Problem with reimporting modules

[Gabriel Genellina]

En Sun, 11 Feb 2007 15:56:16 -0300, Christoph Zwerschke <cito at online.de>  
escribió:

> Yes I know about reload(), but TurboGears (TurboKid) does not use it and
> the docs say that removing modules from sys.module is possible to force
> reloading of modules. I don't want to rewrite everything since it's a
> pretty complex thing with modules which are compiled from templates
> which can depend from other templates etc...

If you remove the module from sys.modules, when you import it again you  
end up with a *new*, fresh, module object, unrelated to the original one.

Quoting your original message again:

> I tracked it down to the following behavior of Python. Assume you have a
> module hello.py like that:
>  ---- hello. py ----
> greeting = 'Hello!'
> def print_hello():
>      print greeting
> -------------------
>  Now run the following code:
>  from hello import print_hello
> print_hello()
> import sys
> del sys.modules['hello'] # delete module
> import hello # recreate module
> print_hello()
>  The second print_hello() prints "None" instead of "Hello!". Why is that?

Notice that you are mixing references here. You first import print_hello  
 from hello, and after deleting the module, you import hello (not  
print_hello). And you expect that your old reference to print_hello now  
refers to the new function. The whole point of reloading/reimporting a  
module is to get the *new* contents, but that only works if you refer to  
things using the module.function notation, not if you hold a reference to  
the function (which will always be the original function).

In short, your code should be:

import hello
hello.print_hello()
import sys
del sys.modules['hello']
import hello
hello.print_hello()

or, using reload:

import hello
hello.print_hello()
reload(hello)
hello.print_hello()

If you think that always typing module.function is too much - well, don't  
try to reload modules then :)
Somewhere I read that at Google the policy is to always import modules,  
never functions, and this may be a good reason for it.

If you want to know the details: print_hello doesn't hold a reference to  
the containing module, only to its namespace, in the func_globals  
attribute. When you delete the last reference to the module, it gets  
destroyed. At that time, all values in the module namespace are set to  
None (for breaking possible cycles, I presume). print_hello now has a  
func_globals with all names set to None. (Perhaps the names could have  
been deleted instead, so print_hello() would raise a NameError, but I'm  
just describing the current CPython implementation)

-- 
Gabriel Genellina