We Keep Coding

Creating a python package (deb/rpm) from cmake

I am trying to create a python package (deb & rpm) from cmake, ideally using cpack. I did read
https://cmake.org/cmake/help/latest/cpack_gen/rpm.html and,
https://cmake.org/cmake/help/latest/cpack_gen/deb.html
The installation works just fine (using component install) for my shared library. However I cannot make sense of the documentation to install the python binding (glue) code. Using the standard cmake install mechanism, I tried:
install(
FILES __init__.py library.py
DESTINATION ${ACME_PYTHON_PACKAGE_DIR}/project_name
COMPONENT python)
And then using brute-force approach ended-up with:
# debian based package (relative path)
set(ACME_PYTHON_PACKAGE_DIR lib/python3/dist-packages)
and
# rpm based package (full path required)
set(ACME_PYTHON_PACKAGE_DIR /var/lang/lib/python3.8/site-packages)
The above is derived from:
debian % python -c 'import site; print(site.getsitepackages())'
['/usr/local/lib/python3.9/dist-packages', '/usr/lib/python3/dist-packages', '/usr/lib/python3.9/dist-packages']
while:
rpm % python -c 'import site; print(site.getsitepackages())'
['/var/lang/lib/python3.8/site-packages']
It is pretty clear that the brute-force approach will not be portable, and is doomed to fail on the next release of python. The only possible solution that I can think of is generating a temporary setup.py python script (using setuptools), that will do the install. Typically cmake would call the following process:
% python setup.py install --root ${ACME_PYTHON_INSTALL_ROOT}
My questions are:
Did I understand the cmake/cpack documentation correctly for python package ? If so this means I need to generate an intermediate setup.py script.
I have been searching through the cmake/cpack codebase (git grep setuptools) but did not find helper functions to handle generation of setup.py and passing the result files back to cpack. Is there an existing cmake module which I could re-use ?
I did read, some alternative solution, such as:
How to build debian package with CPack to execute setup.py?
Which seems overly complex, and geared toward Debian-only based system. I need to handle RPM in my case.

As mentionned in my other solution, the ugly part is dealing with absolute path in cmake install() commands. I was able to refactor the code to avoid usage of absolute path in install(). I simply changed the installation into:
install(
# trailing slash is important:
DIRECTORY ${SETUP_OUTPUT}/
# "." syntax is a reliable mechanism, see:
# https://gitlab.kitware.com/cmake/cmake/-/issues/22616
DESTINATION "."
COMPONENT python)
And then one simply needs to:
set(CMAKE_INSTALL_PREFIX "/")
set(CPACK_PACKAGING_INSTALL_PREFIX "/")
include(CPack)
At this point all install path now need to include explicitely /usr since we've cleared the value for CMAKE_INSTALL_PREFIX.
The above has been tested for deb and rpm packages. CPACK_BINARY_TGZ does properly run with the above solution:
https://gitlab.kitware.com/cmake/cmake/-/issues/22925

I am going to post the temporary solution I am using at the moment, until someone provide something more robust.
So I eventually manage to stumble upon:
https://alioth-lists.debian.net/pipermail/libkdtree-devel/2012-October/000366.html and,
Using CMake with setup.py
Re-using the above to do an install step instead of a build step can be done as follow:
find_package(Python COMPONENTS Interpreter)
set(SETUP_PY_IN "${CMAKE_CURRENT_SOURCE_DIR}/setup.py.in")
set(SETUP_PY "${CMAKE_CURRENT_BINARY_DIR}/setup.py")
set(SETUP_DEPS "${CMAKE_CURRENT_SOURCE_DIR}/project_name/__init__.py")
set(SETUP_OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/build-python")
configure_file(${SETUP_PY_IN} ${SETUP_PY})
add_custom_command(
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/setup_timestamp
COMMAND ${Python_EXECUTABLE} ARGS ${SETUP_PY} install --root ${SETUP_OUTPUT}
COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_CURRENT_BINARY_DIR}/setup_timestamp
DEPENDS ${SETUP_DEPS})
add_custom_target(target ALL DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/setup_timestamp)
And then the ugly part is:
install(
# trailing slash is important:
DIRECTORY ${SETUP_OUTPUT}/
DESTINATION "/" # FIXME may cause issues with other cpack generators
COMPONENT python)
Turns out that the documentation for install() is pretty clear about absolute paths:
https://cmake.org/cmake/help/latest/command/install.html#introduction
DESTINATION
[...]
As absolute paths are not supported by cpack installer generators,
it is preferable to use relative paths throughout.
For reference, here is my setup.py.in:
from setuptools import setup
if __name__ == '__main__':
setup(name='project_name_python',
version='${PROJECT_VERSION}',
package_dir={'': '${CMAKE_CURRENT_SOURCE_DIR}'},
packages=['project_name'])
You can be fancy and remove the __pycache__ folder using the -B flag:
COMMAND ${Python_EXECUTABLE} ARGS -B ${SETUP_PY} install --root ${SETUP_OUTPUT}
You can be extra fancy and add debian option such as:
if(CPACK_BINARY_DEB)
set(EXTRA_ARG "--install-layout" "deb")
endif()
use as:
COMMAND ${Python_EXECUTABLE} ARGS -B ${SETUP_PY} install --root ${SETUP_OUTPUT} ${EXTRA_ARG}

