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A few months ago a friend and I were discussing a bug in
dnspython
when I first learned about the trouble of trying to delete items from
a dictionary while iterating over that dictionary. If you try to do this,
you'll get a RuntimeError exception explaining that 'dictionary changed
size during iteration'. Try it yourself:
d = { 1: 'one', 2: 'two', 3: 'trois'}
for key in d:
del d[key]
I bring this up now because a similar issue came up recently and bit
me. I was trying to enumerate a list and delete some of its elements.
Python lets you do this without raising an exception, but it's probably
not going to work the way you think it does.
>>> l = range(3) # take a little list
>>> e = enumerate(l) # enumerate it
>>> e.next() # ok
(0, 0)
>>> del l[0] # get rid of this element
>>> e.next() # oops, we skipped one...
(1, 2)
>>> e.next() # The flowers are still standing!
>>> e.next()Traceback (most recent call last):
File "", line 1, in ?
StopIteration
The list shrinks in place when you delete an element, but the index
part of your iterator moves ahead and causes you to skip an element. In
this case, we skip the element with the value of '1' because it resides
at l[0] after the deletion.
So, if you wanted to do this properly, use range, start at the end
of your list, and count backwards...
Take care.
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