`ModuleDirImport` only finds distribution metadata if module has already been loaded

[chrisnovakovic]

In the pex bootstrap code, the ModuleDirImport class is able to act as a meta path finder owing to its implementation of find_distributions, but only does so if the distribution metadata lookup (e.g. via importlib.metadata.version) occurs after the module in question has been imported by the interpreter. The root cause is the check on whether the module name exists in sys.modules, which for pex-packed modules will have been added in load_module:

python-rules/tools/please_pex/pex/pex_main.py

Line 207 in fba6378

sys.modules[fullname] = module

python-rules/tools/please_pex/pex/pex_main.py

Line 253 in fba6378

if context.name in sys.modules:

Ultimately, it means that while this succeeds:

import importlib.metadata
import somemodule

importlib.metadata.version("somemodule")

This fails:

import importlib.metadata

importlib.metadata.version("somemodule")
# importlib.metadata.PackageNotFoundError: No package metadata was found for somemodule