Understanding the Python Ecosystem

This project focuses on understanding the language ecosystem, not getting into programming details.

Summary

🌄 Python's Habitat

This topic describes how to set up the environment for Python development.

• Preparing the Environment for the Python
• Check Python Configuration
• Advanced settings of Python
• What is a virtual environment and how it works

🐍 Python's Taxonomy

This topic describes features of the pattern of Python projects.

• Package manager
• Requirements file
• Deterministic build

💢 Python's Behavior

This topic describes how the language is designed and how it works.

• Compiler and interpreter
• How Python runs a program
• How Python search path module
• How Python manages process and threads
• How Python manages memory
• How to deeply understand Python code execution (debug)

🐛 Python's Feeding

This topic describes static code analysis, formatting patterns and style guides.

• Static code analysis
• Principal style guides
• Knobs (Google YAPF)
• My Knobs
• Docstrings

🔍 Python's Other Features

Extra topics.

• Awesome Python
• Why Is CPython Written in C and Not Python?

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Preparing the Environment for the Python

Linux

Python needs a set of tools that are system requirements. If necessary, install these requirements with this command:

sudo apt update

sudo apt install\
  software-properties-common\
  build-essential\
  libffi-dev\
  python3-pip\
  python3-dev\
  python3-venv\
  python3-setuptools\
  python3-pkg-resources

Now, the environment is done to install Python

sudo apt install python

Windows

On Windows, I recommend using the package manager chocolatey and setting your Powershell to work as admin. See this tutorial.

Now, install Python

choco install python 

Test

python --version 

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Check Python Configuration

Check current version

Watch

python --version

Check where installed Python

Watch

which python

Check which Python versions are installed

Watch

sudo update-alternatives --list python

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Advanced settings of Python

Install multiples Python versions

Sometimes you might work on different projects simultaneously with different versions of Python. Normally using Anaconda is the easiest solution, however, there are restrictions.

1. Add repository

   Watch

   

   This PPA contains more recent Python versions packaged for Ubuntu.

   sudo add-apt-repository ppa:deadsnakes/ppa -y

2. Update packages

   sudo apt update -y

3. Check which Python version is installed

   python --version

4. Install Python

   sudo apt install python3.<VERSION>

Install multiples Python versions Using Pyenv

1. Add dependencies

   sudo apt install curl -y

2. Update packages

   sudo apt update -y

3. Install pyenv

   curl https://pyenv.run | bash

4. Add these three lines from .bashrc or .zhshrc

   export PATH="$HOME/.pyenv/bin:$PATH"
   eval "$(pyenv init --path)"
   eval "$(pyenv virtualenv-init -)"

5. Open a new terminal and execute

   exec $SHELL
   pyenv --version

Change system's Python

Before installing other versions of Python it's necessary to set which system's Python will be used.

1. Use update-alternatives

   It's possible use the update-alternatives command to set priority to different versions of the same software installed in Ubuntu systems. Now, define priority of versions:

   sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.11 1
   
   sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.10 2
    
   sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.8 3
   
   sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.6 4

   In directory /usr/bin will be create simbolic link: /usr/bin/python -> /etc/alternatives/python*

2. Choose version

   Watch

   

   sudo update-alternatives --config python

3. Test

   python --version

Change Python2 to Python3

If return Python 2, try set a alias in /home/$USER/.bashrc, see this example.

alias python=python3

NOTE: The important thing to realize is that Python 3 is not backwards compatible with Python 2. This means that if you try to run Python 2 code as Python 3, it will probably break.

Set Python's Environment Variables

• PYTHONPATH is an environment variable which you can set to add additional directories where python will look for modules and packages. Example: Apache Airflow read dag/ folder and add automatically any file that is in this directory.
• To interpreter PYTHONHOME indicate standard packages.

