README.md for p3pack

p3pack is a short Python program to demonstrate differences in how Python 2 and Python 3 handle packages.

Here I am using Python 2.7.6 and 3.6.5 to issustrate the difference.

In these tests I have installed p3pack/ under ~/sproj/.

Structure of application

p3pack/
    top.py
    mymodule/
        __init__.py    # this is empty
        moda.py
        modb.py

Let's say that top.py is the entry point of my main program.

I have a library in mymodule/ which is called by top.py. Inside mymodule/ I also have test code in modb.py which tests moda.py, so I want to run modb.py as the entry point of my program when testing and debugging the library.

This works in Python 2 but not in Python 3.

Running in Python 2.7

(We don't need to create a virtual ewnvironment because python 2.7 is the default python on this system).

Run p3pack/mymodule/modb.py:

~/sproj/p3pack$ cd mymodule/
~/sproj/p3pack/mymodule$ python modb.py
----- testing moda.py -----
Result=124, should be 124

This demonstrates that I can run with modb.py as the main module.

Run p3pack/top.py:

~/sproj/p3pack/mymodule$ cd ..
~/sproj/p3pack$ python top.py
=== top.py ===
result is 17
r2 is 21

And I can also run with top.py as the main module. This is what I want to be able to do.

Running in Python 3.6

Create a a python 3 virtual environment

~/sproj/p3pack$ python3 -m venv v3
~/sproj/p3pack$ . v3/bin/activate

Now run p3pack/mymodule/modb.py:

(v3) ~/sproj/p3pack$ cd mymodule/
(v3) ~/sproj/p3pack/mymodule$ python modb.py
----- testing moda.py -----
Result=124, should be 124

Run p3pack/top.py:

(v3) ~/sproj/p3pack/mymodule$ cd ..
(v3) ~/sproj/p3pack$ python top.py
Traceback (most recent call last):
  File "top.py", line 3, in <module>
    from mymodule import moda, modb
  File "/home/phil/sproj/p3pack/mymodule/modb.py", line 3, in <module>
    import moda
ModuleNotFoundError: No module named 'moda'

And this is where Python 3 goes wrong. It doesn't let me run modb.py both as the entry point, and also as a module within the package mymodule.

I can get it to work by changing line 3 of modb.py to:

from . import moda

I get this (correct) output):

(v3) ~/sproj/p3pack$ python top.py
=== top.py ===
result is 17

But then, when I try to run with modb.py as the entry point, it falls over:

(v3) ~/sproj/p3pack$ cd mymodule/
(v3) ~/sproj/p3pack/mymodule$ python modb.py
Traceback (most recent call last):
  File "modb.py", line 3, in <module>
    from . import moda
ImportError: cannot import name 'moda'

Now obviously I could get round this my putting all my test scripts in the top-level directory p3pack/. Is this how it is indended to do this in Python 3?

cabalamat/p3pack

README.md for p3pack

Structure of application

Running in Python 2.7

Running in Python 3.6