为什么以及如何：缺少 python3 的符号链接？

Question

I am on macOS 10.15.7.

I need both python2 and python3 . I am a beginner which is why I often struggle to understand many terms. A fellow student told me that the -> are symlinks.

When running me$ which python3 I get:

/Library/Frameworks/Python.framework/Versions/3.9/bin/python3

But when checking ll /usr/bin/python*

lrwxr-xr-x  1 root  wheel    75B Oct  9  2019 /usr/bin/python -> ../../System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7
lrwxr-xr-x  1 root  wheel    82B Oct  9  2019 /usr/bin/python-config -> ../../System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7-config
lrwxr-xr-x  1 root  wheel    75B Oct  9  2019 /usr/bin/python2 -> ../../System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7
lrwxr-xr-x  1 root  wheel    75B Oct  9  2019 /usr/bin/python2.7 -> ../../System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7
lrwxr-xr-x  1 root  wheel    82B Oct  9  2019 /usr/bin/python2.7-config -> ../../System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7-config
-rwxr-xr-x  1 root  wheel    31K Oct 30  2020 /usr/bin/python3
lrwxr-xr-x  1 root  wheel    76B Oct  9  2019 /usr/bin/pythonw -> ../../System/Library/Frameworks/Python.framework/Versions/2.7/bin/pythonw2.7
lrwxr-xr-x  1 root  wheel    76B Oct  9  2019 /usr/bin/pythonw2.7 -> ../../System/Library/Frameworks/Python.framework/Versions/2.7/bin/pythonw2.7

Everything looks the same except for the line concerning python3.

1. Why does which python3 return a different path than the one I see when ll ing /usr/bin/python* . I mean both exist... So, why is /Library/Frameworks/Python.framework/Versions/3.9/bin/python3 preferred?
2. Why is there no symlink for python3? What went wrong during the installation?
3. Is it "safe" to just manually generate a symlink between the /usr/bin/python3 and the /Library/Frameworks/Python.framework/Versions/3.9/bin/python3 as shown here . ln -s /path/to/original/ /path/to/link Being a beginner I'd rather ask the internet first;)

I'm happy to edit my question following your suggestions as I'm here to learn - and hopefully soon also to help.

Thanks for your help.

Answer 1

There's actually a fair bit of research here.

The 2.7 Python is the Apple-used Python, best leave it alone. I would also leave the existing /usr/bin/python3 alone dont replace it with a symlink. As to results of which, they depend on the contents of the PATH variable. wherever something is first found is where it is gotten from. /Library/Frameworks/Python.framework/Versions/3.9/bin/ must precede /usr/bin in PATH and they both have a python3 entry.

The easiest way to have a stable Python to work with (and no, I don't recommend Python 2.7 as part of that) is to use macport or brew to install Python, which install Python symlinks in a given directory, separate from system directories like /usr/bin .

(but do note chepner's comment about brew, he looks to be on to something and he's probably forgotten more about Python than I ever knew)

Then use virtualenv or an equivalent to pick the correct Python to use for a given project.

I would keep as a general policy leaving the contents of /usr/bin alone, unless you have some really good reason.

This is a primer on the macos directory structure, with emphasis on PATH.

I you want to see what's in your PATH, you can echo $PATH or do the following:

create file myshellutils.sh :

paste in the following:

showpath ()
{
    IFS=':' read -a array <<< "$PATH";
    for index in "${!array[@]}";
    do
        echo "$index ${array[index]}";
    done
}

save it, then type source myshellutils.sh . That creates a new shell function, showpath .

which you can use as follows (some stuff edited out).

(venv38) me@bin$ showpath
0 /Users/me/.cargo/bin
1 /Users/me/venv38/bin   👈 this is where the VirtualEnv-activated version of Python lives
3 /opt/local/bin 👈 where macports puts things 
4 /usr/local/bin 👈 where brew puts things.  Yes, I had to use both.
5 /usr/bin 👈 Apple stuff, leave alone.
6 /bin
7 /usr/sbin
8 /sbin
10 /opt/local/sbin
12 /Users/me/bin

Last, but not least... a symlink? Whuzzat? A pretty clever idea actually, it's basically a pointer, or a marker if you wish, to another file somewhere else with a potentially different name.

so, say your programs point to which Python to run

foo.py :

#!/usr/bin/env python
print("hello")

so, first let's which -a python (nice call on -a, L.Grozinger )

which gets

/Users/me/venv38/bin/python

let's cd there, then ls -l python* with a cut to get rid of all the user and size stuff.

$ls -l  python* | cut -c 46-

python -> python3.8  👈 1️⃣ your entry point for where foo.py looks for "python".  now look for "python3.8"
python3 -> python3.8
👇 2️⃣ which resolves to where macport installs 
python3.8 -> /opt/local/Library/Frameworks/Python.framework/Versions/3.8/bin/python3.8  binaries.

The clever thing? As you install Python 3.9, it will be linked to by python -> python3.9 and foo.py will never care.

Answer 2

From the which man page

which returns the pathnames of the files (or links) which would be executed in the current environment, had its arguments been given as commands in a strictly POSIX-conformant shell

and in the options list

 -a print all matching pathnames of each argument

So, without the -a , which simply prints the first 'hit' on your PATH . So for example, if /Library/Frameworks/Python.framework/Versions/3.9/bin is before /usr/bin in your PATH , which python3 will return only one.

I would be willing to bet that your running which -a python3 will return:

/Library/Frameworks/Python.framework/Versions/3.9/bin
/usr/bin/python3

Your install went fine, you just have more than one python3 executable installed. There is also no need to do any extra symlinking.