[Python-Dev] Proposal: defaultdict (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Sat Feb 18 09:08:00 CET 2006

Adam Olsen wrote:

Demo/metaclass/Meta.py:55

That wouldn't break. If you had actually read the code, you would have seen it is

    try:
        ga = dict['__getattr__']
    except KeyError:
        pass

How would it break if dict had a default factory? ga would get the getattr value, and everything would be fine. The KeyError is ignored, after all.

Demo/tkinter/guido/AttrDialog.py:121 # Subclasses override self.classes

Hmm

        try:
            cl = self.classes[c]
        except KeyError:
            cl = 'unknown'

So cl wouldn't be 'unknown'. Why would that be a problem?

Lib/ConfigParser.py:623

           try:
                v = map[var]
            except KeyError:
                raise InterpolationMissingOptionError(
                    option, section, rest, var)

So there is no InterpolationMissingOptionError. Of course not. The whole point would be to provide a value for all interpolation variables.

Lib/random.py:315

This entire functions samples k elements with indices between 0 and len(population). Now, people "shouldn't" be passing dictionaries in in the first place; that specific code tests whether there valid values at indices 0, n//2, and n. If the dictionary isn't really a sequence (i.e. if it doesn't provide values at all indices), the function may later fail even if it passes that test.

With a default-valued dictionary, the function would not fail, but a large number of samples might be the default value.

Lib/string.py:191

Same like ConfigParser: the intperpolation will always succeed, interpolating all values (rather than leaving $identifier in the string). That would be precisely the expected behaviour.

Lib/weakref.py:56 # Currently uses UserDict but I assume it will switch to dict eventually

Or, rather, UserDict might grow the on_missing feature as well.

That is irrelevant for this issue, though:

    o = self.data[key]()
    if o is None:
        raise KeyError, key      # line 56
    else:
        return o

So we are looking for lookup failures in self.data, here: self.dict is initialized to {} in UserDict, with no default factory. So there cannot be a change in behaviour.

Perhaps the KeyError shouldn't ever get triggered in this case, I'm not sure. I think that's besides the point though. The programmer clearly expected it would.

No. I now see your problem: An "except KeyError" does not mean that the programmer "clearly expects it will" raise an KeyError. Instead, the programmer expects it might raise a KeyError, and tries to deal with this situation.

If the situation doesn't arise, the code continue just fine.

Regards, Martin

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