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For this blog post, I am going to use python 3.6. Download it from its official site and install it. Given below are the steps you need to follow to build OpenCV on macOS Catalina.
On my system, I had installed Python various times from DMG installers on python.org. When I switched to Homebrew Python, I wanted to get rid of all the extra Python installs. This is how I did it. Remove all the symbolic links in /usr/local/bin that point to Python related files: $ sudo rm /usr/local/bin/python. $ sudo rm /usr/local/bin/pip.

Note - Python 3.9 will not work at this time (2 November 2020) because not all libraries were ported to 3.9, please use Python 3.8 for now.How to Install Pyt. Maria Campbell provides a post on installing the latest version of Python on Mac OS Catalina and overriding the old default pre-installed version. And it even uses Homebrew and not some gnarly steps. I finally did it. I successfully installed Python version 3.7.7 via Homebrew on my Mac laptop with OS Catalina installed. Install OpenCV 4 with Python 3 on macOS Catalina Posted on October 21, 2019 by Paul. Updated 11 February 2020. In this article, I will show you how to install OpenCV 4 with Python 3 on macOS Catalina.

Pip broken under Mac OS X Catalina? TLDR;

pip install --global-option=build_ext --global-option='-I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/' --global-option='-L/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/' --prefer-binary -r requirements.txt

As Apple has announced, Mac OS Catalina now runs under its own read only file system:

macOS Catalina runs on a dedicated, read-only system volume called Macintosh HD. This volume is completely separate from all other data to help prevent the accidental overwriting of critical operating system files. [1]

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Sounds great. But if you need to change that volume, or if you use software that expects to be able to find files on that read only volume that aren't there and can't be added, you're kind of stuck.

One example is Python's pip which, for some packages, will expect to find header files located in /usr/include. Apple gave developers a get-out-of-jail-free card in Mojave:

As a workaround, an extra package is provided which will install the headers to the base system. In a future release, this package will no longer be provided. [2]

and in true Apple fashion, the package was taken away. This creates problems if you want to install commonly used, but maybe not so commonly maintained, libraries. One example is Pillow, an image processing library. This will well and truly fail to compile if you just try pip install Pillow. It looks for the headers and libraries, and doesn't find them.

What can we do? Well, for pip there's a few options. The first is simply not to compile at all, but to prefer a binary. This is as simple in some cases as the --prefer-binary option. But what if, like Pillow, there is no precompiled binary for your platform? Or for reasons of information security, you have to compile it yourself?

Fortunately, pip can be directed to look in different places using the 'global-options' flag. To get Pillow to build, use the command at the top of this article, reproduced below:

pip install --global-option=build_ext --global-option='-I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/' --global-option='-L/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib/' --prefer-binary -r requirements.txt

To understand this more fully, take a look at https://stackoverflow.com/questions/18783390/python-pip-specify-a-library-directory-and-an-include-directory.

If you need to do a similar function as part of other install tools, common environment variables are LD_LIBRARY_PATH (deprecated[3], but still commonly used), LIBRARY_PATH and INCLUDE_PATH. Setting these to the library paths (/Library ... /usr/lib) and include paths (/Library ... /usr/include) respectively may help.

[1]https://support.apple.com/en-gb/HT210650
[2]https://developer.apple.com/documentation/xcode_release_notes/xcode_10_release_notes#3035624
[3]https://stackoverflow.com/questions/18241517/c-include-path-vs-ld-library-path

Posted on October 21, 2019 by Paul

Updated 11 February 2020

In this article, I will show you how to install OpenCV 4 with Python 3 on macOS Catalina.

There is also a video version of this tutorial:

MacOS comes by default with Python 2.7 which, at this point, receives only bug fixes and will be EOL by 2020. Python 3.x is the future and it is supported by all major Python libraries. In this tutorial, we’ll use the Python 3.8.

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Start by installing the Command Line Tools for macOS. Please note, that you will need the Command Line Tools even if you’ve already installed Xcode. Open a Terminal and write:

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Once the Command Line Tools are installed, we can install Python.

The official installer of Python is a pkg file that will start a GUI installer which will guide you through the installation. You can also check the video version of this tutorial if you want to see how I did it.

As a side note, you can have multiple Python 3 versions installed on your macOS machine. If this is the case, you can select which version you want to use by specifying the version number, e.g.:

or:

After the above, you can invoke Python 3.8 using the python3.8 command. python3 will also invoke the latest installer version of Python 3. This is what I see if I run python3.8 on my machine:

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Next, let’s follow best practices and create a new Python environment in which we can install NumPy and OpenCV:

At this point, your prompt should indicate that you are using the work environment. You can read more about Python environments in the documentation.

Once an environment is activated, all the install commands will apply only to the current environment. By default, if you close your Terminal, the environment is deactivated. If you want to be able to use it, use the source work/bin/activate command.

We can install NumPy and OpenCV with:

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Please note that the above will install the slightly older OpenCV 4.1, latest binary version doesn’t seem to work on macOS Catalina!

As a side note, when you are in a new environment you can use python to invoke the Python interpreter, no need to use the version number.

At this point, you should have OpenCV 4 and Python installed on your Mac. We can write a small test program that will print the OpenCV version, load an image from the disk, convert the image to gray and show the result. Start by downloading the next image:

Save it as clouds.jpg. In the same folder where you’ve saved the above image, create a new file demo.py and write the next code:

Run the code with:

(You can close the two windows by pressing ESC!)

You should see something like in the next figure. By default, the last image (the gray one) will be over the first one. You need to move the window in order to see both images:

If you want to learn more about OpenCV and Python I would recommend reading OpenCV with Python Blueprints by M. Beyeler:

or, OpenCV with Python By Example by G. Garrido and P. Joshi:

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