5 8 13 21 34 55 89 144 233 377

This imports all names except those beginning with an underscore (``_``).

.. note::

   For efficiency reasons, each module is only imported once per interpreter

   session.  Therefore, if you change your modules, you must restart the

   interpreter -- or, if it's just one module you want to test interactively,

   use :func:`reload`, e.g. ``reload(modulename)``.

.. _tut-modulesasscripts:

Executing modules as scripts

----------------------------

When you run a Python module with ::

   python fibo.py <arguments>

the code in the module will be executed, just as if you imported it, but with

the ``__name__`` set to ``"__main__"``.  That means that by adding this code at

the end of your module::

   if __name__ == "__main__":

       import sys

       fib(int(sys.argv[1]))

you can make the file usable as a script as well as an importable module,

because the code that parses the command line only runs if the module is

executed as the "main" file::

   $ python fibo.py 50

   1 1 2 3 5 8 13 21 34

If the module is imported, the code is not run::

   >>> import fibo

   >>>

This is often used either to provide a convenient user interface to a module, or

for testing purposes (running the module as a script executes a test suite).

.. _tut-searchpath:

The Module Search Path

----------------------

.. index:: triple: module; search; path

When a module named :mod:`spam` is imported, the interpreter searches for a file

named :file:`spam.py` in the current directory, and then in the list of

directories specified by the environment variable :envvar:`PYTHONPATH`.  This

has the same syntax as the shell variable :envvar:`PATH`, that is, a list of

directory names.  When :envvar:`PYTHONPATH` is not set, or when the file is not

found there, the search continues in an installation-dependent default path; on

Unix, this is usually :file:`.:/usr/local/lib/python`.

Actually, modules are searched in the list of directories given by the variable

``sys.path`` which is initialized from the directory containing the input script

(or the current directory), :envvar:`PYTHONPATH` and the installation- dependent

default.  This allows Python programs that know what they're doing to modify or

replace the module search path.  Note that because the directory containing the

script being run is on the search path, it is important that the script not have

the same name as a standard module, or Python will attempt to load the script as

a module when that module is imported. This will generally be an error.  See

section :ref:`tut-standardmodules` for more information.

"Compiled" Python files

-----------------------

As an important speed-up of the start-up time for short programs that use a lot

of standard modules, if a file called :file:`spam.pyc` exists in the directory

where :file:`spam.py` is found, this is assumed to contain an

already-"byte-compiled" version of the module :mod:`spam`. The modification time

of the version of :file:`spam.py` used to create :file:`spam.pyc` is recorded in

:file:`spam.pyc`, and the :file:`.pyc` file is ignored if these don't match.

Normally, you don't need to do anything to create the :file:`spam.pyc` file.

Whenever :file:`spam.py` is successfully compiled, an attempt is made to write

the compiled version to :file:`spam.pyc`.  It is not an error if this attempt

fails; if for any reason the file is not written completely, the resulting

:file:`spam.pyc` file will be recognized as invalid and thus ignored later.  The

contents of the :file:`spam.pyc` file are platform independent, so a Python

module directory can be shared by machines of different architectures.

Some tips for experts:

* When the Python interpreter is invoked with the :option:`-O` flag, optimized

  code is generated and stored in :file:`.pyo` files.  The optimizer currently

  doesn't help much; it only removes :keyword:`assert` statements.  When

  :option:`-O` is used, *all* :term:`bytecode` is optimized; ``.pyc`` files are

  ignored and ``.py`` files are compiled to optimized bytecode.

* Passing two :option:`-O` flags to the Python interpreter (:option:`-OO`) will

  cause the bytecode compiler to perform optimizations that could in some rare

  cases result in malfunctioning programs.  Currently only ``__doc__`` strings are

  removed from the bytecode, resulting in more compact :file:`.pyo` files.  Since

  some programs may rely on having these available, you should only use this

  option if you know what you're doing.

* A program doesn't run any faster when it is read from a :file:`.pyc` or

  :file:`.pyo` file than when it is read from a :file:`.py` file; the only thing

  that's faster about :file:`.pyc` or :file:`.pyo` files is the speed with which

  they are loaded.

