About one class/function per module

Peng Yu pengyu.ut at gmail.com
Sun Nov 1 23:11:48 CET 2009

I recently asked how to support one class/function per module under
the title 'How to import only one module in a package when the package
__init__.py has already imports the modules?' I summarize my key
points below. In particular, I have to two questions:
1. What disadvantages there are to enforce one class/function per file?
2. How to better support one class/function per file in python?

I prefer organized my code one class/function per file (i.e per module
in python). I know the majority of programmers don't use this
approach. Therefore, I'm wondering what its disadvantage is.

The advantages of one-function/class-per-file that I am aware of are
the following items (maybe incomplete).
  1. It is easy to see what functions and classes there are just by
browsing the source directory, without the need of opening the file by
an editor.
  2. Testing code for a function/class can be organized along with the
module for the function/class, which also makes it easier to keep
track of the testing code for any function/class.
  3. It is easy to move a function/class from one package to another
by just moving files around.
  4. It is easy change variable names, etc., for a function/class by
replacement in a file without worrying accidentally change variable
names in other functions.
I have used the above approach on a C++ project. And I use vim + ctags
to navigate from a reference of a class/funciton to its definition. So
far it works fine.

Some people mentioned an good IDE can do 1 and 4. But I'm not aware of
an IDE that can allow me to change file name freely. I tried Visual
Studio long time ago, I have to delete a file, change the file name
and add the file back in order to change the file.

Now let us consider how well python can support one function/class per
file style. I encounter the following problems. Suppose that the
parent directory of 'test' is in $PYTHONPATH and __init__.py is empty,

|-- __init__.py
|-- A.py
`-- B.py

where A.py has only class A and B.py has only class B.

The end user of the test package has to either

  import test.A


  from test.A import A

Both of the two cases are not satisfactory. In the first case, a end
user has to type A twice in test.A.A(), which is a burden to the end
user. In the second case, 'from test.A import A' screws up the
namespace, which might cause potential conflicts with classes of the
same name but from different packages.

I was also suggested to put the following line in __init__.py.

  from A import A
  from B import B

Then the end user can use the following code.

  import test

But the disadvantage is that I can not import individule file test.A
and test.B any more. Suppose that A is modified but not done yet, then
I modify B. Since the modification of A is not finished yet, the test
code of B won't run because the test code import test.A.

I'm looking for if there is a solution, so that a end user can use the
following code

  import test.A

So far, I haven't find one. It seems impossible in python, but I want
to double check if there is one solution.

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