Attribute Name Qualification

Now that you’re becoming more familiar with modules, we should look at the notion of name qualification (fetching attributes) in more depth. In Python, you can access the attributes of any object that has attributes using the qualification syntax object.attribute.

Qualification is really an expression that returns the value assigned to an attribute name associated with an object. For example, the expression module2.sys in the previous example fetches the value assigned to sys in module2. Similarly, if we have a built-in list object L, L.append returns the append method object associated with that list.

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So, what does attribute qualification do to the scope rules we studied in Chapter 17? Nothing, really: it’s an independent concept. When you use qualification to access names, you give Python an explicit object from which to fetch the specified names. The LEGB rule applies only to bare, unqualified names. Here are the rules:

Simple variables

X means search for the name X in the current scopes (following the LEGB rule).

Qualification

X.Y means find X in the current scopes, then search for the attribute Y in the object X (not in scopes).

Qualification paths

X.Y.Z means look up the name Y in the object X, then look up Z in the object X.Y.

Generality

Qualification works on all objects with attributes: modules, classes, C extension types, etc.

In Part VI, we’ll see that qualification means a bit more for classes (it’s also the place where something called inheritance happens), but in general, the rules outlined here apply to all names in Python.