Why are underscores better than hyphens for file names?

Question

From Building Skills in Python:

A file name like exercise_1.py is better than the name exercise-1.py. We can run both programs equally well from the command line, but the name with the hyphen limits our ability to write larger and more sophisticated programs.

Why is this?

Answer 1

The issue here is that importing files with the hyphen-minus (the default keyboard key -; U+002D) in their name doesn't work since it represents minus signs in Python. So, if you had your own module you wanted to import, it shouldn't have a hyphen in its name:

>>> import test-1
  File "<stdin>", line 1
    import test-1
               ^
SyntaxError: invalid syntax
>>> import test_1
>>>

Larger programs tend to be logically separated into many different modules, hence the quote

the name with the hyphen limits our ability to write larger and more sophisticated programs.

Answer 2

From that very document (p.368, Section 30.2 'Module Definition'):

Note that a module name must be a valid Python name... A module's name is limited to letters, digits and "_"s.