Online Modification of Python Code

Diez B. Roggisch deets at
Mon Sep 5 11:53:09 CEST 2005

> I seem to recall a post by Diez Roggish that reload() doesn't always
> work as it should. Any news on this? At least im my preliminary test it
> works.

Read the docs on reload:


If a module is syntactically correct but its initialization fails,  the 
first import statement for it does not bind its name  locally, but does 
store a (partially initialized) module object in  sys.modules. To reload 
the module you must first  import it again (this will bind the name to 
the partially  initialized module object) before you can reload() it.

When a module is reloaded, its dictionary (containing the module's 
global variables) is retained. Redefinitions of names will override  the 
old definitions, so this is generally not a problem. If the new  version 
of a module does not define a name that was defined by the  old version, 
the old definition remains. This feature can be used  to the module's 
advantage if it maintains a global table or cache of  objects -- with a 
try statement it can test for the  table's presence and skip its 
initialization if desired:

except NameError:
     cache = {}

It is legal though generally not very useful to reload built-in or 
dynamically loaded modules, except for sys,  __main__ and __builtin__. 
In  many cases, however, extension modules are not designed to be 
initialized more than once, and may fail in arbitrary ways when  reloaded.

If a module imports objects from another module using from  ... import 
..., calling reload() for  the other module does not redefine the 
objects imported from it --  one way around this is to re-execute the 
from statement,  another is to use import and qualified names 
( instead.

If a module instantiates instances of a class, reloading the module 
that defines the class does not affect the method definitions of the 
instances -- they continue to use the old class definition. The  same is 
true for derived classes.

So - expect strange things to happen, sometimes.

I personally _never_ felt the need for reload - after all, writing more 
than totally trivial crap on the commandline justifies a small 
testscript - which I then just execute on the commandline.


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