why pip reinstalls standard packages listed in install_requires

Question

There is a python package that I need to install using pip install SomePackage.tar.gz. In the setup.py of this package, a few libraries that are listed under install_requires have become part of Python 2.7 Standard Library (like argparse).

The problem is that when I install the package on Python 2.7, pip does not realize that, for example, argparse is already included in the standard library, and reinstalls it under site-packages.

Is there a way to have pip download and install packages listed under install_requires only if they are not included in the standard library?

Please note that changing setup.py is not an option, as the package might be installed under Python 2.6 as well.

Answer 1

You say changing setup.py is not an option because it might be used under Python 2.6. Change it anyway, with a conditional:

import sys
from setuptools import setup

install_requires = [
    # ...general modules...
]

if sys.hexversion < 0x02070000:
    install_requires.append('argparse')

setup(
    # ...
    install_requires=install_requires,
)