Identical branches: one build successful, the other not. How to debug?

For a Python package, I have two branches (master and dev) that are identical; however Travis runs successfully on dev but not on master (ModuleNotFoundError: No module named 'zipp').

I’m a bit puzzled, and it is not clear to me how to debug it.

master build:

dev build:

Diffing the raw logs (I use Winmerge) is useful to see what’s different.

I see that in Python 3.6 jobs, you are testing with a different Python version than your Travis config specifies.
That’s because in your Pipfile.lock, there are modules that require Python 3.7 – I reckon you have last saved it from a later version of Python and some module has dropped support for 3.6 in its latest version. Due to this effect, since you’re testing with multiple Python versions, I suggest to delete Pipfile.lock from version control.
Pipenv must be second-guessing you and choosing an appropriate Python version if it’s present in the VM.

That’s all I can say from the logs. To see what’s further up, need to see what Pipenv is doing while setting up its automatically-created virtual environment. I reckon you need to run pipenv with --verbose to see that.

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