Check if my Python has all required packages

Question

I have a requirements.txt file with a list of packages that are required for my virtual environment. Is it possible to find out whether all the packages mention

Answer 1

UPDATE:

An up-to-date and improved way to do this is via distutils.text_file.TextFile. See Acumenus' answer below for details.

ORIGINAL:

The pythonic way of doing it is via the pkg_resources API. The requirements are written in a format understood by setuptools. E.g:

Werkzeug>=0.6.1
Flask
Django>=1.3

The example code:

import pkg_resources
from pkg_resources import DistributionNotFound, VersionConflict

# dependencies can be any iterable with strings, 
# e.g. file line-by-line iterator
dependencies = [
  'Werkzeug>=0.6.1',
  'Flask>=0.9',
]

# here, if a dependency is not met, a DistributionNotFound or VersionConflict
# exception is thrown. 
pkg_resources.require(dependencies)