6. Modules

If you quit from the Python interpreter and enter it againthe definitions you have made (functions and variables) are lost. Thereforeif you want to write a somewhat longer programyou 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 longeryou 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 thisPython 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 .py appended. Within a modulethe module’s name (as a string) is available as the value of the global variable __name__. For instanceuse your favorite text editor to create a file called 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
    print()

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 add the names of the functions defined in fibo directly to the current namespace (see Python Scopes and Namespaces for more details); it only adds 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)
[01123581321345589]
>>> 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

6.1. 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. [1] (They are also run if the file is executed as a script.)

Each module has its own private namespacewhich is used as the global namespace by all functions defined in the module. Thusthe 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 handif you know what you are doing you can touch a module’s global variables with the same notation used to refer to its functionsmodname.itemname.

Modules can import other modules. It is customary but not required to place all import statements at the beginning of a module (or scriptfor that matter). The imported module namesif placed at the top level of a module (outside any functions or classes)are added to the module’s global namespace.

There is a variant of the import statement that imports names from a module directly into the importing module’s namespace. 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 namespace (so in the examplefibo 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 interpreterpossibly hiding some things you have already defined.

Note that in general the practice of importing * from a module or package is frowned uponsince it often causes poorly readable code. Howeverit is okay to use it to save typing in interactive sessions.

If the module name is followed by asthen the name following 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 dowith the only difference of it being available as fib.

It can also be used when utilising 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 reasonseach module is only imported once per interpreter session. Thereforeif you change your modulesyou must restart the interpreter – orif it’s just one module you want to test interactively, use importlib.reload()e.g. import importlib; importlib.reload(modulename).

6.1.1. Executing modules as scripts

When you run a Python module with

python fibo.py <arguments>

the code in the module will be executedjust as if you imported itbut 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
0 1 1 2 3 5 8 13 21 34

If the module is importedthe code is not run:

>>> import fibo
>>>

This is often used either to provide a convenient user interface to a moduleor for testing purposes (running the module as a script executes a test suite).

6.1.2. The Module Search Path

When a module named spam is importedthe interpreter first searches for a built-in module with that name. These module names are listed in sys.builtin_module_names. If not foundit then searches for a file named spam.py in a list of directories given by the variable sys.path. sys.path is initialized from these locations:

• The directory containing the input script (or the current directory when no file is specified).

• PYTHONPATH (a list of directory nameswith the same syntax as the shell variable PATH).

• The installation-dependent default (by convention including a site-packages directoryhandled by the site module).

More details are at The initialization of the sys.path module search path.

Note

On file systems which support symlinksthe 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 initializationPython programs can modify sys.path. The directory containing the script being run is placed at the beginning of the search pathahead 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 Standard Modules for more information.

6.1.3. “Compiled” Python files

To speed up loading modulesPython caches the compiled version of each module in the __pycache__ directory under the name module.version.pyc, where the version encodes the format of the compiled file; it generally contains the Python version number. For examplein 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. Alsothe compiled modules are platform-independentso the same library can be shared among systems with different architectures.

Python does not check the cache in two circumstances. Firstit always recompiles and does not store the result for the module that’s loaded directly from the command line. Secondit does not check the cache if there is no source module. To support a non-source (compiled only) distributionthe compiled module must be in the source directoryand there must not be a source module.

Some tips for experts:

• You can use the -O or -OO switches on the Python command to reduce the size of a compiled module. The -O switch removes assert statementsthe -OO switch removes both assert statements and __doc__ strings. Since some programs may rely on having these availableyou 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 change the effects of optimization.

• 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 compileall can create .pyc files for all modules in a directory.

• There is more detail on this processincluding a flow chart of the decisionsin PEP 3147.

