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 there a reason I would do one over the other?

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

Solution 1

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"

Solution 2

Think it this way.

In java what you write is a Class where in the case of Python, you write a module instead of a class. So a module can contain several classes. Whenever you want to use a particular class, import the respective module first and then call the class to make objects.

Here's an example.

Classes.py (This is a module named 'Classes')

---

class MyClass(object):  
    def greet(self):
        print("Hello World")
class MyNextClass(object):
    def greetAgain(self):
        print("Hello again")

---

Now I can import this module from anywhere I wish

---

import Classes
if __name__ == '__main__':
    a=Classes.MyClass()
    a.greet()
    b=Classes.MyNextClass()
    b.greetAgain()

---

Solution 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.

Solution 4

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.)

Solution 5

It is certainly a normal thing to do in Python. When and why 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.

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Comments

[Mike Graham]

Note, most style standards have __init__.py being empty or close to empty. It turns out it can be confusing and feel disorganized to have modules that contain both submodules and normal stuff at the same levels.

[blokeley]

@Mike Graham: django places lots of functions and classes in __init__.py files and django seems to be well-regarded.

[Mike Graham]

@blokeley, Django is certainly popular, but I don't know if it is widely regarded as adhering to Python best principles. I have never used Django myself, but it sounds like it requires people to do a log of ugly, awful things to get their apps working. See jcalderone.livejournal.com/39794.html and effbot.org/pyfaq/what-is-init-py-used-for.htm and various SO threads to read about __init__.py usually being emptyish. Projects in my site-packages that do this include Twisted, PyMeta, and PyFlakes.

[Joel M]

Coming from outside of python its really hard for me to put multiple classes in the same file, or to have filenames not be the same as classes.

[Joel M]

This also seems to work, from subproject.Foo import *. Is there a reason why we wouldn't just do it this way, and then have one class per file, and many classes in a module? It's a bit annoying to write and you can't import all classes in one go. Is there a way to import all classes in a module using __init__.py and __all__ (without having to use the class as Foo.Foo())?