Attributeerror

[init_self]

extreme newbie question:

I have a filename.py file with an attribute named a.
I`s a dictionary example, and when I start the interpreter
and call the attribute its value prints out fine.

Then I change the name of the attribute from a to b and save 
filename.py

Calling the attribute now produces an errormessage:
Attributeerror: 'module has no attribute 'b'

This irritates me. Why does it happen?