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+.. _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 (``_``).
+
+
+.. _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* 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 \code{__all__} Attribute
+
+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__"``,
+modules intended for use as the main module of a Python application should
+always use absolute imports.
+
+
+Packages in Multiple Directories
+--------------------------------
+
+Packages support one more special attribute, :attr:`__path__`.  This is
+initialized to be a list containing the name of the directory holding the
+package's :file:`__init__.py` before the code in that file is executed.  This
+variable can be modified; doing so affects future searches for modules and
+subpackages contained in the package.
+
+While this feature is not often needed, it can be used to extend the set of
+modules found in a package.
+
+
+.. rubric:: Footnotes
+
+.. [#] In fact function definitions are also 'statements' that are 'executed'; the
+   execution enters the function name in the module's global symbol table.
+