wheres pythonmonks wrote:
> Instead of defaultdict for hash of lists, I have seen something like:
>
>
> m={}; m.setdefault('key', []).append(1)
>
> Would this be preferred in some circumstances?
In some circumstances, sure. I just can't think of them at the moment.
Maybe if your code has to work in Python 2.4.
> Also, is there a way to upcast a defaultdict into a dict?
dict(some_defaultdict)
> I have also
> heard some people use exceptions on dictionaries to catch key
> existence, so passing in a defaultdict (I guess) could be hazardous to
> health. Is this true?
A problem could arise when you swap a "key in dict" test with a
"try...except KeyError". This would be an implementation detail for a dict
but affect the contents of a defaultdict:
>>> from collections import defaultdict
>>> def update(d):
... for c in "abc":
... try: d[c]
... except KeyError: d[c] = c
...
>>> d = defaultdict(lambda:"-")
>>> update(d)
>>> d
defaultdict(<function <lambda> at 0x7fd4ce32a320>, {'a': '-', 'c': '-', 'b':
'-'})
>>> def update2(d):
... for c in "abc":
... if c not in d:
... d[c] = c
...
>>> d = defaultdict(lambda:"-")
>>> update2(d)
>>> d
defaultdict(<function <lambda> at 0x7fd4ce32a6e0>, {'a': 'a', 'c': 'c', 'b':
'b'})
Peter
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