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MacOS Monterey Breaks Python, How to Fix it?

After I updated my MacOS to Monterey, every time I changed the directory in the terminal this message below popped up:

env: python: No such file or directory

But I've already installed Python3. I googled out that Monterey removed Python2.7, but it cannot automatically point to Python3...

It took me 2 hours to find the solution, here they are:

Step 1: Make Sure Python3 Is Installed

Use whereis python3 to make sure it is installed. At this time, it would show something like this:

python3: /usr/bin/python3

Step 2: Find Out the Brew Info

When installing Python3 with Home-brew, it also creates a libexec folder with unversioned symlinks, which can be found by this command: brew info python.

After executing this command, there will be something like the below print out:

python@3.9: stable 3.9.7
Interpreted, interactive, object-oriented programming language
https://www.python.org/

/usr/local/Cellar/python@3.9/3.9.7 (2,882 files, 49.3MB)

Poured from bottle on 2021-09-15 at 22:09:14

From: https://github.com/Homebrew/homebrew-core/blob/HEAD/Formula/python@3.9.rb

License: Python-2.0

==> Dependencies

Build: pkg-config ✔

Required: gdbm ✔, mpdecimal ✔, openssl@1.1 ✔, readline ✔, sqlite ✔, xz ✔

==> Caveats

Python has been installed as

/usr/local/bin/python3

Unversioned symlinks `python`, `python-config`, `pip` etc. pointing to

`python3`, `python3-config`, `pip3` etc., respectively, have been installed into

/usr/local/opt/python@3.9/libexec/bin

I omit the rest of the print-out because the MOST IMPORTANT part of this is the last line.

Step 3: Update Your $PATH

Put the directory path in that last line to your .bash_profile or .zshrc, depending on which one of them you're using. The complete syntax is:

export PATH="/usr/local/opt/python@3.9/libexec/bin:$PATH"

Save the file and update the shell environment:

source ~/.bash_profile

# OR

source ~/.zshrc

Voilà! The env: python: No such file or directory problem is solved!

Now type whereis python3 again, the result would be something like this: python3: /usr/bin/python3 /usr/local/share/man/man1/python3.1.

Summary

Your situation may be different from mine, so for more references, I put some links here and hope these would help you.