One class per file?

Tony Meyer tony.meyer at
Sun Oct 5 06:58:52 CEST 2008

>> The book "Code Complete" recommends that you put only one class in a
>> source file, which seems a bit extreme for me.

IMO this is a misunderstanding (by the author).  In Python, a file is  
not equivalent to a class, it is equivalent to a module.  A module  
might contain a single class or many, just as a class might contain a  
single method or many.  You can tell that files are not classes,  
because you have have variables, methods, etc at the module level,  
outside of class definitions.

If all of the module's functionality is in a single class, then of  
course the file only contains one class.  If the module's  
functionality is split over many classes (or no classes at all), then  
the file will not contain exactly one class.  Rigidly putting every  
class in a separate file would mean that the next level higher than  
classes wouldn't be modules, it would be packages.

[Roy Smith]
> Consider this.  You're slogging through some code in a large project  
> trying
> to debug a problem when you come upon the line (in pseudo-code):
>  foo = SomeClass::SomeFunction(bar)
> You want to go look at the source for SomeClass.  What file do you  
> open to
> find it?  If you follow the "one class per file" rule, the answer is  
> easy;
> it's in!

Alternatively, most IDEs will let you go to the source very simply.   
If you don't have that facility available, then you just do "import  
X;print X.__file__".


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