* When a script is run by giving its name on the command line, the bytecode for

  the script is never written to a :file:`.pyc` or :file:`.pyo` file.  Thus, the

  startup time of a script may be reduced by moving most of its code to a module

  and having a small bootstrap script that imports that module.  It is also

  possible to name a :file:`.pyc` or :file:`.pyo` file directly on the command

  line.

* It is possible to have a file called :file:`spam.pyc` (or :file:`spam.pyo`

  when :option:`-O` is used) without a file :file:`spam.py` for the same module.

  This can be used to distribute a library of Python code in a form that is

  moderately hard to reverse engineer.

  .. index:: module: compileall

* The module :mod:`compileall` can create :file:`.pyc` files (or :file:`.pyo`

  files when :option:`-O` is used) for all modules in a directory.

.. _tut-standardmodules:

Standard Modules

================

.. index:: module: sys

Python comes with a library of standard modules, described in a separate

document, the Python Library Reference ("Library Reference" hereafter).  Some

modules are built into the interpreter; these provide access to operations that

are not part of the core of the language but are nevertheless built in, either

for efficiency or to provide access to operating system primitives such as

system calls.  The set of such modules is a configuration option which also

depends on the underlying platform For example, the :mod:`winreg` module is only

provided on Windows systems. One particular module deserves some attention:

:mod:`sys`, which is built into every Python interpreter.  The variables

``sys.ps1`` and ``sys.ps2`` define the strings used as primary and secondary

prompts::

   >>> import sys

   >>> sys.ps1

   '>>> '

   >>> sys.ps2

   '... '

   >>> sys.ps1 = 'C> '

   C> print 'Yuck!'

   Yuck!

   C>

These two variables are only defined if the interpreter is in interactive mode.

The variable ``sys.path`` is a list of strings that determines the interpreter's

search path for modules. It is initialized to a default path taken from the

environment variable :envvar:`PYTHONPATH`, or from a built-in default if

:envvar:`PYTHONPATH` is not set.  You can modify it using standard list

operations::

   >>> import sys

   >>> sys.path.append('/ufs/guido/lib/python')

.. _tut-dir:

The :func:`dir` Function

========================

The built-in function :func:`dir` is used to find out which names a module

defines.  It returns a sorted list of strings::

   >>> import fibo, sys

   >>> dir(fibo)

   ['__name__', 'fib', 'fib2']

   >>> dir(sys)

   ['__displayhook__', '__doc__', '__excepthook__', '__name__', '__stderr__',

    '__stdin__', '__stdout__', '_getframe', 'api_version', 'argv', 

    'builtin_module_names', 'byteorder', 'callstats', 'copyright',

    'displayhook', 'exc_clear', 'exc_info', 'exc_type', 'excepthook',

    'exec_prefix', 'executable', 'exit', 'getdefaultencoding', 'getdlopenflags',

    'getrecursionlimit', 'getrefcount', 'hexversion', 'maxint', 'maxunicode',

    'meta_path', 'modules', 'path', 'path_hooks', 'path_importer_cache',

    'platform', 'prefix', 'ps1', 'ps2', 'setcheckinterval', 'setdlopenflags',

    'setprofile', 'setrecursionlimit', 'settrace', 'stderr', 'stdin', 'stdout',

    'version', 'version_info', 'warnoptions']

Without arguments, :func:`dir` lists the names you have defined currently::

   >>> a = [1, 2, 3, 4, 5]

   >>> import fibo

   >>> fib = fibo.fib

   >>> dir()

   ['__builtins__', '__doc__', '__file__', '__name__', 'a', 'fib', 'fibo', 'sys']

Note that it lists all types of names: variables, modules, functions, etc.