6.2. Standard Modules

Python comes with a library of standard modulesdescribed in a separate documentthe 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 ineither 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 examplethe winreg module is only provided on Windows systems. One particular module deserves some attention: syswhich 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 PYTHONPATHor from a built-in default if PYTHONPATH is not set. You can modify it using standard list operations:

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

6.3. The dir() Function

The built-in function 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)
['__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_''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 argumentsdir() 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: variablesmodulesfunctionsetc.

dir() does not list the names of built-in functions and variables. If you want a list of thosethey are defined in the standard module builtins:

>>> import builtins
>>> dir(builtins)
['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']

6.4. Packages

Packages are a way of structuring Python’s module namespace by using “dotted module names”. For examplethe module name 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 namesthe 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 extensionfor example: .wav, .aiff.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 mixingadding echoapplying an equalizer functioncreating 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 packagePython searches through the directories on sys.path looking for the package subdirectory.

The __init__.py files are required to make Python treat directories containing the file as packages (unless using a namespace packagea relatively advanced feature). This prevents directories with a common name, such as stringfrom unintentionally hiding valid modules that occur later on the module search path. In the simplest case__init__.py can just be an empty filebut it can also execute initialization code for the package or set the __all__ variabledescribed later.

Users of the package can import individual modules from the packagefor example:

import sound.effects.echo

This loads the submodule 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 echoand makes it available without its package prefixso 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

Againthis loads the submodule echobut this makes its function echofilter() directly available:

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

Note that when using from package import itemthe item can be either a submodule (or subpackage) of the packageor some other name defined in the packagelike a functionclass or variable. The import statement first tests whether the item is defined in the package; if notit assumes it is a module and attempts to load it. If it fails to find itan ImportError exception is raised.

Contrarilywhen using syntax like import item.subitem.subsubitemeach 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.

6.4.1. Importing * From a Package

Now what happens when the user writes from sound.effects import *? Ideally, one would hope that this somehow goes out to the filesystemfinds which submodules are present in the packageand 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 import statement uses the following convention: if a package’s __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 itif they don’t see a use for importing * from their package. For examplethe 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 sound.effects package.

Be aware that submodules might become shadowed by locally defined names. For exampleif you added a reverse function to the sound/effects/__init__.py filethe from sound.effects import * would only import the two submodules echo and surroundbut not the reverse submodulebecause it is shadowed by the locally defined reverse function:

__all__ = [
    "echo",      # refers to the 'echo.py' file
    "surround",  # refers to the 'surround.py' file
    "reverse",   # !!! refers to the 'reverse' function now !!!
]

def reverse(msg: str):  # <-- this name shadows the 'reverse.py' submodule
    return msg[::-1]    #     in the case of a 'from sound.effects import *'

If __all__ is not definedthe statement from sound.effects import * does not import all submodules from the package sound.effects into the current namespace; it only ensures that the package sound.effects has been imported (possibly running any initialization code in __init__.py) and then imports whatever names are defined in the package. This includes any names defined (and submodules explicitly loaded) by __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 examplethe echo and surround modules are imported in the current namespace because they are defined in the sound.effects package when the from...import statement is executed. (This also works when __all__ is defined.)

Although certain modules are designed to export only names that follow certain patterns when you use import *it is still considered bad practice in production code.

Rememberthere is nothing wrong with using from package import specific_submodule! In factthis is the recommended notation unless the importing module needs to use submodules with the same name from different packages.

6.4.2. Intra-package References

When packages are structured into subpackages (as with the sound package in the example)you can use absolute imports to refer to submodules of siblings packages. For exampleif the module sound.filters.vocoder needs to use the echo module in the sound.effects packageit can use from sound.effects import echo.

You can also write relative importswith 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 surround module for exampleyou 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’s package. Since the main module does not have a packagemodules intended for use as the main module of a Python application must always use absolute imports.

6.4.3. Packages in Multiple Directories

Packages support one more special attribute__path__. This is initialized to be a sequence of strings containing the name of the directory holding the package’s __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 neededit can be used to extend the set of modules found in a package.

Footnotes

[1]

In fact function definitions are also ‘statements’ that are ‘executed’; the execution of a module-level function definition adds the function name to the module’s global namespace.