Python has a strong community which releases lot's of modules(packages), which you can use for free and focus on "get job done", instead of reinventing the wheel.

pip is the tool for installing Python packages from PyPI (Python Package Index)

Note, that if you do not use a Virtual Environment, a package installed by using pip will be installed in your system's python distribution.

pip basic commands

get help on pip:

pip help

install a package from PyPI:

pip install package_name

uninstall a package

pip uninstall package_name

list installed packages:

pip list

upgrade a package:

pip install --upgrade SomePackage

show information about installed packages.:

pip show package_name

output installed packages in requirements format

pip freeze

Reference: pip commands

More readings

pip - The PyPA recommended tool for installing Python packages.

PyPI - the Python Package Index

Virtual environment - Why?

Python apps usually depends on many modules(packages), which depends on other packages, and very often - from a specific version of a package.

Many system tools also depends on specific versions of installed Python packages.

You don't want to pollute your system's Python environment or to deal with dependency hell!

Virtual environment - What?

A virtual environment is an isolated Python environment that allows packages to be installed for use by a particular application, rather than being installed system wide

A virtual environment is a directory tree which contains Python interpreter, libraries and scripts installed into it, and other files which indicate that it is a virtual environment.

There are many tools for working with virtual environments in Python (virtualenv, pipenv, poetry, conda, etc.)

Starting with Python 3.4, venv module is the preferred way to create and manage virtual environments.

venv module is included in the Python standard library and requires no additional installation.

Create Virtual Environment

Open Terminal and make sure that current working directory is the one where you want the virtual environment to be created

                ### make sure you are in project folder, where you want to use virtual environment
                # Command Prompt:
                cd

                # Linux/MacOS:
                pwd

                ### create virtual environment with name ".venv"
                python -m venv .venv
            

a folder '.venv' is created in the project folder

.venv is a common name for a virtual environment as it keeps the directory typically hidden in your shell.

Note, that you need to create a virtual environment per project only once. But each time you work in your project you must activate it.

In order to install and use python packages in your virtiual env, you must activate it:

                # On Windows PowerShell run:
                .venv\Scripts\Activate.ps1

                # On Windows CommandPrompt run:
                .venv\Scripts\activate.bat

                # On Windows GitBash run:
                source .venv\Scripts\activate

                # On Linux, MacOS, run:
                source .venv/bin/activate
            

If on PowerShell you receive an error File ... cannot be loaded because running scripts is disabled on this system you must first set the PowerShell execution policies (Set-ExecutionPolicy @microsoft.com):

                Set-ExecutionPolicy Unrestricted -Scope Process
                # now try again to activate:
                .venv\Scripts\Activate.ps1
            

You can confirm you’re in the virtual environment by checking the location of your Python interpreter:

                # On Windows, run:
                where python

                # On Unix or MacOS, run:
                which python

                # It should be in the .venv directory
            

If you want to switch projects or otherwise leave your virtual environment, simply run

                deactivate
            

References

Virtual Environments and Packages @python3 tutorial

Install Project Dependencies

                # Create virt.env - just once per project
                python -m venv .venv

                # Activate it - every time you will work on the project:
                # On Windows CommandPrompt run:
                .venv\Scripts\activate.bat

                # On Linux, MacOS, run:
                source .venv/bin/activate

                # Install module
                pip install module_name
            

Once the virtual environment is activated, whenever we use pip install to install any library, if we don’t specify the desired version of the library, pip will automatically install the latest version that is compatible with the python version inside the virtual environment

Reference: Installing packages using pip and virtual environments

Overview

"Freezing the dependency versions" in the context of Python development refers to the practice of creating a snapshot of all the packages and their exact versions that your project depends on.

This is crucial for ensuring consistency, reproducibility, and stability across different environments (development, testing, production) and when sharing the project with others.

It helps to avoid the "it works on my machine" problem by ensuring that everyone working on the project uses the same versions of dependencies.

the pip freeze

Before freezing dependencies, ensure that your virtual environment is activated. This ensures that the dependency list is specific to your project and not your global Python installation.

Once your virtual environment is activated, use pip freeze to list all installed packages and their exact versions. Then, redirect the output to a requirements.txt file:

                pip freeze > requirements.txt
            

This requirements.txt file now contains a snapshot of all packages (with versions) installed in your virtual environment.

Using the 'requirements.txt' file

When you or someone else needs to set up the project environment, you can use the requirements.txt file to install all the dependencies at their frozen versions. Run:

                pip install -r requirements.txt
            

This command installs the exact versions of the packages listed in requirements.txt, ensuring that the environment is consistent with the original setup.