Python: "import ... as" causes incorrect reference

Question

I've noticed a peculiar behavior in how python handles module names.

If I write:

import a.b
import a

and then attempt to reference something in module a.b, it works without a problem.

However, if I attempt something only slightly different:

import a.b
import c as a

and then attempt to reference something in module a.b, I get an error, saying:

AttributeError: 'module' object has no attribute 'b'

I understand that the obvious solution is simply to not have two different modules I refer to by the same name. However, I'm still curious what caused this change in behaviour. Why does one work and not the other?

Answer 1

Why do you expect it to work? All 4 of your import statements are assignments to the variable a. Thus if you assign c to a, it overwrites the contents with something that doesn't have a .b attribute.

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Here's what happens in the first case:

1. When import a.b is run, Python loads module a, and then loads the module a.b into the loaded a module's attribute b. It then assigns the result to the variable a.
2. The import a doesn't import anything new, because module a is already loaded. It simply reassigns the same module from sys.modules['a'] to a.

Here's what happens in the second case:

1. When import a.b is run, Python loads module a, and then loads the module a.b into the loaded a module's attribute b. It then assigns the result to the variable a.
2. When import c as a is run, it loads the module c, and then assigns that module to variable a, overwriting the contents (a = sys.modules['c']). You could verify this by comparing the contents of sys.modules['a'] with the contents of a - they'll be different in this case.