Set PYTHONPATH

1. Open profile

   sudo vim ~/.bashrc

2. Insert Python PATH

   export PYTHONHOME=/usr/bin/python<NUMER_VERSION>

3. Update profile/bashrc

   source ~/.bashrc

4. Test

   >>> import sys
   >>> from pprint import pprint
   >>> pprint(sys.path)
   ['',
    '/usr/lib/python311.zip',
    '/usr/lib/python3.11',
    '/usr/lib/python3.11/lib-dynload',
    '/usr/local/lib/python3.11/dist-packages',
    '/usr/lib/python3/dist-packages']

   Example with Apache Airflow

   >>> import sys
   >>> from pprint import pprint
   >>> pprint(sys.path)
   ['',
    '/home/project_name/dags',
    '/home/project_name/config',
    '/home/project_name/utilities',
    ...
    ]

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What is a virtual environment and how it works

Python can run in a virtual environment with isolation from the system.

Image source: https://vincenttechblog.com/fix-change-python-virtualenv-settings/

Arquitecture of Execution

Virtualenv enables us to create multiple Python environments which are isolated from the global Python environment as well as from each other.

When Python is initiating, it analyzes the path of its binary. In a virtual environment, it's actually just a copy or Symbolic link to your system's Python binary. Next, set the sys.prefix location which is used to locate the site-packages (third party packages/libraries)

Symbolic link

• sys.prefix points to the virtual environment directory.
• sys.base.prefix points to the non-virtual environment.

Folder of virtual environment

ll

# random.py -> /usr/lib/python3.6/random.py
# reprlib.py -> /usr/lib/python3.6/reprlib.py
# re.py -> /usr/lib/python3.6/re.py
# ...

tree

├── bin
│   ├── activate
│   ├── activate.csh
│   ├── activate.fish
│   ├── easy_install
│   ├── easy_install-3.8
│   ├── pip
│   ├── pip3
│   ├── pip3.8
│   ├── python -> python3.8
│   ├── python3 -> python3.8
│   └── python3.8 -> /Library/Frameworks/Python.framework/Versions/3.8/bin/python3.8
├── include
├── lib
│   └── python3.8
│       └── site-packages
└── pyvenv.cfg

Create Virtual Environment

Watch

Create virtual environment

virtualenv -p python3  <NAME_ENVIRONMENT>

Activate

source <NAME_ENVIRONMENT>/bin/activate

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Package manager

Pipenv

Create and manage automatically a virtualenv for your projects, as well as adds/removes packages from your Pipfile as you install/uninstall packages. It also generates the ever-important Pipfile.lock, which is used to produce deterministic builds.

Features

• Deterministic builds
• Separates development and production environment packages into a single file Pipefile
• Automatically adds/removes packages from your Pipfile
• Automatically create and manage a virtualenv
• Check PEP 508 requirements
• Check installed package safety

Pipfile X requirements

# Pipfile

[[source]]
name = "pypi"
url = "https://pypi.org/simple"
verify_ssl = true

[dev-packages]

[packages]
requests = "*"
numpy = "==1.18.1"
pandas = "==1.0.1"
wget = "==3.2"

[requires]
python_version = "3.8"
platform_system = 'Linux'

# requirements.txt

requests
matplotlib==3.1.3
numpy==1.18.1
pandas==1.0.1
wget==3.2

Install

pip3 install --user pipenv

Create Pipfile and virtual environment

1. Create environment

   Watch

   

   pipenv --python 3

2. See where virtual environment is installed

   pipenv --venv

3. Activate environment

   pipenv run

4. Install packages with Pipefile

   pipenv install flask
   # or
   pipenv install --dev flask

5. Create lock file

   Watch

   

   pipenv lock

   
   

Python Package Index

Doc Python Package Index

Poetry

Doc Poetry

Conda

Doc Conda

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Requirements File

Requirements.txt is file containing a list of items to be installed using pip install.

Principal Comands

1. Visualize instaled packages

pip3 freeze

2. Generate file requirements.txt

pip3 freeze > requirements.txt

3. Test

cat requirements.txt

4. Install packages in requirements

pip3 install -r requirements.txt

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Deterministic Build

Using pip and requirements.txt file, have a real issue here is that the build isn’t deterministic. What I mean by that is, given the same input (the requirements.txt file), pip does not always produce the same environment.

pip-tools

A set of command line tools to help you keep your pip-based packages fresh and ensure the deterministic build.

