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macOS Big Sur; Command Line Tools and Python 3.8.2

I am experiencing difficulty with installing Python correctly on my Mac (Fresh Install, I wiped my PC clean). I installed firstly
Home Brew
Xcode through App Store
Command line tools using command: "xcode-select --install"
On typing the following command:
which python3
Output is as follows:
gaurangsmacbookpro#Gaurangs-New-MacBook-Pro ~ % which python
/usr/bin/python
gaurangsmacbookpro#Gaurangs-New-MacBook-Pro ~ % which python3
/usr/bin/python3
upon running the following command which I was following off a blog from this site:
https://www.pyimagesearch.com/2016/12/05/macos-install-opencv-3-and-python-3-5/
brew install eigen tbb
Home brew installed python 3.9 dependencies so no when I type the following commands:
brew list python3
I get the following output:
gaurangsmacbookpro#Gaurangs-New-MacBook-Pro ~ % brew list python3
/usr/local/Cellar/python#3.9/3.9.1_8/bin/2to3
/usr/local/Cellar/python#3.9/3.9.1_8/bin/2to3-3.9
/usr/local/Cellar/python#3.9/3.9.1_8/bin/easy_install-3.9
/usr/local/Cellar/python#3.9/3.9.1_8/bin/idle3
/usr/local/Cellar/python#3.9/3.9.1_8/bin/idle3.9
/usr/local/Cellar/python#3.9/3.9.1_8/bin/pip3
/usr/local/Cellar/python#3.9/3.9.1_8/bin/pip3.9
/usr/local/Cellar/python#3.9/3.9.1_8/bin/pydoc3
/usr/local/Cellar/python#3.9/3.9.1_8/bin/pydoc3.9
/usr/local/Cellar/python#3.9/3.9.1_8/bin/python3
/usr/local/Cellar/python#3.9/3.9.1_8/bin/python3-config
/usr/local/Cellar/python#3.9/3.9.1_8/bin/python3.9
/usr/local/Cellar/python#3.9/3.9.1_8/bin/python3.9-config
/usr/local/Cellar/python#3.9/3.9.1_8/bin/wheel3
/usr/local/Cellar/python#3.9/3.9.1_8/Frameworks/Python.framework/ (3019 files)
/usr/local/Cellar/python#3.9/3.9.1_8/IDLE 3.app/Contents/ (8 files)
/usr/local/Cellar/python#3.9/3.9.1_8/lib/pkgconfig/ (4 files)
/usr/local/Cellar/python#3.9/3.9.1_8/libexec/bin/ (7 files)
/usr/local/Cellar/python#3.9/3.9.1_8/libexec/pip/ (480 files)
/usr/local/Cellar/python#3.9/3.9.1_8/libexec/setuptools/ (334 files)
/usr/local/Cellar/python#3.9/3.9.1_8/libexec/wheel/ (44 files)
/usr/local/Cellar/python#3.9/3.9.1_8/Python Launcher 3.app/Contents/ (16 files)
/usr/local/Cellar/python#3.9/3.9.1_8/share/man/ (2 files)
The contents of my ~./bash_profile file are as follows:
# Add Homebrew's executable directory to the front of the PATH
export PATH=/usr/local/bin:$PATH
export PATH="/usr/local/sbin:$PATH"
export PATH="/usr/local/opt/python#3.8/bin:$PATH"
What am I doing wrong? I just want a fresh clean install of Python 3.8 not 3.9 as there is no support for TF Lite on 3.9 yet. Can someone help me fix this mess? I will be very grateful!

I'm a bit late at the game, but I hope this will be useful to somebody. The best solution here is to use a virtual environment, but for the sake of argument and because it is a little bit more involved, I won't discuss that as an option here and will assume you just want a specific version installed on your machine where you can just add all the dependencies you need.
From looking at the page OP was using, the easiest solution here is probably to use a distribution like Anaconda if you are particularly interested in OpenCV or that type of libraries. Then it's just a case of setting up your path like export PATH=$HOME/anaconda/bin:$PATH or whatever path you used to install Anaconda with your shell of choice.
But let's imagine that Anaconda doesn't help you and you still need a specific version of Python, and you figure 3.8.2 is good enough for your needs and a newer version wouldn't work. Using the Python version that comes with the Xcode command line tools is probably a bad idea, just like using the system installed Python is. That version is really meant to support the Apple toolchain and while I've never run into any issues with it, I would not recommend it.
Next option is is to use homebrew. Great option since most people will already be using it, and you might already have python installed as part of a dependency but maybe like OP it's not the version you were hoping for. You can still make it work by installing an older version like so brew install python#3.8 however, it comes with a few caveats. Whenever one of the python dependencies is updated, you might need to unlink and relink it so that you can continue to use the correct version. Believe me, it's no way to live.
What I've found works (in combination with virtual environments is to use pyenv. Install it through homebrew, set your shell as described in the install doc and use it to install the python version you want (very easy) and then set the global interpreter to be whatever version you want and your system will stick to it until you set it to something else. It also works great in combination with tools like pipenv which allow you to manage virtual environments.

