i even tried with venv, pip install azure-cli, pip freeze >reqs, pex setuptools -r reqs -m azure.cli -o az, still azure.common not found

I can repro this, digging deeper

It looks like you sussed the underlying issue. The relevant difference beteen pip and pex here is that the pip install installs all distributions in 1 chroot. As such the

azure/_init_.py

is layed down by one of these:

$ zipinfo -1 az | grep /azure/__init__.py | while read path; do echo -e "> In $path:\n===\n$(unzip -qc az $path | grep -E -v "^#")\n==="; done
> In .deps/azure_cli-2.5.1-py3-none-any.whl/azure/__init__.py:
===
import pkg_resources
pkg_resources.declare_namespace(__name__)
===
> In .deps/azure_cli_core-2.5.1-py3-none-any.whl/azure/__init__.py:
===
import pkg_resources
pkg_resources.declare_namespace(__name__)
===
> In .deps/azure_cli_telemetry-1.0.4-py2.py3-none-any.whl/azure/__init__.py:
===
import pkg_resources
pkg_resources.declare_namespace(__name__)
===
> In .deps/azure_cosmos-3.1.2-py3-none-any.whl/azure/__init__.py:
===
__import__('pkg_resources').declare_namespace(__name__)
===
> In .deps/azure_multiapi_storage-0.3.1-py2.py3-none-any.whl/azure/__init__.py:
===
__import__('pkg_resources').declare_namespace(__name__)
===

In a pex, each dependency is installed in its own isolated chroot. As such azure-common gets no help from any of the distributuions above and does not have an

azure/_init_.py

file and so its

azure/common

contents cannot be found by any load sequence that imports

azure/*

via a pkg_resources namespace. So, that's the reason for the difference. Pip does mask bad namespace package hygiene as @happy-kitchen-89482 suggested. As for a fix for Pex for this sort of problem that does not break multiplatform PEXes (these are the reason for the isolated dependency chroots) ... it's not immediately clear to me what the best approach is.