How to tell Python to prefer module from $HOME/lib/python over /usr/lib/python?

In Python, I'm getting an error because it's loading a module from /usr/lib/python2.6/site-packages but I'd like it to use my version in $HOME/python-modules/lib/python2.6/site-packages, which I installed using pip-python --install-option="--prefix=$HOME/python-modules --ignore-installed
How can I tell Python to use my version of the library? Setting PYTHONPATH to $HOME/python-modules/lib/python2.6/site-packages doesn't help, since /usr/lib/... apparently has precedence.

Take a look at the site module for ways to customize your environment.
One way to accomplish this is to add a file to a location currently on sys.path called usercustomize.py, when Python is starting up it will automatically import this file, and you can use it to modify sys.path.
First, set $PYTHONPATH to $HOME (or add $HOME if $PYTHONPATH has a value), then create the file $HOME/usercustomize.py with the following contents:
import sys, os
my_site = os.path.join(os.environ['HOME'],
'python-modules/lib/python2.6/site-packages')
sys.path.insert(0, my_site)
Now when you start Python you should see your custom site-packages directory before the system default on sys.path.

Newer Python versions now have built-in support to search the opendesktop location:
$HOME/.local/lib/pythonX.Y/site-packages
If you put your local modules there you don't have to any sys.path manipulations.

If one has multiple versions of a package installed, say e.g. SciPy:
>>> import scipy; print(scipy.__version__); print(scipy.__file__)
0.17.0
/usr/lib/python3/dist-packages/scipy/__init__.py
and one would like the user installed version (installed e.g. using pip install --user --upgrade scipy) to be prefered, one needs a usercustomize.py file in ~/.local/lib/python3.5/site-packages/ with e.g. this content:
import sys, os
my_site = os.path.join(
os.environ['HOME'], '.local/lib/python%d.%d/site-packages' % (
sys.version_info[0], sys.version_info[1]))
for idx, pth in enumerate(sys.path):
if pth.startswith('/usr'):
sys.path.insert(idx, my_site)
break
else:
raise ValueError("No path starting with /usr in sys.path")
(the for loop selecting index ensures that packages installed in "develop mode" takes precedence) now we get our user specific version of SciPy:
>>> import scipy; print(scipy.__version__); print(scipy.__file__)
0.18.1
/home/user/.local/lib/python3.5/site-packages/scipy/__init__.py

to prefer packages installed to userbase (e.g. pip install --user --upgrade cool_thing )
in ~/.bashrc,~/.profile, or whatever the init file for your shell is, add
export PYTHONUSERBASE="$HOME/python-modules"
in $PYTHONUSERBASE/usercustomize.py
#!/usr/bin/env python
import sys, site
sys.path.insert(0, site.getusersitepackages())

How can I add autocomplete to Vim for non-standard Python libraries?

For example, I have a Python script using the Google App Engine SDK:
from google.appengine.ext import db
from google.appengine.ext import webapp
from google.appengine.ext.webapp import template
from google.appengine.ext.webapp.util import run_wsgi_app
The module db has a submodule Key, so I try to use autocomplete on it:
db.KTab
But at the bottom of the Vim window, I get the following:
-- Omni completion (^O^N^P) Pattern not found
How do I include the path to non-standard Python libraries so that Vim autocompletion can find them? And also display their docstrings?

