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pythondictionarylambdaclosuresdictionary-comprehension

Dictionary comprehension with lambda expression fails to produce desired result

I'm creating a one liner to map the string of int to a function testing if the values are matched. Ideally, the result dictionary d behaves like d['0'](0) is True and d['0'](1) is False. But instead, I get the following output:

>>> d = { str(i): lambda v: v == i for i in range(3) }
>>> d['0'](0)
False
>>> d['0'](2)
True

I'm guessing the reason being lazy evaluation. I guess I could build the dictionary with a for loop correctly but I want a one line expression instead.

Can anyone explain why this approach fails and how I do it right?

Solution

  • You need to capture the current value of i for each lambda which can be done via the default argument i=i. See:

    >>> d = { str(i): lambda v, i=i: v == i for i in range(3) }
>>> d['0'](0)
True
>>> d['0'](2)
False