When should a Python script be split into multiple files/modules?

In Java, this question is easy (if a little tedious) - every class requires its own file. So the number of .java files in a project is the number of classes (not counting anonymous/nested classes).

In Python, though, I can define multiple classes in the same file, and I'm not quite sure how to find the point at which I split thing开发者_开发百科s up. It seems wrong to make a file for every class, but it also feels wrong just to leave everything in the same file by default. How do I know where to break a program up?

Remember that in Python, a file is a module that you will most likely import in order to use the classes contained therein. Also remember one of the basic principles of software development "the unit of packaging is the unit of reuse", which basically means:

If classes are most likely used together, or if using one class leads to using another, they belong in a common package.

As I see it, this is really a question about reuse and abstraction. If you have a problem that you can solve in a very general way, so that the resulting code would be useful in many other programs, put it in its own module.

For example: a while ago I wrote a (bad) mpd client. I wanted to make configuration file and option parsing easy, so I created a class that combined ConfigParser and optparse functionality in a way I thought was sensible. It needed a couple of support classes, so I put them all together in a module. I never use the client, but I've reused the configuration module in other projects.

EDIT: Also, a more cynical answer just occurred to me: if you can only solve a problem in a really ugly way, hide the ugliness in a module. :)

In Java ... every class requires its own file.

On the flipside, sometimes a Java file, also, will include enums or subclasses or interfaces, within the main class because they are "closely related."

not counting anonymous/nested classes

Anonymous classes shouldn't be counted, but I think tasteful use of nested classes is a choice much like the one you're asking about Python.

(Occasionally a Java file will have two classes, not nested, which is allowed, but yuck don't do it.)

Python actually gives you the choice to package your code in the way you see fit.

The analogy between Python and Java is that a file i.e., the .py file in Python is equivalent to a package in Java as in it can contain many related classes and functions.

For good examples, have a look in the Python built-in modules. Just download the source and check them out, the rule of thumb I follow is when you have very tightly coupled classes or functions you keep them in a single file else you break them up.