Here the answer that worked, which is basically what has been explained in the comments of the question. However, I thought it would be useful to have it explained as a clear and well structured answer.

As highlighted, the problem was that I was not using the interpreter that pip was installing for. The command which shows where pip was installing the modules:

$ which -a pip
/usr/local/bin/pip

and where the different python versions were located:

$ which -a python
/usr/bin/python
/usr/local/bin/python

That is, my system/default python was

/usr/bin/python

while pip was installing for

/usr/local/bin/python

Therefore, I could not import anything I installed when I just typed python, because the /usr/bin/python interpreter was the one started.

Solution

Install pip again specifying the destination of the modules that will be installed. This must be the destination for the system/default python.

This has been done in two steps:

1. Downloding get-pip.py from bootstrap.pypa.io/get-pip.py. (You may need to use the deprecated one for Python 2: bootstrap.pypa.io/2.7/get-pip.py)

2. Installing it with the following command

   sudo /usr/bin/python get-pip.py

Note that without the sudo I got an error and was not able to install pip.

It sounds like you've installed a Homebrew Python 2.7, you don't actually need it, and you'd rather not deal with having two conflicting copies of Python 2.7 lying around. In that case, here's what you should do:

• pip --list > packages.txt. This gets you a list of all the packages you've installed.
• brew unlink python.
• sudo easy_install pip && sudo pip install --upgrade setuptools.
• Open packages.txt in a text editor.
• sudo pip install [all the packages you want from packages.txt].
  • Or just pip install -r packages.txt if you want all of them.

You may want to consider setting up virtualenv, or a user site-packages directory, so you don't have to keep sudoing everything. Especially if you're used to Homebrew.

Using either brew unlink or brew uninstall will get the extra Python 2.7 out of the way, so only the pre-installed Apple version is accessible, solving your problem. But with brew unlink, it doesn't actually delete anything, so if you change your mind, you can get it back with brew link python. (Or, if you're later sure you never want to go back, you can always brew uninstall python later.)

Also, in any other situation this is the wrong way to get pip, but for the Python versions Apple has distributed with 10.7 through 10.9, it seems to work best. That may change with later versions of setuptools.

Finally, one last pitch: Do you actually need Python 2.7 instead of 3.x? There are all kinds of reasons the answer may be "yes", but if not, you can install a Homebrew 3.4 alongside the Apple 2.7 and they won't conflict at all; you'll get separate programs named python3, python3-config, pip3, ipython3, etc.

You probably installed it with sudo, so it's not accessible when you run the script without sudo. Anyhow, it's really messy to install modules directly. And it's dangerous to install modules to systems python.

Install a brew version of python and pip. then create a virtualenv and install your packages in it. It's much safer and portable.

Read this answer No module named 'pafy'

Please don't run the script as sudo to make it work, especially if you are not the one who wrote it. Use a virtualenv instead.

The first thing you'll notice when viewing the Installation instructions is a large warning:

            Pillow and PIL cannot co-exist in the same environment. Before installing Pillow, please uninstall PIL.

You appear to have PIL installed, so uninstall it first, then see if that solves the problem.

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> or pip3 install <package>for python 3 packages.

So if you are running older python 2 script, install required packages and run as below :

pip install <package>

python <script.py>

If you wish to run python 3 script, install required packages and run as below :

pip3 install <package>

python3 <script.py>

All relevant packages will get installed as per the required environment and help run the script.

This could happen if you have multiple versions of Python installed on your computer. For example you have a package installed on Python 3.5 but you run your script on Python 3.8. On cmd or Powershell you could try something like py -3.8 -m pip list or py -3.5 -m pip list to check which libraries you have installed on each version of Python. Then if the library is missing from the version that you used on your script you can install it specifically for this version using something like py -3.5 -m pip install library_name.

Use pip show -f cookiecutter to show the location of everything it installed (which I found from this answer on how to list the files installed by pip

When I ran this, it shows that it's installed in ../../../../usr/local/bin/cookiecutter. Looking at the Location information above it (/Library/Python/2.7/site-packages in my case), I can see that the string of .. are used to get to the root directory.

On my Mac, it installed it in /usr/local/bin. So I can access it by updating my path environmental:

export PATH=$PATH:/usr/local/bin