.. index:: module: __builtin__

:func:`dir` does not list the names of built-in functions and variables.  If you

want a list of those, they are defined in the standard module

:mod:`__builtin__`::

   >>> import __builtin__

   >>> dir(__builtin__)

   ['ArithmeticError', 'AssertionError', 'AttributeError', 'DeprecationWarning',

    'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False',

    'FloatingPointError', 'FutureWarning', 'IOError', 'ImportError',

    'IndentationError', 'IndexError', 'KeyError', 'KeyboardInterrupt',

    'LookupError', 'MemoryError', 'NameError', 'None', 'NotImplemented',

    'NotImplementedError', 'OSError', 'OverflowError', 

    'PendingDeprecationWarning', 'ReferenceError', 'RuntimeError',

    'RuntimeWarning', 'StandardError', 'StopIteration', 'SyntaxError',

    'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'True',

    'TypeError', 'UnboundLocalError', 'UnicodeDecodeError',

    'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError',

    'UserWarning', 'ValueError', 'Warning', 'WindowsError',

    'ZeroDivisionError', '_', '__debug__', '__doc__', '__import__',

    '__name__', 'abs', 'apply', 'basestring', 'bool', 'buffer',

    'callable', 'chr', 'classmethod', 'cmp', 'coerce', 'compile',

    'complex', 'copyright', 'credits', 'delattr', 'dict', 'dir', 'divmod',

    'enumerate', 'eval', 'execfile', 'exit', 'file', 'filter', 'float',

    'frozenset', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex',

    'id', 'input', 'int', 'intern', 'isinstance', 'issubclass', 'iter',

    'len', 'license', 'list', 'locals', 'long', 'map', 'max', 'min',

    'object', 'oct', 'open', 'ord', 'pow', 'property', 'quit', 'range',

    'raw_input', 'reduce', 'reload', 'repr', 'reversed', 'round', 'set',

    'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super',

    'tuple', 'type', 'unichr', 'unicode', 'vars', 'xrange', 'zip']

.. _tut-packages:

Packages

========

Packages are a way of structuring Python's module namespace by using "dotted

module names".  For example, the module name :mod:`A.B` designates a submodule

named ``B`` in a package named ``A``.  Just like the use of modules saves the

authors of different modules from having to worry about each other's global

variable names, the use of dotted module names saves the authors of multi-module

packages like NumPy or the Python Imaging Library from having to worry about

each other's module names.

Suppose you want to design a collection of modules (a "package") for the uniform

handling of sound files and sound data.  There are many different sound file

formats (usually recognized by their extension, for example: :file:`.wav`,

:file:`.aiff`, :file:`.au`), so you may need to create and maintain a growing

collection of modules for the conversion between the various file formats.

There are also many different operations you might want to perform on sound data

(such as mixing, adding echo, applying an equalizer function, creating an

artificial stereo effect), so in addition you will be writing a never-ending

stream of modules to perform these operations.  Here's a possible structure for

your package (expressed in terms of a hierarchical filesystem)::

   sound/                          Top-level package

         __init__.py               Initialize the sound package

         formats/                  Subpackage for file format conversions

                 __init__.py

                 wavread.py

                 wavwrite.py

                 aiffread.py

                 aiffwrite.py

                 auread.py

                 auwrite.py

                 ...

         effects/                  Subpackage for sound effects

                 __init__.py

                 echo.py

                 surround.py

                 reverse.py

                 ...

         filters/                  Subpackage for filters

                 __init__.py

                 equalizer.py

                 vocoder.py

                 karaoke.py

                 ...

When importing the package, Python searches through the directories on

``sys.path`` looking for the package subdirectory.

The :file:`__init__.py` files are required to make Python treat the directories

as containing packages; this is done to prevent directories with a common name,

such as ``string``, from unintentionally hiding valid modules that occur later

on the module search path. In the simplest case, :file:`__init__.py` can just be

an empty file, but it can also execute initialization code for the package or

set the ``__all__`` variable, described later.

Users of the package can import individual modules from the package, for

example::

   import sound.effects.echo

This loads the submodule :mod:`sound.effects.echo`.  It must be referenced with

its full name. ::

   sound.effects.echo.echofilter(input, output, delay=0.7, atten=4)

An alternative way of importing the submodule is::

   from sound.effects import echo

This also loads the submodule :mod:`echo`, and makes it available without its

package prefix, so it can be used as follows::

   echo.echofilter(input, output, delay=0.7, atten=4)

Yet another variation is to import the desired function or variable directly::

   from sound.effects.echo import echofilter

Again, this loads the submodule :mod:`echo`, but this makes its function

:func:`echofilter` directly available::

   echofilter(input, output, delay=0.7, atten=4)

Note that when using ``from package import item``, the item can be either a

submodule (or subpackage) of the package, or some  other name defined in the

package, like a function, class or variable.  The ``import`` statement first

tests whether the item is defined in the package; if not, it assumes it is a

module and attempts to load it.  If it fails to find it, an :exc:`ImportError`

exception is raised.

