Multiple classes in a Python module

Question

I'm very new to Python (I'm coming from a JAVA background) and I'm wondering if anyone could help me with some of the Python standards. Is it a normal or "proper" practice to put multiple class in a module? I have been working with Django and started with the tutorials and they place their database model classes in the same module. Is this something that is normally done or should I stick with 1 class per module? Is their a reason I would do one over the other?

Hope I'm being clear and not to generic. Thanks to everyone in advance!

Answer 1

It is absolutely proper to do so. A module groups related functionality. If that functionality is implemented in several classes (e.g., Tree, Node, Leaf) then it is appropriate to place them together.

A module is more closely associated with a Java package than a Java class. You can also implement a module as a folder, named for the module, with an __init__.py file inside (so Python can identify the module as such; the __init__.py may also optionally include initialization code and lists of classes, functions, and sub-packages to export.)

Answer 2

It is certainly a normal thing to do in Python. When and way you choose one over the other is partly a matter of taste, and partly convention.

If you're still getting to know Python, and therefore its conventions, reading the style guide is well worth your time.

Answer 3

When in doubt, just look at Python's standard libraries :)

For example, the standard calendar module contains 31 classes. So yes, it is ok.

Answer 4

Here is a useful rule of thumb from what I have seen of typical Java projects:

The bottom-most package in Java should be a file in Python

What does that mean? If your Java project was organized:

toplevel/
   subproject/
        Foo.java
        Bar.java
   subproject2/
        Baz.java
        Qux.java

Then your Python project should look like:

toplevel/
    subproject.py <-- put class Foo, Bar here
    subproject2.py <-- put class Baz, Qux here

Things to notice re: organization:

• Do not use inner classes. Just put classes in the same module
• By convention, things that start with _ are "private"
• It's OK to have "public variables"