1. Check Python environment# 

❯ ls /usr/bin/python* & ls /usr/local/bin/python*
/usr/local/bin/python3            /usr/local/bin/python3.10
/usr/bin/python3

/usr/bin belongs to the system. Mess with it at your peril. /usr/local/bin is yours to fool with; if you mess something up in there, you can trash /usr/local and the system will chug along just fine. If you trash /usr/bin, you’ll probably end up reinstalling the OS.

1.1. Multiple versions of Python interpreter#

As you can see, there are two python3 located at /usr/bin/ and /usr/local/bin/ respectively, and both of these dirs added into the $PATH, so when I input python3 on my terminal, which "python3" will be chosen?

It depends on the PATH environment variable, the PATH variable determines the order in which directories are searched to find executable files. If /usr/local/bin appears before /usr/bin in the PATH, then /usr/local/bin/python3 will be chosen when you enter “python3” in the terminal.

2. Environment variable $PATH#

The PATH environment variable is an essential component of any Linux system. If you ever use the command line at all, the system is relying on the PATH variable to find the location of the commands you are entering.

echo $path
/opt/homebrew/bin /opt/homebrew/sbin /usr/local/bin /usr/bin /bin /usr/sbin /sbin

Learn more: https://linuxconfig.org/linux-path-environment-variable

3. Virtual environment#

venv (for Python 3) and virtualenv (for Python 2) allow you to manage separate package installations for different projects. They essentially allow you to create a “virtual” isolated Python installation and install packages into that virtual installation. When you switch projects, you can simply create a new virtual environment and not have to worry about breaking the packages installed in the other environments. It is always recommended to use a virtual environment while developing Python applications.

If you are using Python 2, replace venv with virtualenv in the below commands:

python3 -m venv env_310
source env/bin/activate

This command will create a virtual environment in your current directory. The version of Python in the virtual environment using this command will be the same is the version of the python interpreter you used.

Here the python3 is 3.10, so the created env will use python 3.10 as interpreter, you can find the interpreter at ./env_310/bin/ folder.

You can also create different virtual env in a same project so that you can switch different version of Python interpreter.

# python3 is python 3.10 on my machine
$ python3 -m venv env_310

#  /usr/bin/python3 is python 3.9
$  /usr/bin/python3 -m venv env_309

Then there will be two virtual environments, you can switch to any version of the Python virtual environment by deactivating one and activating another.

$ source env_310/bin/activate
$ which python 
/Users/David/Codes/PyCharm/tgpt/env_310/bin/python
$ env_310 ❯ python --version      
Python 3.10.0
# switch to python 3.9
$ deactivate
$ source env_309/bin/activate
...

4. Command python -m <module-name># 

python3 -m notebook
python3 -m pip ...

# if no -m, there will be an error
python3 notebook
// /Library/Developer/CommandLineTools/usr/bin/python3: can't open file '/Users/shaowen/notebook': [Errno 2] No such file or directory

python pip 
// python: can't open file '/Users/PyCharm/pythonProject/pip': [Errno 2] No such file or directory

What does -m in python -m pip install <package> mean? or while upgrading pip using python -m pip install --upgrade pip. What is difference when just running pip install.

-m: run library module as a script (terminates option list)

python3 main.py
python3 -m main
python3 main // error
python3 -m main.py // warning remove '.py'

5. if __name__ == "__main__"#

This is useful because you do not want this code block to run when importing into other files, but you do want it to run when invoked from the command line.

//cat.py:
if __name__ == "__main__":
    print('mow~')

//main.py:
import cat
print('hello')

Execute on shell:

python main.py
hello
# After the first line in cat.py is removed
python main.py
mow~
hello

6. pip uninstall xxx#

use pip-autoremove to remove a package plus unused dependencies.

pip list
# install pip-autoremove
pip install pip-autoremove
# remove "somepackage" plus its dependencies:
pip-autoremove xxx -y

https://stackoverflow.com/a/10284948/16317008