Python module (Python learning memo ④)

About Python modules

point

• Module = File containing Python definitions and statements
• The file name is * .py
• In the module, the module name is set as the value of the global variable __name__

sample_module.py

def func1(n):
    return n

def func2(n):
    return n

In the Python interpreter

>>> import sample_module
>>> sample_module.func1('kawauso') 
'kawauso'

About module import

point

• Modules can contain executable statements as well as function definitions
• These statements are executed at the time of import, so they are used as initialization.
• Each module has a private symbol table, so you don't have to worry about name conflicts with the caller.
• To access global variables in a module, write it in the form modulename.itemname
• You can call other modules from the module
• By convention, the import statement should be placed at the beginning of the file
• It is also possible to call the names defined in the module individually

>>> from sample_module import func1, func2
>>> func1('kawauso')
'kawauso'
>>> func2('kawauso')
'kawauso'

• Use * to capture all names defined by the module except those that start with _
• Generally, it is difficult to understand at first glance what is imported, so it is not preferred.

>>> from sample_module import *
>>> func1('kawauso')
'kawauso'

Run the module as a script

point

• You can also run the module in the python sample_module.py argument
• At this time, __name__ becomes __main__
• You can use this to write a branch when it is executed as a script.

sample_module.py

def func1(n):
    return n

def func2(n):
    return n

#This code does not run when imported as a module
if __name__ == "__main__":
    import sys
    func1(sys.argv[1])

$ python sample_module.py kawauso
kawauso

Module search path

point

• When a module is imported, the Python interpreter first looks for a built-in module.
• Then search for the filename using the list of directories obtained with the sys.path variable
• sys.path can be modified programmatically after initialization
• The directory containing the running script is placed at the beginning of the search path
• Therefore, if there is a module with the same name as the standard library, that will be loaded preferentially.

About compiled Python files

point

• Python caches compiled modules in the __pycache__ directory for faster module loading
• Compiled modules are cached with the name module.version.pyc
• This naming convention allows different releases and versions of Python to coexist.
• Python compares the last modified date and time of the source file with the compiled version to determine if recompilation is needed

Standard module

point

• Python comes with a standard module library
• Some modules are built into the interpreter and are built in for performance needs and OS primitive processing.
• These depend on the installation settings and platform
• For example, the winreg module is only available on Windows systems
• sys is typical of these modules (this is built into all Python interpreters)
• Variables such as sys.ps1, sys.ps2 are defined only when the interpreter is in interactive mode.
• sys.path is a list of strings that specify the interpreter's module search path
• sys.path is either PYTHONPATH or the built-in default path
• This can be modified with standard list operations

dir () function

• The built-in function dir () is used to check the name defined by the module (return value is a list of strings)

>>> import module
>>> dir(module) 
['__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', 'func1', 'func2']

• Argumentless dir () returns a list of names defined at the current level

>>> hoge = 'hoge' 
>>> dir()
['__annotations__', '__builtins__', 
'__doc__', '__loader__', '__name__', '__package__', '__spec__', 'hoge', 
'module']
>>>

• dir () does not list built-in function and variable names
• If you want to check, check 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', 'ModuleNotFoundError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented', 'NotImplementedError', 'OSError', 'OverflowError', 'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError', 
'RecursionError', 'ReferenceError', 
'ResourceWarning', 'RuntimeError', 'RuntimeWarning', 'StopAsyncIteration', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning', 'ValueError', 'Warning', 'WindowsError', 'ZeroDivisionError', '_', '__build_class__', '__debug__', '__doc__', '__import__', '__loader__', '__name__', '__package__', '__spec__', 'abs', 'all', 'any', 'ascii', 'bin', 'bool', 'breakpoint', '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']

package

point

• Package is how to build a Python module using "dot delimited module name"
• For example, the module name ʻA.B` refers to the submodule "B] in package "A".
• Requires a __init__.py file to treat a directory as containing packages
• __init__.py can be just an empty file, but you can also run the package initialization code and set the __all__ variable.

Package configuration example

A/ ->Top level package
    __init__.py ->Package initialization
    A1/  ->Subpackage 1
        __init__.py
        hoge1.py
        hoge2.py
        hoge3.py
    A2/ ->Subpackage 2
        __init__.py
        hoge1.py
        hoge2.py
        hoge3.py

Load package

import A.A1.hoge1
#References to this must be done with full name
A.A1.hoge1.func1()

There is also another loading method

Load package 2

from A.A1 import hoge1
hoge1.func1()

There is also a way to directly import the desired function or variable.

from A.A1.hoge1 import func1
func1()

Import * from package

point

• When from A.A1 import *, if the code of __init__.py of the package defines the __all__ variable, import the package defined there.
• If __all__ is not defined, only import ʻA.A1` and import all names defined in the package body
• Names defined in __init__.py are also imported

__init__.py

__all__ = ["module1", "module2", "module3"]
module4 = func4() #module4 is also imported

• If there is a submodule that was explicitly loaded in the previous import statement, it will also be imported.

#echo and surround modules are imported
import sound.effects.echo
import sound.effects.surround
from sound.effects import *

Cross-reference within the package

point

• When a package is composed of subpackages, sister package references can be imported relative to absolute as well as imported.
• Relative import is based on the current module, so be sure to use absolute import from the main module
• The name of the main module will always be __main__

from . import echo #Same hierarchy
from .. import formats #One level up
from ..filters import equalizer #In the filters submodule one level up

Packages that span multiple directories

point

• By rewriting __path__, it affects the search for modules and subpackages included in the package.
• Can be used when you want to extend the set of modules that a package has