Features

• Distinguish direct dependencies and versions
• Freeze a set of exact packages and versions that we know work
• Make it reasonably easy to update packages
• Take advantage of pip's hash checking to give a little more confidence that packages haven't been modified (DNS attack)
• Stable

Principal Comands

1. Install

pip install pip-tools

2. Get packages's version

pip3 freeze > requirements.in

3. Generate hashes and list dependeces

pip-compile --generate-hashes requirements.in

output: requirements.txt

4. Install packages and hash checking

pip-compile --generate-hashes requirements.in

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Compiler and interpreter

CPython can be defined as both an interpreter and a compiler.

• The compiler converts the .py source file into a .pyc bytecode for the Python virtual machine.
• The interpreter executes this bytecode on the virtual machine.

CPython's Design

The principal feature of CPython, is that it makes use of a global interpreter lock (GIL). This is a mechanism used in computer-language interpreters to synchronize the execution of threads so that only one native thread can execute at a time.
Therefore, for a CPU-bound task in Python, single-process multi-thread Python program would not improve the performance. However, this does not mean multi-thread is useless in Python. For a I/O-bound task in Python, multi-thread could be used to improve the program performance.

Multithreading in Python

The Python has multithreads despite the GIL. Using Python threading, we are able to make better use of the CPU sitting idle when waiting for the I/O bound, how memory I/O, hard drive I/O, network I/O.

This can happen when multiple threads are servicing separate clients. One thread may be waiting for a client to reply, and another may be waiting for a database query to execute, while the third thread is actually processing Python code or other example is read multiples images from disk.

NOTE: we would have to be careful and use locks when necessary. Lock and unlock make sure that only one thread could write to memory at one time, but this will also introduce some overhead.

Community Consensus

Removing the GIL would have made Python 3 slower in comparison to Python 2 in single-threaded performance. Other problem if remove the GIL it's would broke the existing C extensions which depend heavily on the solution that the GIL provides.
Although many proposals have been made to eliminate the GIL, the general consensus has been that in most cases, the advantages of the GIL outweigh the disadvantages; in the few cases where the GIL is a bottleneck, the application should be built around the multiprocessing structure.

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How Python runs a program

1. Tokenize the source code: Parser/tokenizer.c
2. Parse the stream of tokens into an Abstract Syntax Tree (AST): Parser/parser.c
3. Transform AST into a Control Flow Graph: Python/compile.c
4. Emit bytecode based on the Control Flow Graph: Python/compile.c

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How Python search path module

When Python executes this statement:

import my_lib

The interpreter searches my_lib.py a list of directories assembled from the following sources:

• Current directory
• The list of directories contained in the PYTHONPATH environment variable
• In directory which Python was is installed. E.g.

The resulting search can be accessed using the sys module:

import sys

sys.paths
# ['', '/usr/lib/python38.zip', 
# '/usr/lib/python3.8',
# '/usr/lib/python3.8/lib-dynload',
# '/home/campos/.local/lib/#python3.8/site-packages',
# '/usr/local/lib/python3.8/dist-packages',
# '/usr/lib/python3/dist-packages']

Now, to see where a packeage was imported from you can use the attribute __file__:

import zipp

zipp.__file__
# '/usr/lib/python3/dist-packages/zipp.py'

NOTE: you can see that the __file__ directory is in the list of directories searched by the interpreter.

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How Python manages process and threads

TODO

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How Python manages memory

TODO

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How to deeply understand Python code execution

TODO

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Static code analysis

The static code analysis serves to evaluate the coding. This analysis must be done before submitting for a code review. The static code analysis can check:

• Code styling analysis
• Comment styling analysis
• Error detection
• Duplicate code detection
• Unused code detection
• Complexity analysis
• Security linting

The characteristics of a static analysis are:

• Provides insight into code without executing it
• Can automate code quality maintenance
• Can automate the search for bugs at the early stages
• Can automate the finding of security problems

A lint, is a static code analysis tool.