You need to add your library files to your tags file. For instance, if you have installed the Google App Engine via pip in a virtual environment located in env/:
virtualenv --no-site-package env/
source env/bin/activate
pip install google_appengine
... then you should execute:
ctags -R --python-kinds=-i -o tags env/
If you did not install google_appengine through pip, then you should locate the path to your python libraries (hint: it should be indicated by $PYTHONPATH. And according to this reference page: "on Unix, this is usually .:/usr/local/lib/python.") and replace env/ by the path you found.
Finally, your .vimrc file should parse your tags file. For instance, in my .vimrc, I have:
set tags+=/path/to/my/tags

I grabbed this from natw's vimrc (I think...maybe sontek), but it should do the trick, so long as your packages are findable by your current install of Python. This lets you use gf, but also sets up searching these files for autocompletion. Note the py <<EOF part, which starts a section interpreted in Python. This means you'd have to have the python interpreter installed in vim to use it.
function! LoadPythonPath()
py <<EOF
# load PYTHONPATH into vim, this lets you hover over a module name
# and type 'gf' (for goto file) and open that file in vim. Useful
# and easier than rope for simple tasks
import os.path
import sys
import vim
for p in sys.path:
if os.path.isdir(p):
vim.command(r"set path+=%s" % (p.replace(" ", r"\ ")))
EOF
endfunction
Btw, I don't like to have this load automatically, so I set it to a function that intelligently loads/unloads when I call it/first enter a Python doc. And I add a let g:PythonPathLoaded=1 to the previous function.
function! GetPythonPath()
if !exists("g:PythonPathLoaded")
call LoadPythonPath()
return
elseif g:PythonPathLoaded
return
else
call LoadPythonPath()
endif
endfunction
And I have an unload function too...though I'm not sure whether this makes a huge difference.
function! UnloadPythonPath()
py <<EOF
# load PYTHONPATH into vim, this lets you hover over a module name
# and type 'gf' (for goto file) and open that file in vim. Useful
# and easier than rope for simple tasks
for p in sys.path:
if os.path.isdir(p):
vim.command(r"set path-=%s" % (p.replace(" ", r"\ ")))
EOF
let g:PythonPathLoaded = 0
endfunction
Hope this helps! Plus, an added bonus is that this will load your packages regardless of whether you are using virtualenv (since it, I believe, runs whatever is set as 'python' at the moment).

compile python .py file without executing

Is there a way to compile a Python .py file from the command-line without executing it?
I am working with an application that stores its python extensions in a non-standard path with limited permissions and I'd like to compile the files during installation. I don't need the overhead of Distutils.

The py_compile module provides a function to generate a byte-code file from a source file, and another function used when the module source file is invoked as a script.
python -m py_compile fileA.py fileB.py fileC.py

Yes, there is module compileall. Here's an example that compiles all the .py files in a directory (but not sub-directories):
python -m compileall -l myDirectory

In fact if you're on Linux you may already have a /usr/bin/py_compilefiles command in your PATH. It wraps the the py_compile module mentioned by other people. If you're not on Linux, here's the script code.

$ python -c "import py_compile; py_compile.compile('yourfile.py')"
or
$ python -c "import py_compile; py_compile.compileall('dir')"

In addition to choose the output location of pyc (by #Jensen Taylor's answer),
you can also specify a source file name you like for traceback if you don't want the absolute path of py file to be written in the pyc:
python -c "import py_compile; py_compile.compile('src.py', 'dest.pyc', 'whatever_you_like')"
Though "compileall -d destdir" can do the trick too, it will limit your working directory sometimes.
For example, if you want source file name in pyc to be "./src.py", you have to move working directory
to the folder of src.py, which is undesirable in some cases, then run something like "python -m compileall -d ./ ."

I would say something like this, so you can compile it to your chosen location:
import py_compile
py_compile(filename+".py",wantedlocation+wantedname+".pyc")
As I have now done in my Python project on github.com/lolexorg/Lolex-Tools

How do I find the location of my Python site-packages directory?

How do I find the location of my site-packages directory?

