Overview

Modules allows to split a long program into several files

for instance - to put function/class definitions in one or more files, and the main program to be written in a separate file.

Python modules

Module == Python file

Each module creates its own namespace.

A module can contain definitions, as well as executable statements

Creating a custom Python module

Its just simple as creating a Python file - do not forget the .py extension

      ├── my_app
      │   ├── app.py           # the main file
      │   ├── helper_module.py # a module file
    

Import module: import X

To use a module, you have to import it first.

As a module generates its own namespace, you have to prefix the imported names with the module name:

        # import the helper_module:
        import helper_module

        # use helper_module
        user_name = helper_module.get_user_name()
        helper_module.greet(user_name)
      

Module alias: import X as Y

      import helper_module as hm

      user_name = hm.get_user_name()
      hm.greet(user_name)
    

Import object from module: from X import Y

You can import an object from module, using the
from X import Y notation. After that, you can use the name (Y), without having to prefixing it.

You can import more that one object, if you separate them with comma

      from helper_module import get_user_name, greet

      user_name = get_user_name()
      greet(user_name)
    

Import object from module: from X import *

Imports all objects from a module.

Not recommended to use it, as a name collision could occurs.

      from helper_module import *

      user_name = get_user_name()
      greet(user_name)
    

Where import looks for a module?

The directory containing the input script

the directories specified in the PYTHONPATH environment variable

The installation-dependent default.

Python's built-in modules

or why Python is "Batteries Included"

Python Module Index

to use a built-in (standard) module, you have to import it!

Help on Python's built-in modules

Using the help('module_name') function provides useful information about a module.

If you want to check all built-in modules, just use help("modules")

Most modules have defined the __file__ attribute (discussed in next slides), which contain the path where the module is installed

        import datetime
        print( datetime.__file__)

        # /usr/lib/python3.5/datetime.py
      

Python's third-party modules

The Python Packaging Index PyPIis a public repository of open source licensed packages made available for use by other Python users.

Installing Python Modules @python3 docs

Creating Virtual Environments

Managing Application Dependencies

Overview

A Python package is simply a group of Python module(s), usually stored in one directory

Packages are a way of structuring Python’s module namespace by using “dotted module names”.

make directory a package: __init__.py

You should put an __init__.py file in the directory, which you want it to be treated as package from Python.

__init__.py can be an empty file

or you can put some initialization code for the package

a package - example

  		$ tree my_app
  		my_app/
  		├── app.py
  		└── packA
  		    ├── greet.py
  		    ├── __init__.py
  		    └── packB
  		        ├── get_data.py
  		        └── __init__.py
  	

The __pycahce__ folder and .pyc files

To speed up loading modules, Python caches the compiled version of each module in the automatically created __pycache__ directory under the name module.version.pyc

If you really need to tell python not to generate the compiled versions, you can set the PYTHONDONTWRITEBYTECODE env. var.

Compile and decompile pyc files

Importing packages

You can import a package using the same mechanism as importing a module. But do not forget - a package makes its namespace, as well.

Overview

Python modules are objects and as such they have attributes.

Most useful module attributes are:

__name__

__file__

__name__

When Python interpreter reads a .py file, it executes the code in it!

If a .py file is executed as a module:

__name__ is set to module's own filename

If a .py file is executed as stand-alone program:

__name__ is set to "__main__"

__name__ - examples

Create in same directory next Python files:

      import helper_module
    

      if __name__ == "__main__":
        print("helper_module is executed as stand-alone py file")
      else:
        print("helper_module is imported as module")
    

Run the main.py and look at the output

Run the helper_module.py and look at the output

__file__

Stores the full filename (including path) of the file/module being executed

Will be same, no matter if the file is executed as a stand-alone or as a module.

If we need to strip the path, and get just the filename,

__file__ - examples

Create in same directory next Python files:

      import helper_module
    

      print( "__file__:", __file__)
    

Run the main.py and look at the output

Run the helper_module.py and look at the output

pip way - requirements.txt

When you need to move your project and

Readings

The Definitive Guide to Python import Statements