Handling missing dict keys, revisited

A while back, I wrote a post showing how to handle missing dict keys. In summary:

• Using setdefault
• Using defaultdict
• Implementing __missing__

While the advice there still stands, I've been reading through Fluent Python and came across two things worth being aware of when subclassing dict:

1. __contains__ doesn't call __missing__, so k in d returns False for keys not yet set, even if __missing__ would handle them
2. dict.get doesn't call __missing__, so .get(k) won't use your custom default

Both are by design and often what you want. __missing__ is only used by d[key], not other dict methods. Even defaultdict follows the same rule: only d[key] triggers the default factory and methods like .get() and in do not.

Overriding get is tempting but tricky. A naive implementation might look like this:

class M(dict):
    def __missing__(self, key):
        value = "my default value"
        self[key] = value
        return value

    def get(self, key, default=None):
        try:
            return self[key]  # triggers __missing__ if key is absent
        except KeyError:
            return default    # dead code, never reached

This doesn't work. self[key] calls __missing__ instead of raising KeyError, so the except block is unreachable and default is effectively useless:

>>> m = M()
>>> m.get("x", "fallback")
'my default value'          # expected "fallback", got __missing__ value instead
>>> m
{'x': 'my default value'}  # side effect: key was set in the dict

There's no clean way to honour both the __missing__ default and the default parameter in get - they pull in opposite directions. It's better to accept that get and __missing__ serve different purposes and leave get alone.

As for __contains__, overriding it is a design decision. If your subclass provides a value for every possible key, returning True always can be reasonable:

def __contains__(self, key):
    return True

Just be aware that "x" in m returns True even when m is {}, which looks confusing.