looping through the keys of a dictionary

[Tim Peters]

[Tist Verdonck]
> Is there a way to loop through the keys of a dictionary (whitout using
> a list containing all those keys)?

Not before the not-yet-released 2.2, but why do you care?  Most people
worried about speed or space in this context haven't measured either and are
simply irrational <0.7 wink>.

Under 2.2 you'll be able to do:

    for k in dict:  # a synonym for the next one
    for k in dict.iterkeys():
    for k in dict.iteritems():
    for k in dict.itervalues():

None of those create a list.  OTOH, if you try to *mutate* the dict while
iterating over it in these ways, you deserve whatever you get.  This is the
best you can hope for:

>>> d = {1:2, 2:3}
>>> for k in d:
...    d[k+2] = 3
...
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
RuntimeError: dictionary changed size during iteration
>>>

The great advantage to materializing a list is that it captures a snapshot
of the keys, and mutating the set of dict keys later doesn't mutate the
snapshot.