Check if my Python has all required packages

Here's a one-liner based on Zaur Nasibov's answer for if you don't care to know which packages are not installed:

python3 -c "import pkg_resources; pkg_resources.require(open('requirements.txt',mode='r'))" &>/dev/null

Whether the command finishes successfully can then be used to do a pip install.

As an equivalent to Ruby's bundle check || bundle install, we're doing:

python3 -c "import pkg_resources; pkg_resources.require(open('requirements.txt',mode='r'))" &>/dev/null || pip3 install --ignore-installed -r requirements.txt

I realise this is not addressing the exact question, but this page comes up first when Googling for that. And anyway, you'd only really be wanting to know what the missing dependencies are in order to then satisfy them.

We can't just use pip3 check for this, since it does not look at the requirements.txt

Based on the answer by Zaur, assuming you indeed use a requirements file, you may want a unit test, perhaps in tests/test_requirements.py, that confirms the availability of packages.

Moreover, this approach uses a subtest to independently confirm each requirement. This is useful so that all failures are documented. Without subtests, only a single failure is documented.

"""Test availability of required packages."""

import unittest
from pathlib import Path

import pkg_resources

_REQUIREMENTS_PATH = Path(__file__).parent.with_name("requirements.txt")


class TestRequirements(unittest.TestCase):
    """Test availability of required packages."""

    def test_requirements(self):
        """Test that each required package is available."""
        # Ref: https://stackoverflow.com/a/45474387/
        requirements = pkg_resources.parse_requirements(_REQUIREMENTS_PATH.open())
        for requirement in requirements:
            requirement = str(requirement)
            with self.subTest(requirement=requirement):
                pkg_resources.require(requirement)

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)

You can run pip freeze to see what you have installed and compare it to your requirements.txt file.

If you want to install missing modules you can run pip install -r requirements.txt and that will install any missing modules and tell you at the end which ones were missing and installed.