Install python 2.6 on Mac

I have to use an API which uses Python 2.6 and am having a very difficult time getting it installed on my mac. I have downloaded it but figuring out what I am supposed to write in my bash_profile is exceedingly difficult. For all versions of Python that I have used so far I write
PATH="/Library/Frameworks/Python.framework/Versions/3.7/bin:${PATH}"
export Path
in my bash profile. But I can't do that for python 2.6 because it does not have a bin folder. Does anyone know what I'm suppoed to write on my bash profile in order to get Python to use version 2.6?
I'm aware that one solution is pyenv but I tried that a long time ago and there were too many modules that I could not install with it so I don't like that program.
I'm also hoping brew will work but according to this page
https://formulae.brew.sh/formula/
python 2.6 is no longer available for brew install. And when I run brew install python#2.6 no formulae are found.
My current strategy is to try macports so we'll see how that goes.

'python' points to anaconda installation

I've installed anaconda, but I still want to use python as normal within a Terminal window (zsh). At the moment, running which python shows /Users/USERNAME/opt/anaconda3/bin/python.
I don't want this command to point to my anaconda installation. I want to use python as I have in the past, and for it to point to my python 2.7 installation (/usr/bin/python).
The same issue is present with pip and with python3. How do I change these paths back to their default locations?

I want to start out by saying I personally have never used Anaconda, but I have many friends who have and they always seem to wind up getting into hot water as far as configurations go. If you don't need the data-science specific tools that Anaconda offers (most of which I imagine you can get just as easily by using pip install <package_name>), then I STRONGLY urge you to not use Anaconda. I would suggest you instead use homebrew to install python 3.7 and leave the system version of python 2.7 alone... Anyways...
I recently fixed an issue similar to the one you're describing for a friend. I ended up deleted his anacaonda build and reinstalling fresh from homebrew (link: https://brew.sh/), so this is how I helped him and hopefully this will help you too.
Firstly, I would check the configuration of your ~/.bash_profile (or potentially ~/.bashrc) file. Comment out any lines you find that have to do with Anaconda by preceding with the # character. I believe when anaconda installs it puts the location of the anaconda-version-of-python higher in the PATH hierarchy, therefore you computer sees it first before it has the chance to look in /usr/bin...
Secondly, you need to move the anaconda-python to the trash... I would start by trashing the whole '/Users/USERNAME/opt/anaconda3' directory. If memory serves me correctly anaconda stores other python related things in the /MacitoshHD/Library/Frameworks directory, but don't quote me on that. I would advise looking there and trashing whatever seems to be related to the anaconda install.
After that, in a new terminal (so that your PATH refreshes), I installed homebrew, and then ran the command brew install python3 to install python3. I think perhaps I ran the command recommended in the installer output, something like "brew link python3" to symlink python3 where it needs to be linked after the installation.
Finally, everything is done python2 and python3 are installed and working with no issues. python2 is the system default version and python3 is the homebrew installed version. Feel free to ask any questions and I will try to help more, hopefully those instructions will get you started though.

Installing MatplotLib in mac osx lion

I was trying to install matplotlib in Mac OSX Lion. Tried to used the binary that is in the sourcefourge site, but I got this error: "matplotlib requires System Python 2.7 to install".
I went to the Terminal and typed python --version and its check the requirements.
After this I tried to used the pip, and while doing so é got the following error:
"src/_image.cpp:908: error: invalid use of incomplete type ‘struct png_info_def’"
Can you give me clues in how to install it?
Many thanks.
EDIT: I found a way, without having to install a new version of python:
http://the.taoofmac.com/space/blog/2011/07/24/2222

This is because of a change in the API for libpng v1.5 (which is included with Mac OSX Lion). The changes has already been pushed upstream, so you will need to compile from the upstream repository until the next release.
You can follow the instructions on http://jholewinski.wordpress.com/2011/07/21/installing-matplotlib-on-os-x-10-7-with-homebrew/

I followed this page's instructions. I got stuck at
pip install -e git+https://github.com/matplotlib/matplotlib#egg=matplotlib-dev
Then I did:
git clone https://github.com/matplotlib/matplotlib.git
cd matplotlib
python setup.py build
python setup.py install
Checked my installation by typing in terminal:
python
import matplotlib
print matplotlib.__version__
print matplotlib.__file__
I got version 0.10.0 dev (as of this writing) and path /usr/local/Cellar/...

