Python Language Modularization: Difference between revisions
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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 <code>.py</code> appended. Modules are loaded into Python by the process of [[#Import|importing]], where the code in one module is made available to Python code in another 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 <code>.py</code> appended. Modules are loaded into Python by the process of [[#Import|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 Managers#Overview|Python package manager]] like [[Pip#Overview|pip]], [[Conda#Overview|Conda]], [[Pipenv#Overview|Pipenv]] and [[Poetry#Overview|Poetry]]. | ||
<font color=darkkhaki>Modules have a [[Python_Language#Variables_Namespace_and_Scope|namespace]] containing arbitrary Python objects.</font> | <font color=darkkhaki>Modules have a [[Python_Language#Variables_Namespace_and_Scope|namespace]] containing arbitrary Python objects.</font> |
Revision as of 23:06, 19 June 2022
External
Internal
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:
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:
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
__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
Requires
Required-by
Importing
Importing All The Code from a Module
import mymodule
Clarify the difference between importing a module and a package. Multiple modules can be imported on the same line in a comma-separated list:
import mymodule, mymodule2 # The style checker signals this as a violation
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
Notable Modules and Packages
These modules are available in the Python Standard Library, hence are referred to as "standard libraries".
json
os
urllib
venv
webbrowser
shutil
collections
Python Module Index
Python Package Index PyPI
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.