Perform Requires-Python check before downloading dependencies

[tekumara]

What's the problem this feature will solve?

When installing a package that requires a specific python version its dependencies are installed first, which

1. downloads a bunch of dependencies unnecessarily
2. can result in a confusing message when those dependencies don't have a distribution

eg:

myapi has python_requires=">=3.7" and depends on mylib. mylib has only been built for python 3.6.

pip install myapi with the Python 3.6 interpreter will download all dependencies and error with:

pip._internal.exceptions.DistributionNotFound: No matching distribution found for mylib==1.2.3

Describe the solution you'd like

pip install myapi with the Python 3.6 interpreter won't download dependencies and will error with the more helpful:

ERROR: Package 'myapi' requires a different Python: 3.6.10 not in '>=3.7'

Alternative Solutions

Additional context

[uranusjr]

Are you using PyPI.org? This is already done for all packages from PyPI.org, but an alternative index implementation may not provide enough information to make this possible.

[tekumara]

Thanks @uranusjr for looking into this. Yes, this happens when using pypi.org. The undesired behaviour seems to have been introduced in pip 20.3.4.

Example

setup.py

from setuptools import find_packages, setup

setup(
    name="myapp",
    version="0.0.0",
    python_requires=">=3.7",
    install_requires=["black==20.8b1"]
)

❯ python --version
Python 3.6.10

❯ python -m venv --clear .venv
❯ . .venv/bin/activate

pip 20.3.3 (and earlier) will check the package's python requires before downloading dependencies (yay!):

❯ pip install pip==20.3.3
...
❯ pip install -e .
Obtaining file:///private/tmp/repo-name
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
ERROR: Package 'myapp' requires a different Python: 3.6.10 not in '>=3.7'
WARNING: You are using pip version 20.3.3; however, version 21.1.1 is available.
You should consider upgrading via the '/private/tmp/repo-name/.venv/bin/python -m pip install --upgrade pip' command.

pip 20.3.4 will download dependencies first and then check the package's python requires:

❯ pip install pip==20.3.4
...
❯ pip install -e .
Obtaining file:///private/tmp/repo-name
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
Collecting black==20.8b1
  Using cached black-20.8b1.tar.gz (1.1 MB)
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
ERROR: Package 'myapp' requires a different Python: 3.6.10 not in '>=3.7'
WARNING: You are using pip version 20.3.4; however, version 21.1.1 is available.
You should consider upgrading via the '/private/tmp/repo-name/.venv/bin/python -m pip install --upgrade pip' command.

Same behavior in pip 21.1.1

❯ pip install pip==21.1.1
...
❯ pip install -e .       
Obtaining file:///private/tmp/repo-name
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
Collecting black==20.8b1
  Using cached black-20.8b1.tar.gz (1.1 MB)
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
ERROR: Package 'myapp' requires a different Python: 3.6.10 not in '>=3.7'

[uranusjr]

Ah OK, you meant checking Requires-Python of a package before downloading the dependencies of that package, not dependnecies in general. That makes sense, thanks for the additional context. I think that’s a reasonable optimisation.