Modules in Python

S.Madhumalar

Assistant Professor

C.P.A College, Bodinayakanur

What are Modules?

• Modules are files containing Python definitions and statements (ex. name.py)
• A module’s definitions can be imported into other modules by using “import name”
• The module’s name is available as a global variable value
• To access a module’s functions, type “name.function()”

More on Modules

• Modules can contain executable statements along with function definitions
• Each module has its own private symbol table used as the global symbol table by all functions in the module
• Modules can import other modules
• Each module is imported once per interpreter session
• reload(name)
• Can import names from a module into the importing module’s symbol table
• from mod import m1, m2 (or *)
• m1()

Executing Modules

• python name.py <arguments>
• Runs code as if it was imported
• Setting _name_ == “_main_” the file can be used as a script and an importable module

The Module Search Path

• The interpreter searches for a file named name.py
• Current directory given by variable sys.path
• List of directories specified by PYTHONPATH
• Default path (in UNIX - .:/usr/local/lib/python)
• Script being run should not have the same name as a standard module or an error will occur when the module is imported

“Compiled” Python Files

• If files mod.pyc and mod.py are in the same directory, there is a byte-compiled version of the module mod
• The modification time of the version of mod.py used to create mod.pyc is stored in mod.pyc
• Normally, the user does not need to do anything to create the .pyc file
• A compiled .py file is written to the .pyc
• No error for failed attempt, .pyc is recognized as invalid
• Contents of the .pyc can be shared by different machines

Some Tips

• -O flag generates optimized code and stores it in .pyo files
• Only removes assert statements
• .pyc files are ignored and .py files are compiled to optimized bytecode
• Passing two –OO flags
• Can result in malfunctioning programs
• _doc_ strings are removed
• Same speed when read from .pyc, .pyo, or .py files, .pyo and .pyc files are loaded faster
• Startup time of a script can be reduced by moving its code to a module and importing the module
• Can have a .pyc or .pyo file without having a .py file for the same module
• Module compileall creates .pyc or .pyo files for all modules in a directory

Standard Modules

• Python comes with a library of standard modules described in the Python Library Reference
• Some are built into interpreter
• >>> import sys

>>> sys.s1

‘>>> ‘

>>> sys.s1 = ‘c> ‘

c> print ‘Hello’

Hello

c>

• sys.path determines the interpreters’s search path for modules, with the default path taken from PYTHONPATH
• Can be modified with append() (ex. Sys.path.append(‘SOMEPATH’)

The dir() Function

• Used to find the names a module defines and returns a sorted list of strings
• >>> import mod

>>> dir(mod)

[‘_name_’, ‘m1’, ‘m2’]

• Without arguments, it lists the names currently defined (variables, modules, functions, etc)
• Does not list names of built-in functions and variables
• Use _bulltin_to view all built-in functions and variables

Packages

• “dotted module names” (ex. a.b)
• Submodule b in package a
• Saves authors of multi-module packages from worrying about each other’s module names
• Python searches through sys.path directories for the package subdirectory
• Users of the package can import individual modules from the package
• Ways to import submodules
• import sound.effects.echo
• from sound.effects import echo
• Submodules must be referenced by full name
• An ImportError exception is raised when the package cannot be found

Importing * From a Package

• * does not import all submodules from a package
• Ensures that the package has been imported, only importing the names of the submodules defined in the package
• import sound.effects.echo

import sound.effects.surround

from sound.effects import *

Intra-package References

• Submodules can refer to each other
• Surround might use echo module
• import echo also loads surround module
• import statement first looks in the containing package before looking in the standard module search path
• Absolute imports refer to submodules of sibling packages
• sound.filters.vocoder uses echo module

from sound.effects import echo

• Can write explicit relative imports
• from . import echo
• from .. import formats
• from ..filters import equalizer

Packages in Multiple Directories

• _path_ is a list containing the name of the directory holding the package’s _init_.py
• Changing this variable can affect futute searches for modules and subpackages in the package
• Can be used to extend the set of modules in a package
• Not often needed