When a module is run under pdb in Python 3, __package__ is set to the empty string instead of None.  The attached minimum working example depicts this behavior.  The results are summarized in the following table.

=============== ====== ======
    Command      Py 2   Py 3
=============== ====== ======
 python          None   None
 python -m        ''     ''
 python -m pdb   None    ''
 pdb             None    ''
=============== ====== ======

The behavior I expected was for the Python executable to treat the input file as a script and not a module.  My rationale is that pdb is loaded as the executing module that accepts a script as its argument.  Per the help, only one -m option is allowed when launching python.

pdb imports the module with importlib, and populates __main__ with data from its spec, including the package. This doesn't contradict the fact that the python command line can have only one "-m".

What is the actual problem here?

Caveat: It's been a few years so I'm trying to recall what I was doing at the time and what my original problem was.

I understand and expect pdb to populate __package__ and __main__ to be what it expects when run as a module with -m.  However, I believe my expectation was for it to not alter the relevant __package__ or __main__ with respect to the script being debugged.  This was the behavior in Python 2.7 which changed (unexpectedly to me) in Python 3.4 (or earlier.  I obviously didn't look too hard).  I must have been trying to debug a script that expected to run using the -m option but could be run as a script too.

A few things have changed since the original report:

1.  I have forgotten which script/tool I was working on so I wouldn't be able to find it if I tried.
2.  I was trying to support Python 2.7 and Python 3 (which I have 0 interest in doing ever again),
3.  This small gotcha apparently hasn't tripped up anyone else.

At this point, I suspect this has been overcome by events and can be closed.

Thanks Keith. Closing and we can look into it if someone else has an issue.