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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 a < n:
               print(a, end=' ')
    
               a, b = b, a+b
    
       def fib2(n):   # return Fibonacci series up to n
    
           result = []
           a, b = 0, 1
    
           while a < n:
               result.append(a)
    
               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)
    
       0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
    
       >>> fibo.fib2(100)
    
       [0, 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)
    
       0 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 name is encountered in an import statement. [#]_
    (They are also run if the file is executed as a script.)
    
    
    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)
    
       0 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)
    
       0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
    
    
    This imports all names except those beginning with an underscore (``_``).
    
    In most cases Python programmers do not use this facility since it introduces
    an unknown set of names into the interpreter, possibly hiding some things
    
    you have already defined.
    
    Benjamin Peterson's avatar
    Benjamin Peterson committed
    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.
    
    
    If the module name is followed by :keyword:`!as`, then the name
    following :keyword:`!as` is bound directly to the imported module.
    
    
    ::
    
       >>> import fibo as fib
       >>> fib.fib(500)
       0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
    
    This is effectively importing the module in the same way that ``import fibo``
    will do, with the only difference of it being available as ``fib``.
    
    It can also be used when utilising :keyword:`from` with similar effects::
    
       >>> from fibo import fib as fibonacci
       >>> fibonacci(500)
       0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
    
    
    
    .. 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:`importlib.reload`, e.g. ``import importlib;
       importlib.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:
    
    .. code-block:: shell-session
    
    
       $ python fibo.py 50
    
    
    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 first searches for
    
    a built-in module with that name. These module names are listed in
    :data:`sys.builtin_module_names`. If not found, it then searches for a file
    
    named :file:`spam.py` in a list of directories given by the variable
    :data:`sys.path`.  :data:`sys.path` is initialized from these locations:
    
    
    * The directory containing the input script (or the current directory when no
      file is specified).
    
    * :envvar:`PYTHONPATH` (a list of directory names, with the same syntax as the
      shell variable :envvar:`PATH`).
    
    * The installation-dependent default (by convention including a
      ``site-packages`` directory, handled by the :mod:`site` module).
    
    
    .. note::
       On file systems which support symlinks, the directory containing the input
       script is calculated after the symlink is followed. In other words the
       directory containing the symlink is **not** added to the module search path.
    
    
    After initialization, Python programs can modify :data:`sys.path`.  The
    directory containing the script being run is placed at the beginning of the
    search path, ahead of the standard library path. This means that scripts in that
    directory will be loaded instead of modules of the same name in the library
    directory. This is an error unless the replacement is intended.  See section
    :ref:`tut-standardmodules` for more information.
    
    .. %
        Do we need stuff on zip files etc. ? DUBOIS
    
    
    "Compiled" Python files
    -----------------------
    
    
    To speed up loading modules, Python caches the compiled version of each module
    
    in the ``__pycache__`` directory under the name :file:`module.{version}.pyc`,
    
    where the version encodes the format of the compiled file; it generally contains
    the Python version number.  For example, in CPython release 3.3 the compiled
    version of spam.py would be cached as ``__pycache__/spam.cpython-33.pyc``.  This
    naming convention allows compiled modules from different releases and different
    versions of Python to coexist.
    
    Python checks the modification date of the source against the compiled version
    to see if it's out of date and needs to be recompiled.  This is a completely
    automatic process.  Also, the compiled modules are platform-independent, so the
    same library can be shared among systems with different architectures.
    
    Python does not check the cache in two circumstances.  First, it always
    recompiles and does not store the result for the module that's loaded directly
    from the command line.  Second, it does not check the cache if there is no
    source module.  To support a non-source (compiled only) distribution, the
    compiled module must be in the source directory, and there must not be a source
    module.
    
    
    Some tips for experts:
    
    
    * You can use the :option:`-O` or :option:`-OO` switches on the Python command
      to reduce the size of a compiled module.  The ``-O`` switch removes assert
      statements, the ``-OO`` switch removes both assert statements and __doc__
      strings.  Since some programs may rely on having these available, you should
      only use this option if you know what you're doing.  "Optimized" modules have
    
      an ``opt-`` tag and are usually smaller.  Future releases may
    
    * A program doesn't run any faster when it is read from a ``.pyc``
    
      file than when it is read from a ``.py`` file; the only thing that's faster
    
      about ``.pyc`` files is the speed with which they are loaded.
    
    * The module :mod:`compileall` can create .pyc files for all modules in a
      directory.
    
    
    * There is more detail on this process, including a flow chart of the
    
      decisions, in :pep:`3147`.
    