There are two types of site-packages directories, global and per user.
Global site-packages ("dist-packages") directories are listed in sys.path when you run:
python -m site
For a more concise list run getsitepackages from the site module in Python code:
python -c 'import site; print(site.getsitepackages())'
Caution: In virtual environments getsitepackages is not available with older versions of virtualenv, sys.path from above will list the virtualenv's site-packages directory correctly, though. In Python 3, you may use the sysconfig module instead:
python3 -c 'import sysconfig; print(sysconfig.get_paths()["purelib"])'
The per user site-packages directory (PEP 370) is where Python installs your local packages:
python -m site --user-site
If this points to a non-existing directory check the exit status of Python and see python -m site --help for explanations.
Hint: Running pip list --user or pip freeze --user gives you a list of all installed per user site-packages.
Practical Tips
<package>.__path__ lets you identify the location(s) of a specific package: (details)
$ python -c "import setuptools as _; print(_.__path__)"
['/usr/lib/python2.7/dist-packages/setuptools']
<module>.__file__ lets you identify the location of a specific module: (difference)
$ python3 -c "import os as _; print(_.__file__)"
/usr/lib/python3.6/os.py
Run pip show <package> to show Debian-style package information:
$ pip show pytest
Name: pytest
Version: 3.8.2
Summary: pytest: simple powerful testing with Python
Home-page: https://docs.pytest.org/en/latest/
Author: Holger Krekel, Bruno Oliveira, Ronny Pfannschmidt, Floris Bruynooghe, Brianna Laugher, Florian Bruhin and others
Author-email: None
License: MIT license
Location: /home/peter/.local/lib/python3.4/site-packages
Requires: more-itertools, atomicwrites, setuptools, attrs, pathlib2, six, py, pluggy

>>> import site; site.getsitepackages()
['/usr/local/lib/python2.7/dist-packages', '/usr/lib/python2.7/dist-packages']
(or just first item with site.getsitepackages()[0])

A solution that:
outside of virtualenv - provides the path of global site-packages,
insidue a virtualenv - provides the virtualenv's site-packages
...is this one-liner:
python -c "from distutils.sysconfig import get_python_lib; print(get_python_lib())"
Formatted for readability (rather than use as a one-liner), that looks like the following:
from distutils.sysconfig import get_python_lib
print(get_python_lib())
Source: an very old version of "How to Install Django" documentation (though this is useful to more than just Django installation)

For Ubuntu,
python -c "from distutils.sysconfig import get_python_lib; print get_python_lib()"
...is not correct.
It will point you to /usr/lib/pythonX.X/dist-packages
This folder only contains packages your operating system has automatically installed for programs to run.
On ubuntu, the site-packages folder that contains packages installed via setup_tools\easy_install\pip will be in /usr/local/lib/pythonX.X/dist-packages
The second folder is probably the more useful one if the use case is related to installation or reading source code.
If you do not use Ubuntu, you are probably safe copy-pasting the first code box into the terminal.

This is what worked for me:
python -m site --user-site

A modern stdlib way is using sysconfig module, available in version 2.7 and 3.2+. Unlike the current accepted answer, this method still works regardless of whether or not you have a virtual environment active.
Note: sysconfig (source) is not to be confused with the distutils.sysconfig submodule (source) mentioned in several other answers here. The latter is an entirely different module and it's lacking the get_paths function discussed below. Additionally, distutils is deprecated in 3.10 and will be unavailable soon.
Python currently uses eight paths (docs):
stdlib: directory containing the standard Python library files that are not platform-specific.
platstdlib: directory containing the standard Python library files that are platform-specific.
platlib: directory for site-specific, platform-specific files.
purelib: directory for site-specific, non-platform-specific files.
include: directory for non-platform-specific header files.
platinclude: directory for platform-specific header files.
scripts: directory for script files.
data: directory for data files.
In most cases, users finding this question would be interested in the 'purelib' path (in some cases, you might be interested in 'platlib' too). The purelib path is where ordinary Python packages will be installed by tools like pip.
At system level, you'll see something like this:
# Linux
$ python3 -c "import sysconfig; print(sysconfig.get_path('purelib'))"
/usr/local/lib/python3.8/site-packages
# macOS (brew installed python3.8)
$ python3 -c "import sysconfig; print(sysconfig.get_path('purelib'))"
/usr/local/Cellar/python#3.8/3.8.3/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages
# Windows
C:\> py -c "import sysconfig; print(sysconfig.get_path('purelib'))"
C:\Users\wim\AppData\Local\Programs\Python\Python38\Lib\site-packages
With a venv, you'll get something like this
# Linux
/tmp/.venv/lib/python3.8/site-packages
# macOS
/private/tmp/.venv/lib/python3.8/site-packages
# Windows
C:\Users\wim\AppData\Local\Temp\.venv\Lib\site-packages
The function sysconfig.get_paths() returns a dict of all of the relevant installation paths, example on Linux:
>>> import sysconfig
>>> sysconfig.get_paths()
{'stdlib': '/usr/local/lib/python3.8',
'platstdlib': '/usr/local/lib/python3.8',
'purelib': '/usr/local/lib/python3.8/site-packages',
'platlib': '/usr/local/lib/python3.8/site-packages',
'include': '/usr/local/include/python3.8',
'platinclude': '/usr/local/include/python3.8',
'scripts': '/usr/local/bin',
'data': '/usr/local'}
A shell script is also available to display these details, which you can invoke by executing sysconfig as a module:
python -m sysconfig
Addendum: What about Debian / Ubuntu?
As some commenters point out, the sysconfig results for Debian systems (and Ubuntu, as a derivative) are not accurate. When a user pip installs a package it will go into dist-packages not site-packages, as per Debian policies on Python packaging.
The root cause of the discrepancy is because Debian patch the distutils install layout, to correctly reflect their changes to the site, but they fail to patch the sysconfig module.
For example, on Ubuntu 20.04.4 LTS (Focal Fossa):
root#cb5e85f17c7f:/# python3 -m sysconfig | grep packages
platlib = "/usr/lib/python3.8/site-packages"
purelib = "/usr/lib/python3.8/site-packages"
root#cb5e85f17c7f:/# python3 -m site | grep packages
'/usr/local/lib/python3.8/dist-packages',
'/usr/lib/python3/dist-packages',
USER_SITE: '/root/.local/lib/python3.8/site-packages' (doesn't exist)
It looks like the patched Python installation that Debian/Ubuntu are distributing is a bit hacked up, and they will need to figure out a new plan for 3.12+ when distutils is completely unavailable. Probably, they will have to start patching sysconfig as well, since this is what pip will be using for install locations.