Contrarily, when using syntax like ``import item.subitem.subsubitem``, each item

except for the last must be a package; the last item can be a module or a

package but can't be a class or function or variable defined in the previous

item.

.. _tut-pkg-import-star:

Importing \* From a Package

---------------------------

.. index:: single: __all__

Now what happens when the user writes ``from sound.effects import *``?  Ideally,

one would hope that this somehow goes out to the filesystem, finds which

submodules are present in the package, and imports them all.  Unfortunately,

this operation does not work very well on Windows platforms, where the

filesystem does not always have accurate information about the case of a

filename!  On these platforms, there is no guaranteed way to know whether a file

:file:`ECHO.PY` should be imported as a module :mod:`echo`, :mod:`Echo` or

:mod:`ECHO`.  (For example, Windows 95 has the annoying practice of showing all

file names with a capitalized first letter.)  The DOS 8+3 filename restriction

adds another interesting problem for long module names.

The only solution is for the package author to provide an explicit index of the

package.  The import statement uses the following convention: if a package's

:file:`__init__.py` code defines a list named ``__all__``, it is taken to be the

list of module names that should be imported when ``from package import *`` is

encountered.  It is up to the package author to keep this list up-to-date when a

new version of the package is released.  Package authors may also decide not to

support it, if they don't see a use for importing \* from their package.  For

example, the file :file:`sounds/effects/__init__.py` could contain the following

code::

   __all__ = ["echo", "surround", "reverse"]

This would mean that ``from sound.effects import *`` would import the three

named submodules of the :mod:`sound` package.

If ``__all__`` is not defined, the statement ``from sound.effects import *``

does *not* import all submodules from the package :mod:`sound.effects` into the

current namespace; it only ensures that the package :mod:`sound.effects` has

been imported (possibly running any initialization code in :file:`__init__.py`)

and then imports whatever names are defined in the package.  This includes any

names defined (and submodules explicitly loaded) by :file:`__init__.py`.  It

also includes any submodules of the package that were explicitly loaded by

previous import statements.  Consider this code::

   import sound.effects.echo

   import sound.effects.surround

   from sound.effects import *

In this example, the echo and surround modules are imported in the current

namespace because they are defined in the :mod:`sound.effects` package when the

``from...import`` statement is executed.  (This also works when ``__all__`` is

defined.)

Note that in general the practice of importing ``*`` from a module or package is

frowned upon, since it often causes poorly readable code. However, it is okay to

use it to save typing in interactive sessions, and certain modules are designed

to export only names that follow certain patterns.

Remember, there is nothing wrong with using ``from Package import

specific_submodule``!  In fact, this is the recommended notation unless the

importing module needs to use submodules with the same name from different

packages.

Intra-package References

------------------------

The submodules often need to refer to each other.  For example, the

:mod:`surround` module might use the :mod:`echo` module.  In fact, such

references are so common that the :keyword:`import` statement first looks in the

containing package before looking in the standard module search path. Thus, the

:mod:`surround` module can simply use ``import echo`` or ``from echo import

echofilter``.  If the imported module is not found in the current package (the

package of which the current module is a submodule), the :keyword:`import`

statement looks for a top-level module with the given name.

When packages are structured into subpackages (as with the :mod:`sound` package

in the example), you can use absolute imports to refer to submodules of siblings

packages.  For example, if the module :mod:`sound.filters.vocoder` needs to use

the :mod:`echo` module in the :mod:`sound.effects` package, it can use ``from

sound.effects import echo``.

Starting with Python 2.5, in addition to the implicit relative imports described

above, you can write explicit relative imports with the ``from module import

name`` form of import statement. These explicit relative imports use leading

dots to indicate the current and parent packages involved in the relative

import. From the :mod:`surround` module for example, you might use::

   from . import echo

   from .. import formats

   from ..filters import equalizer

Note that both explicit and implicit relative imports are based on the name of

the current module. Since the name of the main module is always ``"__main__"``,