Python (and me) getting confused finding keys

[Steven D'Aprano]

On Tue, 22 Dec 2009 17:47:14 +0100, Christian Heimes wrote:

> John schrieb:
>> Hi there,
>> 
>> I have a rather lengthy program that troubles me for quite some time.
>> After some debugging, I arrived at the following assertion error:
>> 
>> for e in edges.keys():
>> 	assert edges.has_key(e)
>> 
>> Oops!? Is there ANY way that something like this can possibly happen?
> 
> Yes, it happens when another part of your program -- most likely a
> thread -- modifies edges while you are iterating over its keys. The
> keys() method of a dict returns a *copy* of its keys. If you had uses
> "for e in edges" you'd have seen a RuntimeError "dictionary changed size
> during iteration".

To be pedantic, you *might* have seen a RuntimeError, as the heuristic 
for detecting modifications during iteration is fairly simple and can 
only detect changes that change the size of the dict.

>>> d = {1: 'a', 2: 'b', 3: 'c'}
>>> n = 1
>>> for key in d:
...     del d[n]
...     d[str(n)] = None
...     n += 1
...
>>> d
{'1': None, '2': None, '3': None}



-- 
Steven