Let's say you have installed the package 'django'. import it and type in dir(django). It will show you, all the functions and attributes with that module. Type in the python interpreter -
>>> import django
>>> dir(django)
['VERSION', '__builtins__', '__doc__', '__file__', '__name__', '__package__', '__path__', 'get_version']
>>> print django.__path__
['/Library/Python/2.6/site-packages/django']
You can do the same thing if you have installed mercurial.
This is for Snow Leopard. But I think it should work in general as well.

As others have noted, distutils.sysconfig has the relevant settings:
import distutils.sysconfig
print distutils.sysconfig.get_python_lib()
...though the default site.py does something a bit more crude, paraphrased below:
import sys, os
print os.sep.join([sys.prefix, 'lib', 'python' + sys.version[:3], 'site-packages'])
(it also adds ${sys.prefix}/lib/site-python and adds both paths for sys.exec_prefix as well, should that constant be different).
That said, what's the context? You shouldn't be messing with your site-packages directly; setuptools/distutils will work for installation, and your program may be running in a virtualenv where your pythonpath is completely user-local, so it shouldn't assume use of the system site-packages directly either.

The native system packages installed with python installation in Debian based systems can be found at :
/usr/lib/python2.7/dist-packages/
In OSX - /Library/Python/2.7/site-packages
by using this small code :
from distutils.sysconfig import get_python_lib
print get_python_lib()
However, the list of packages installed via pip can be found at :
/usr/local/bin/
Or one can simply write the following command to list all paths where python packages are.
>>> import site; site.getsitepackages()
['/usr/local/lib/python2.7/dist-packages', '/usr/lib/python2.7/dist-packages']
Note: the location might vary based on your OS, like in OSX
>>> import site; site.getsitepackages()
['/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages', '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/site-python', '/Library/Python/2.7/site-packages']

pip show will give all the details about a package:
https://pip.pypa.io/en/stable/reference/pip_show/ [pip show][1]
To get the location:
pip show <package_name>| grep Location
In Linux, you can go to site-packages folder by:
cd $(python -c "import site; print(site.getsitepackages()[0])")

All the answers (or: the same answer repeated over and over) are inadequate. What you want to do is this:
from setuptools.command.easy_install import easy_install
class easy_install_default(easy_install):
""" class easy_install had problems with the fist parameter not being
an instance of Distribution, even though it was. This is due to
some import-related mess.
"""
def __init__(self):
from distutils.dist import Distribution
dist = Distribution()
self.distribution = dist
self.initialize_options()
self._dry_run = None
self.verbose = dist.verbose
self.force = None
self.help = 0
self.finalized = 0
e = easy_install_default()
import distutils.errors
try:
e.finalize_options()
except distutils.errors.DistutilsError:
pass
print e.install_dir
The final line shows you the installation dir. Works on Ubuntu, whereas the above ones don't. Don't ask me about windows or other dists, but since it's the exact same dir that easy_install uses by default, it's probably correct everywhere where easy_install works (so, everywhere, even macs). Have fun. Note: original code has many swearwords in it.

