Python Language Modularization

External

• https://realpython.com/python-modules-packages

Internal

• Python Language

Overview

Python code is organized in units like standalone programs, modules and packages.

Standalone Program

A standalone program consists of one or more files of code that is read by the Python interpreter and executed. A typical way to interact with a Python program is command line arguments. For more details on handling command line arguments, see:

Command Line Argument Processing in Python

Python Script

A script is a module whose aim is to be executed. It has the same meaning as "program", standalone program, or "application", but it is usually used to describe simple and small program. It contains a stored set of instructions that can be handed over to the Python interpreter:

python3 ./my-script.py

Python scripts have .py extensions.

The python code can be specified in-line with a here-doc:

python3 <<EOF
print('hello')
print('hello2')
EOF

The same approach can be taken when Python code needs to be executed from within a bash script, for more details see:

Calling Python from bash | Inline Python Code

Top-Level Script

Packages can be run as if they were scripts if the package provides a top-level script __main__.py.

• TO PROCESS: https://docs.python.org/3/library/__main__.html
• Understand setup.py - it defines the entry point.

Module

A module is an organizational unit of Python code. It consists of one file. The module can be imported inside another Python program or executed on its own. The module can define variables, functions and classes. If intended to run on its own, the module will also include runnable code. The file name consists of the module name with the suffix .py appended. Modules are loaded into Python by the process of importing, where the code in one module is made available to Python code in another module. The files of Python modules and packages are managed by a Python package manager like pip, Conda, Pipenv and Poetry.

Modules have a namespace containing arbitrary Python objects.

Module Name

Naming conventions are documented here: https://www.python.org/dev/peps/pep-0008/#id36: Modules should have short, all-lowercase names. Underscores can be used in the module name if it improves readability. The name of the module cannot contain dashes ('-'). The name of the directory the module is stored in can contain dashes.

Module Search Path

When a module is imported from a code file, the runtime looks at a list of directory names and ZIP files stored in the standard sys module as the variable path. This list can be accessed and modified:

import sys
for i in sys.path:
  print(i)

 /opt/brew/Cellar/python@3.9/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python39.zip
 /opt/brew/Cellar/python@3.9/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9
 /opt/brew/Cellar/python@3.9/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/lib-dynload
 /Users/ovidiu/my-project/venv/lib/python3.9/site-packages

The initial blank output line is the empty string '', which stands for current directory. The first match will be used. If a module with the same name as a module from standard library is encountered in the search path before the standard library, it will be used instead of the module coming from the standard library.

Extending Module Search Path

Externally

Set the environment variable PYTHONPATH to a colon-separated list of directories to search for imported modules. A directory declared in PYTHONPATH will be inserted at the beginning of the sys.path list when Python starts up.

Internally from the Program

Append to sys.path

Appending an absolute path works:

import sys
sys.path.append('/path/to/search')

Appending a relative path does not work, unless the script is executed from the directory the path is relative to.

import sys
sys.path.append('./my-module') # This does not work unless the Python script is executed from "my-module"'s parent.

To use a relative directory and make append() insensitive to the location the program is run from, use this pattern:

import os
import sys
sys.path.append(os.path.dirname(__file__) + "/my-module")
import my_module

PyCharm trick: If the module name and the parent directory have the same name, PyCharm will stop issuing static analysis error "No module named ..."

Append to site.addsitedir

Another way is to use site.addsitedir to add a directory to sys.path. The difference between this and just plain appending is that when you use addsitedir, it also looks for .pth files within that directory and uses them to possibly add additional directories to sys.path based on the contents of the files.

Package

A package is a Python module which can contain submodules or recursively, subpackages. Technically, a package is a Python module with a __path__ attribute. Packages allow for a hierarchical structuring of the module namespace using dot notation. In the same way that module avoid collisions between global variable names, packages avoid collision between module names. For example, the urllib package contains several modules: urllib.request, urllib.error, etc. Modules and packages are managed by a Python package manager like pip, Conda, Pipenv and Poetry.

Not clear yet how to define working packages. I've defined a package with an __init__.py and a __main__.py with a function defined inside __main__.py, and after import I get:

ImportError: cannot import name 'my_test_function' from 'my_package' (/Users/ovidiu/.../main/python/my_package/__init__.py)

Regular Package

A traditional package, such as a directory containing an __init__.py file.

__init__.py

https://docs.python.org/3/reference/import.html#regular-packages

__main__.py

The "main" module, which will be imported automatically when the package is imported.

Namespace Package

A PEP 420 package which serves only as a container for subpackages. Namespace packages may have no physical representation, and have no __init__.py file.

Package Metadata

Name: pulumi
Version: 2.11.2
Summary: Pulumi's Python SDK
Home-page: https://github.com/pulumi/pulumi
Author:
Author-email:
License: Apache 2.0
Location: /Users/ovidiu/Library/Python/3.8/lib/python/site-packages
Requires: dill, grpcio, protobuf
Required-by: pulumi-aws, pulumi-kubernetes, pulumi-random, pulumi-tls

Importing

The process of importing is loading code of a module or a package to make it available to other module.

Importing a Module

The entire code of a module is imported into another module by using the import statement, listed at the top of the file, followed by the name of the module, which is the name of the module's Python file, without the .py extension.

import mymodule

Multiple comma-separated module names can be specified in the same import statement, but various static analysis programs flag this as a style violation:

import mymodule, mymodule2 # The style checker flags this as a style violation

Clarify the difference between importing a module and a package. Multiple modules can be imported on the same line in a comma-separated list:

TO PROCESS: The import system: https://docs.python.org/3/reference/import.html

import some_module as blah_some_module
import some_other_module as blah_some_other_module

Importing only a Function

from  urllib.request import urlopen

Library

The term library is simply a generic term for a bunch of code that was designed with the aim of being reused by many applications. It provides some generic functionality that can be used by specific applications. When a module or a package is published, people refer to it as a library.

Standard Library

If the published module or package is is available in the Python Module Index, it is referred to as a "standard library". The standard library modules are included with Python when it is installed.

Python Standard Library

https://docs.python.org/3/library/

Notable Modules and Packages

These modules are available in the Python Standard Library, hence are referred to as "standard libraries".

Python Module Index

https://docs.python.org/3/py-modindex.html

Python Package Index PyPI

https://pypi.org

It can be searched with pip search.

Organizatorium

• Each installation of Python may have different modules installed. Python determines the path to its modules by examining the location of the python3 executable.