Question

How is this implemented at a python level?

I ve got an object that pretends to be a dict for the most part (in retrospect I should have just subclassed dict, but I d rather not refactor the codebase, and I d also like to know this for future reference), which looks something a bit like

class configThinger(object):
    _config = {}
    def __getitem__(self, key):
        return self._config[key]
    def __setitem__(self, key, value):
        self._config[key] = value

Which works exactly as it s supposed to and behaves correctly when I try to access it s elements as configThingerInstance[ whatever ]

But a call like

t = configThinger()
t.populate() # Internal method that fills it with some useful data
if  DEBUG  in t:
    doStuff()

results in a KeyError being raised because presumably the `in protocol does a getitem() lookup for the key in question. Do I need to raise some other exception to tell in that it s not there? I d rather not do something like this.

try:
    t[ DEBUG ]
except KeyError:
    pass
else:
    doStuff()

Also, where in the documentation is this?

I was looking around

http://docs.python.org/tutorial/datastructures.html

http://docs.python.org/library/stdtypes.html

but tragically trying to google for something specific to the word in is folly :(

EDIT 1:

With a stack of trace prints, I can see that the program calls configThingerInstance.getitem(0)

however

t = { rawk : 1,
      rawr : 2,
    }
t[0] # Raises KeyError
 thing  in t # returns False

Answer 1

It sounds like you want to overload the in operator?

You can do that by defining the method __contains__: http://docs.python.org/reference/datamodel.html#object.contains

Answer 2

For the best support for the in operator (containment aka membership checking), implement the __contains__ special method on your configThinger class:

class configThinger(object):
    _config = {}
    def __getitem__(self, key):
        return self._config[key]
    def __setitem__(self, key, value):
        self._config[key] = value
    def __contains__(self, key):
        return key in self._config

the docs are here (also explaining other, lesser ways to support the in operator).

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