UPDATED - Homebrew version after 1.5

According to the official Homebrew page:

On 1st March 2018 the python formula will be upgraded to Python 3.x and a python@2 formula will be added for installing Python 2.7 (although this will be keg-only so neither python nor python2 will be added to the PATH by default without a manual brew link --force). We will maintain python2, python3 and python@3 aliases.

So to install Python 3, run the following command:

brew install python3

Then, the pip or pip3 is installed automatically, and you can install any package by pip install <package>.

The older version of Homebrew

Not only brew install python3 but also brew postinstall python3

So you must run:

brew install python3
brew postinstall python3

Note that you should check the console, as it might get you errors and in that case, the pip3 is not installed.

TLDR. On any modern Mac python3 -m ensurepip

then pip3 --version to check.

pip's documentation lists the supported mechanisms to install it: https://pip.pypa.io/en/stable/installation/#supported-methods

It is generally recommended to avoid installing pip on the OS-provided python commands, and to install Python via the official installers or using something like Homebrew or pyenv.

Python 3.4+ will have ensurepip, so if you're unable to run python3 -m pip -- run python3 -m ensurepip and it'll install pip for you.

If you're using an end-of-life version of Python, you can use get-pip.py instead.

Old answer (outdated, and results in a broken installation)

easy_install pip

If you need admin privileges to run this, try:

sudo easy_install pip

The officially recommended python packaging tool is pipenv. One example of a workflow you could use to make a virtual environment with the exact libraries your project needs as well as ensuring security is this:

$ brew install pipenv
$ cd /path/to/project
$ pipenv --three
$ pipenv install opencv-python

And after you write your code in, say, project.py

$ pipenv run python3 project.py

More info on the pipenv site.

You could have multiple Python versions on your macOS.

You may check that by command, type or which command, like:

which -a python python2 python2.7 python3 python3.6

Or type python in Terminal and hit Tab few times for auto completion, which is equivalent to:

compgen -c python

By default python/pip commands points to the first binary found in PATH environment variable depending what's actually installed. So before installing Python packages with Homebrew, the default Python is installed in /usr/bin which is shipped with your macOS (e.g. Python 2.7.10 on High Sierra). Any versions found in /usr/local (such as /usr/local/bin) are provided by external packages.

It is generally advised, that when working with multiple versions, for Python 2 you may use python2/pip2 command, respectively for Python 3 you can use python3/pip3, but it depends on your configuration which commands are available.

It is also worth to mention, that since release of Homebrew 1.5.0+ (on 19 January 2018), the python formula has been upgraded to Python 3.x and a python@2 formula will be added for installing Python 2.7. Before, python formula was pointing to Python 2.

For instance, if you've installed different version via Homebrew, try the following command:

brew list python python3

or:

brew list | grep ^python

it'll show you all Python files installed with the package.

Alternatively you may use apropos or locate python command to locate more Python related files.

To check any environment variables related to Python, run:

env | grep ^PYTHON

To address your issues:

• Error: No such keg: /usr/local/Cellar/python

  Means you don't have Python installed via Homebrew. However double check by specifying only one package at a time (like brew list python python2 python3).

• The locate database (/var/db/locate.database) does not exist.

  Follow the advice and run:

  sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.locate.plist
  

  After the database is rebuild, you can use locate command.