Pip freeze vs. pip list

To answer the second part of this question, the two packages shown in pip list but not pip freeze are setuptools (which is easy_install) and pip itself.

It looks like pip freeze just doesn't list packages that pip itself depends on. You may use the --all flag to show also those packages.

From the documentation:

--all

Do not skip these packages in the output: pip, setuptools, distribute, wheel

The main difference is that the output of pip freeze can be dumped into a requirements.txt file and used later to re-construct the "frozen" environment.

In other words you can run: pip freeze > frozen-requirements.txt on one machine and then later on a different machine or on a clean environment you can do: pip install -r frozen-requirements.txt and you'll get the an identical environment with the exact same dependencies installed as you had in the original environment where you generated the frozen-requirements.txt.

When you are using a virtualenv, you can specify a requirements.txt file to install all the dependencies.

A typical usage:

$ pip install -r requirements.txt

The packages need to be in a specific format for pip to understand, which is

feedparser==5.1.3
wsgiref==0.1.2
django==1.4.2
...

That is the "requirements format".

Here, django==1.4.2 implies install django version 1.4.2 (even though the latest is 1.6.x). If you do not specify ==1.4.2, the latest version available would be installed.

You can read more in "Virtualenv and pip Basics", and the official "Requirements File Format" documentation.