Way out of Python venv hell.

no_env_loader.py

"""
So you are doing Python on MS Windows. Maybe even deploying your stuff on a Windows Server.
By now you know that virtual environments are HEAVY on Windows, and you have many projects or application with the same
copy of numpy (60MB each) over and over again.

No more. Install a package by unpacking wheels (not pip install) to the corresponding versioned folder:

  __pypackages__
    └── numpy-1.21.0
        └── numpy
        └── numpy-1.21.0.dist-info
    └── dateutil-2.8.1
        └── dateutil
        │   └── parser
        │   └── tz
        │   └── zoneinfo
        └── python_dateutil-2.8.1.dist-info
        
This differs from https://www.python.org/dev/peps/pep-0582, which _locally_ caches packages by _python version_.
See also https://discuss.python.org/t/pep-582-python-local-packages-directory/963

Remarks:
IDE support for finding packages is nil, as VsCode or PyCharm will look into a **fixed** set of configurable folders.
Would symlinks help, or oblitarate this approach in the first place?
Yes it does (both Python and PyCharm) on MS Windows, if you have the right to create symbolic links:
    set PROJECT_DIR=SOMETHING
    mklink /d %PROJECT_DIR%\venv\Lib\site-packages\dateutil C:\__pypackages__\python_dateutil-2.8.1\dateutil
So we could create symbolic links from distributions_by_package_name in site-dir if allowed (most likely on PC),
else use VersionFinder (on Server).  
"""
import importlib.util
import sys
from importlib.machinery import SourceFileLoader
import importlib.metadata
from pathlib import Path
from typing import Dict


class VersionFinder(importlib.machinery.PathFinder):
    path_by_name: Dict[str, Path] = dict()
    @classmethod
    def find_spec(cls, name: str, path, target=None):
        # print(f"Importing {name} {path}")
        if name in cls.path_by_name:
            path = [cls.path_by_name[name].as_posix()]
        return super().find_spec(name, path)

    @classmethod
    def find_distributions(cls, ctx):
        if ctx.name in cls.path_by_name:
            dist_info = next(cls.path_by_name[ctx.name].glob('*.dist-info'))
            return iter([importlib.metadata.PathDistribution(dist_info)])
        else:
            return super().find_distributions(ctx)


def simple_requirements_parser(requirements: str) -> Dict[str, Path]:
    distributions_by_package_name = dict()
    for line in requirements.splitlines():
        line = line.strip()
        if not line:
            continue
        name, version = line.split('==')
        name = name.replace('-', '_').lower()
        dist_folder = Path('__pypackages__') / f'{name}-{version}'
        if not dist_folder.is_dir():
            print(f'Cannot find distribution {dist_folder.as_posix()}', file=sys.stderr)
            continue
        for item in dist_folder.iterdir():
            if item.is_dir() and item.suffix != '.dist-info':
                distributions_by_package_name[item.name] = dist_folder

    return distributions_by_package_name


# Fill this from your fixed requirements.txt. I recommend pip-compile strongly!
VersionFinder.path_by_name = simple_requirements_parser("""
numpy==1.21.0
python-dateutil==2.8.1
""")

sys.meta_path.insert(0, VersionFinder)


import dateutil.parser

print(dateutil.parser.parse('2021-01-01'))
print(importlib.metadata.version('dateutil'))