Pylint

Pylint is a lint that checks for errors in Python code, tries to enforce a coding standard and looks for code smells. The principal features is:

• Pylint follow the PEP8 style guide.
• It's possible automate with Jenkins.
• It is fully customizable through a .pylintrc file where you can choose which errors or agreements are relevant to you.
• Usage

  # Get Errors & Warnings
  pylint -rn <file/dir> --rcfile=<.pylintrc>
  
  # Get Full Report
  pylint <file/dir> --rcfile=<.pylintrc>

Example execution

Pyflakes

Documentation Pyflakes

Mypy

Documentation Mypy

Prospector

Documentation Propector

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Other Tools to make an effective Python style guide

Isort

isort is a Python tool/library for sorting imports alphabetically, automatically divided into sections. It is very useful in projects where we deal with a lot of imports [6].

# sort the whole project
isort --recursive ./src/

# just check for errors
isort script.py --check-only

Unify

Someone likes to write them in single quotes, someone in double ones. To unify the whole project, there is a tool that allows you to automatically align with your style guide — unify [6].

unify --in-place -r ./src/

Work recursively for files in the folder.

docformatter

Docformater is utility helps to bring your docstring under the PEP 257 [6]. The agreement specifies how documentation should be written.

docformatter --in-place example.py

Autoformatters

There are also automatic code formatters now, here are the popular one [6]:

• yapf (here you can make a align with your own style guide)
• black (you don't need a style guide because you don't have a choice)
• autopep8 (makes your python script to conform PEP8 style guide)

Settings files to text editor and IDE

• EditorConfig
• Gitattributes

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Principal style guides

To make the code consistent and make sure it's readable the style guides can help.

• Google style guide: THE BETTER
• Real Python: Naming Conventions
• PEP 08: Style Guide
• PEP 257: Docstrings
• PEP 484: Type Hints

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My Knobs

Identation and Length

• 4 spaces
• Limit all lines to a maximum 72 characteres to docstring or comments
• Limit all lines to a maximum 79 characteres to code

Naming Convention

• Class Name (camelCase): CapWords()
• Variables (snack_case): cat_words
• Constants: MAX_OVERFLOW

Exception

Limit the clausule try: minimal code necessary.

Yes:

try:
    value = collection[key]
except KeyError:
    return key_not_found(key)
else:
    return handle_value(value)

No:

try:
    # Too broad!
    return handle_value(collection[key])
except KeyError:
    # Will also catch KeyError raised by handle_value()
    return key_not_found(key)

• The goal to answer the question "What went wrong?" programmatically rather than just claiming that "There was a problem"

Return

"Should explicitly state this as return None"

• Be consistent in return statements.
• Todas as instruções de retorno em uma função devem retornar uma expressão ou nenhuma delas deve.

Yes:

def foo(x):
    if x >= 0:
        return math.sqrt(x)
    else:
        return None

No:

def foo(x):
    if x >= 0:
        return math.sqrt(x)

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Docstrings

Docstrings must have:

• Args
• Returns
• Raises

Example Google Style Guide

def fetch_bigtable_rows(big_table, keys, other_silly_variable=None):
    """Fetches rows from a Bigtable.

    Retrieves rows pertaining to the given keys from the Table instance
    represented by big_table.  Silly things may happen if
    other_silly_variable is not None.

    Args:
        big_table: An open Bigtable Table instance.
        keys: A sequence of strings representing the key of each table row
            to fetch.
        other_silly_variable: Another optional variable, that has a much
            longer name than the other args, and which does nothing.

    Returns:
        A dict mapping keys to the corresponding table row data
        fetched. Each row is represented as a tuple of strings. For
        example:

        {'Serak': ('Rigel VII', 'Preparer'),
         'Zim': ('Irk', 'Invader'),
         'Lrrr': ('Omicron Persei 8', 'Emperor')}

        If a key from the keys argument is missing from the dictionary,
        then that row was not found in the table.

    Raises:
        IOError: An error occurred accessing the bigtable.Table object.
    """
    return None

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References

• 1: Python 3 Installation & Setup Guide
• 2: An Effective Python Environment: Making Yourself at Home
• 3: Import Scripts as Modules
• 4: Python Modules and Packages – An Introduction
• 5: What Is the Python Global Interpreter Lock (GIL)?
• 6: 7 tips to make an effective python style guide
• 7: Python Static Analysis Tools
• 8: My unpopular opinion about black code formatter

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