You can try with an "official" python distribution, apple might have tweaked the supplied one. You can find the 2.7 here: http://www.python.org/download/
You might have to re-install numpy as well afterwords: http://sourceforge.net/projects/numpy/files/NumPy/1.6.1/
I would suggest to install scipy as well
Let me know if it works ;)

I would recommend using macports, it should take care of dependencies and would be indepedent of the system python version.
EDIT: Just a few clarifications, taking into account comments to this answer.
Why use macports (or another installer)? Because they take care of dependencies, provide functionality to uninstall and switch between versions (I've used the latter successfully for gcc and python). And because the default installation location is not the system executable location. Overriding the system python can break applications that rely on it (this is certainly true in many Linux distributions, maybe less so on mac OS X).
When is it particularly useful? When you want to install on top of a version of python that is different to system python, and when you have non-python extensions (C, C++, Fortran...).
What's the down side? As #Trond has mentioned in the comments, it is good if you're OK with default compilations of packages. You don't have a handle over configuration or compiler flags as you would if you built from source.
A good alternative to macports is fink, which seems to give you more control (build from source). I am not sure it is completely ready for mac OS X Lion yet.

I know it will take a lot more time, but I would reccomend installing python, numpy, and matplotlib from source to ensure that you have consistency in your system. From the error it could look like you have a problem with libPNG support. I have a written a "How to install python dev tools on Mac OSX" that you may find useful. It contains directions for how to install python, numpy, matplotlib and all necessary dependencies. I understand you are working with Python 2.7 and the directions I am listing is for Python 2.6.5, but it would be the same approach. Hope you get it to work.

You could just install the Python 2.6 version of Matplotlib, assuming that Python2.6 is included with Lion (py2.5 & 2.6 are included with Mac OS 10.6 "Snow Leopard" - try typing python2.6 to find out if that version is installed.).
On MacPorts you do this via something like:
sudo port install py26-matplotlib
where py26-... (or py25-..., py27-... etc.) indicates which python version it is meant for. (check the Pallet/ports list to get the correct package name, in case I guessed wrong)

How do I upgrade python 2.5.2 to python 2.6rc2 on ubuntu linux 8.04?

I'd like to how to upgrade the default python installation(2.5.2) supplied with ubuntu 8.04 to python 2.6rc2. I'd like to make 2.6 the default python version on the system and migrate all the other useful installed python libraries installed on 2.5.2 to python 2.6rc2. Please let me know how I can achieve this.
Thanks
Dirk

With the warning that I think it's a tremendously bad idea to replace the default Python with an unreleased beta version:
First, install 2.6rc2. You can download the source from the Python website. Standard ./configure && make && sudo make install installation style.
Next, remove the /usr/bin/python symlink. Do not remove /usr/bin/python2.5. Add a symlink to 2.6 with ln -s /usr/local/bin/python2.6 /usr/bin/python.
Once again, I think this is a terrible idea. There is almost certainly a better way to do whatever you're trying to accomplish.
Migrating installed libraries is a much longer process. Look in the /usr/lib/python2.5/site-packages/ and /usr/local/lib/python2.5/site-packages/ directories. Any libraries installed to them will need to be re-installed with 2.6. Since you're not using a packaged Python version, you cannot use Ubuntu's packages -- you'll have to manually upgrade all the libraries yourself. Most of them can probably be installed with sudo easy_install <name>, but some like PyGTK+ are not so easy. You'll have to follow custom installation procedures for each such library.

I have the same issue, and apparently pre-built binaries can be found here:
# Python 2.6
deb http://ppa.launchpad.net/doko/ubuntu intrepid main
deb-src http://ppa.launchpad.net/doko/ubuntu intrepid main

Is there any need to?
Ubuntu in general doesn't package RC releases. 2.6 will not be available in Ubuntu until Jaunty Jackalope.
However,, if you insist that you need to install it, then, you'll have to do so without a package manager.
Download the package, and unzip it to a directory
run the following commands (waiting for each to finish as you do so)
./configure
make
sudo make install
There, you have it installed.
It's better to wait for it to be packaged first, espescially as Python is used in a lot of ubuntu internals, so may break your system horribly

It would not be wise to change the default version of Python, i.e. what you get when you type "python" into a shell. However, you can have multiple versions of python installed. The trick is to make sure that the program named "python" on the path is the system supplied version. If you want to run your install of Python 2.6 you'd then type python2.6 into a shell to start it.
Download the package and unzip it, then run:
./configure
make
sudo make install
ls -l /usr/local/bin
You should see a python and a python2.6 file, both created on the day you ran make install; delete the python file. Then when python is launched the standard system Python version from /usr/bin will be run, and when python2.6 is run you get your shiny new python 2.6rc2. Python displays the version when it starts an interactive interpreter.