An additional note to the get_python_lib function mentioned already: on some platforms different directories are used for platform specific modules (eg: modules that require compilation). If you pass plat_specific=True to the function you get the site packages for platform specific packages.

This works for me.
It will get you both dist-packages and site-packages folders.
If the folder is not on Python's path, it won't be
doing you much good anyway.
import sys;
print [f for f in sys.path if f.endswith('packages')]
Output (Ubuntu installation):
['/home/username/.local/lib/python2.7/site-packages',
'/usr/local/lib/python2.7/dist-packages',
'/usr/lib/python2.7/dist-packages']

This should work on all distributions in and out of virtual environment due to it's "low-tech" nature. The os module always resides in the parent directory of 'site-packages'
import os; print(os.path.dirname(os.__file__) + '/site-packages')
To change dir to the site-packages dir I use the following alias (on *nix systems):
alias cdsp='cd $(python -c "import os; print(os.path.dirname(os.__file__))"); cd site-packages'

A side-note: The proposed solution (distutils.sysconfig.get_python_lib()) does not work when there is more than one site-packages directory (as recommended by this article). It will only return the main site-packages directory.
Alas, I have no better solution either. Python doesn't seem to keep track of site-packages directories, just the packages within them.

from distutils.sysconfig import get_python_lib
print get_python_lib()

You should try this command to determine pip's install location
Python 2
pip show six | grep "Location:" | cut -d " " -f2
Python 3
pip3 show six | grep "Location:" | cut -d " " -f2

Answer to old question. But use ipython for this.
pip install ipython
ipython
import imaplib
imaplib?
This will give the following output about imaplib package -
Type: module
String form: <module 'imaplib' from '/usr/lib/python2.7/imaplib.py'>
File: /usr/lib/python2.7/imaplib.py
Docstring:
IMAP4 client.
Based on RFC 2060.
Public class: IMAP4
Public variable: Debug
Public functions: Internaldate2tuple
Int2AP
ParseFlags
Time2Internaldate

For those who are using poetry, you can find your virtual environment path with poetry debug:
$ poetry debug
Poetry
Version: 1.1.4
Python: 3.8.2
Virtualenv
Python: 3.8.2
Implementation: CPython
Path: /Users/cglacet/.pyenv/versions/3.8.2/envs/my-virtualenv
Valid: True
System
Platform: darwin
OS: posix
Python: /Users/cglacet/.pyenv/versions/3.8.2
Using this information you can list site packages:
ls /Users/cglacet/.pyenv/versions/3.8.2/envs/my-virtualenv/lib/python3.8/site-packages/

I made a really simple function that gets the job done
import site
def get_site_packages_dir():
return [p for p in site.getsitepackages()
if p.endswith(("site-packages", "dist-packages"))][0]
get_site_packages_dir()
# '/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages'
If you want to retrieve the results using the terminal:
python3 -c "import site;print([p for p in site.getsitepackages() if p.endswith(('site-packages', 'dist-packages')) ][0])"
/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages

I had to do something slightly different for a project I was working on: find the relative site-packages directory relative to the base install prefix. If the site-packages folder was in /usr/lib/python2.7/site-packages, I wanted the /lib/python2.7/site-packages part. I have, in fact, encountered systems where site-packages was in /usr/lib64, and the accepted answer did NOT work on those systems.
Similar to cheater's answer, my solution peeks deep into the guts of Distutils, to find the path that actually gets passed around inside setup.py. It was such a pain to figure out that I don't want anyone to ever have to figure this out again.
import sys
import os
from distutils.command.install import INSTALL_SCHEMES
if os.name == 'nt':
scheme_key = 'nt'
else:
scheme_key = 'unix_prefix'
print(INSTALL_SCHEMES[scheme_key]['purelib'].replace('$py_version_short', (str.split(sys.version))[0][0:3]).replace('$base', ''))
That should print something like /Lib/site-packages or /lib/python3.6/site-packages.

Something that has not been mentioned which I believe is useful, if you have two versions of Python installed e.g. both 3.8 and 3.5 there might be two folders called site-packages on your machine. In that case you can specify the python version by using the following:
py -3.5 -c "import site; print(site.getsitepackages()[1])