Python Modules, Schemes and Mind Maps of Web Programming and Technologies

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PYTHON MODULES

Brandon McKay

Reasons for using modules

• Definitions of functions and variables are lost after quitting each session of the Python interpreter
• Creating a script file of your Python input for each individual program would be cumbersome and redundant
• Modules allow you to import code from an external file and use it in your program/script, in a similar fashion as libraries used in C++

Importing modules (cont.)

• Can import selected portions of the module
• Import selected names in the module as local names

from module import function1, function function1(argument) function2(argument)

• Import all names except those that start with “_”

from module import *

Executing modules as scripts

• When importing modules, the name variable is set to the module’s name
• When running a module directly from the command line, the name variable is instead set to main
• This allows the ability to execute code depending on whether the module is being used as a script or as an imported module

$ python module.py arguments (name=“main”)

import module (name=“module”)

Searching for modules

• When a module is imported, the python interpreter first looks for a built-in module
• If not found, it will then search for “moduleName.py” in the locations defined by the variable “sys.path”, which initially defines these locations: - the directory containing the script currently running - the locations defined in “PYTHONPATH” - the default installation directory
• Python programs can modify “sys.path” after initialization

Pre-compiled Python files

• Python caches the compiled versions of each module,

allows much faster execution of programs

• Stores in “pycache” directory, under the filename

“moduleName.version.pyc”

• This ensures that the compiled versions are used with the

correct version/release of Python being used

• Automatically recompiles any source file that is modified
• Python always recompiles modules ran from the

command line, however

The built-in dir() function

• Is used to find out which names are defined by a module
• Returns the names as a sorted list of strings

import module dir(module) [‘name’, ‘function1’, ‘function2’] a = 5 dir() [‘builtins’, ‘name’, ‘module’, ‘a’, ‘function1’, ‘function2’]

• When called without an argument, dir() lists all currently defined names

Packages

• A package is a collection of modules
• A file named “init.py” must be placed in the directory of the package, to indicate that the directory is indeed a package
• “init.py” can also contain executable code
• Packages can contain other packages, or “subpackages”

import package import package.subpackage.function package.subpackage.function(argument) from package.subpackage import function function(argument)