    
    
    .. _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
    
    
       >>> 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)  # doctest: +NORMALIZE_WHITESPACE
    
       ['__breakpointhook__', '__displayhook__', '__doc__', '__excepthook__',
        '__interactivehook__', '__loader__', '__name__', '__package__', '__spec__',
        '__stderr__', '__stdin__', '__stdout__', '__unraisablehook__',
        '_clear_type_cache', '_current_frames', '_debugmallocstats', '_framework',
        '_getframe', '_git', '_home', '_xoptions', 'abiflags', 'addaudithook',
        'api_version', 'argv', 'audit', 'base_exec_prefix', 'base_prefix',
        'breakpointhook', 'builtin_module_names', 'byteorder', 'call_tracing',
        'callstats', 'copyright', 'displayhook', 'dont_write_bytecode', 'exc_info',
        'excepthook', 'exec_prefix', 'executable', 'exit', 'flags', 'float_info',
        'float_repr_style', 'get_asyncgen_hooks', 'get_coroutine_origin_tracking_depth',
        'getallocatedblocks', 'getdefaultencoding', 'getdlopenflags',
        'getfilesystemencodeerrors', 'getfilesystemencoding', 'getprofile',
        'getrecursionlimit', 'getrefcount', 'getsizeof', 'getswitchinterval',
    
        'gettrace', 'hash_info', 'hexversion', 'implementation', 'int_info',
    
        'intern', 'is_finalizing', 'last_traceback', 'last_type', 'last_value',
        'maxsize', 'maxunicode', 'meta_path', 'modules', 'path', 'path_hooks',
        'path_importer_cache', 'platform', 'prefix', 'ps1', 'ps2', 'pycache_prefix',
        'set_asyncgen_hooks', 'set_coroutine_origin_tracking_depth', 'setdlopenflags',
        'setprofile', 'setrecursionlimit', 'setswitchinterval', 'settrace', 'stderr',
        'stdin', 'stdout', 'thread_info', 'unraisablehook', '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__', '__name__', 'a', 'fib', 'fibo', 'sys']
    
    
    Note that it lists all types of names: variables, modules, functions, etc.
    
    
    .. index:: module: builtins
    
    
    :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:`builtins`::
    
       >>> import builtins
    
       >>> dir(builtins)  # doctest: +NORMALIZE_WHITESPACE
       ['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException',
    
        'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning',
        'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError',
        'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning',
        'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False',
        'FileExistsError', 'FileNotFoundError', 'FloatingPointError',
    
        'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError',
    
        'ImportWarning', 'IndentationError', 'IndexError', 'InterruptedError',
        'IsADirectoryError', 'KeyError', 'KeyboardInterrupt', 'LookupError',
        'MemoryError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented',
        'NotImplementedError', 'OSError', 'OverflowError',
        'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError',
        'ReferenceError', 'ResourceWarning', 'RuntimeError', 'RuntimeWarning',
        'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError',
        'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError',
        'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError',
        'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning',
        'ValueError', 'Warning', 'ZeroDivisionError', '_', '__build_class__',
        '__debug__', '__doc__', '__import__', '__name__', '__package__', 'abs',
        'all', 'any', 'ascii', 'bin', 'bool', 'bytearray', 'bytes', 'callable',
        'chr', 'classmethod', 'compile', 'complex', 'copyright', 'credits',
        'delattr', 'dict', 'dir', 'divmod', 'enumerate', 'eval', 'exec', 'exit',
        'filter', 'float', 'format', 'frozenset', 'getattr', 'globals', 'hasattr',
        'hash', 'help', 'hex', 'id', 'input', 'int', 'isinstance', 'issubclass',
        'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview',
        'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property',
        'quit', 'range', 'repr', 'reversed', 'round', 'set', 'setattr', 'slice',
        'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars',
        '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 Pillow 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):
    
    .. code-block:: text
    
    
       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 directories
    containing the file as packages.  This prevents directories with a common name,
    such as ``string``, 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
    
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    submodules are present in the package, and imports them all.  This could take a
    long time and importing sub-modules might have unwanted side-effects that should
    only happen when the sub-module is explicitly imported.
    
    
    The only solution is for the package author to provide an explicit index of the
    
    package.  The :keyword:`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:`sound/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.effects` 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 :keyword:`import` statements.  Consider this code::
    
    
       import sound.effects.echo
       import sound.effects.surround
       from sound.effects import *
    
    
    In this example, the :mod:`echo` and :mod:`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.)
    
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    Although certain modules are designed to export only names that follow certain
    
    patterns when you use ``import *``, it is still considered bad practice in
    
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    production code.
    
    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
    ------------------------
    
    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``.
    
    
    You can also write relative imports, with the ``from module import name`` form
    of import statement.  These 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 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 must 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 of a module-level function definition enters the function name in
       the module's global symbol table.