[Tutor] packages,modules, and module attributes

[Alan Gauld]

> I've a question, essentially about the import statement. Suppose I have
> two python files, a.py and b.py:

OK, I'll have a go although I'm only 90% sure I've got it right...

> a.py
> ----
> flag = True
> 
> def getFlag():
>    return flag
> 
> b.py
> ----
> from a import *

This imports the names from a.
Thus b now has a name flag that points to TRue and a name 
getFlag that points to the function in a.

> now, in the interpreter:
> 
>>>> import b
>>>> b.flag
> True
>>>> b.flag=False
>>>> b.flag
> False

This first reads the value imported from a then creates a new name 
in b that overwrites the imported value and sets it to False.


>>>> b.getFlag()
> True

This calls the function in a which returns the value from a which is 
still set to True.

> this is probably related to namespaces? I find it very confusing, 

It is to do with namespaces and shows why two bad practices you have 
used are indeed bad practices! :-)

First if you had used 

import a 

in b instead of 

from a import *

All would have worked as you expected because you would have 
been forced to specify 

b.a.flag 

etc.

Secondly relying on a global variable in the function getFlag meant that 
when you imported getFLag the function object remained in a and 
accessed the variable flag in a. If you had passed the name of the flag 
to getFlag as a parameter then it would have worked as expected 
- although only returning the value you already had! (But if Flag were 
a more complex object it would then make sense!)

So as usual the advice "do not use from X import *" holds.

Alan G
Author of the learn to program web tutor
http://www.freenetpages.co.uk/hp/alan.gauld