Classes Are Attributes in Modules

Before we move on, remember that there’s nothing magic about a class name. It’s just a variable assigned to an object when the class statement runs, and the object can be referenced with any normal expression. For instance, if our FirstClass was coded in a module file instead of being typed interactively, we could import it and use its name normally in a class header line:

from modulename import FirstClass           # Copy name into my scope
class SecondClass(FirstClass):              # Use class name directly
    def display(self): ...

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Or, equivalently:

import modulename                           # Access the whole module
class SecondClass(modulename.FirstClass):   # Qualify to reference
    def display(self): ...

Like everything else, class names always live within a module, so they must follow all the rules we studied in Part V. For example, more than one class can be coded in a single module file—like other statements in a module, class statements are run during imports to define names, and these names become distinct module attributes. More generally, each module may arbitrarily mix any number of variables, functions, and classes, and all names in a module behave the same way. The file food.py demonstrates:

# food.py
var = 1                                       # food.var
def func():                                   # food.func
    ...
class spam:                                   # food.spam
    ...
class ham:                                    # food.ham
    ...
class eggs:                                   # food.eggs
    ...

This holds true even if the module and class happen to have the same name. For example, given the following file, person.py:

class person:
   ...

we need to go through the module to fetch the class as usual:

import person                                 # Import module
x = person.person()                           # Class within module

Although this path may look redundant, it’s required: person.person refers to the person class inside the person module. Saying just person gets the module, not the class, unless the from statement is used:

from person import person                     # Get class from module
x = person()                                  # Use class name

As with any other variable, we can never see a class in a file without first importing and somehow fetching it from its enclosing file. If this seems confusing, don’t use the same name for a module and a class within it. In fact, common convention in Python dictates that class names should begin with an uppercase letter, to help make them more distinct:

import person                                 # Lowercase for modules
x = person.Person()                           # Uppercase for classes

Also, keep in mind that although classes and modules are both namespaces for attaching attributes, they correspond to very different source code structures: a module reflects an entire file, but a class is a statement within a file. We’ll say more about such distinctions later in this part of the book.