How does len work?

How does len work on Python?

Look at this example:

class INT(int):
    pass

class STR(str):

    def __len__(self):
        return INT(42)

q = STR('how').__len__()
print q, type(q)
q = len(STR('how'))
print q, type(q)

The output is:

42 <class '__main__.INT'>
42 <type 'int'>

How can I handle it so len returns an INT instance?

Answers suggest that the only solution is overriding len

This is my alternative implementation. It doesn't seem very harmful.

original_len = len
def len(o):
    l开发者_Go百科 = o.__len__()
    if isinstance(l, int):
        return l
    original_len(o)

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Do not do this. You need to learn when the best answer really is not to do what you are trying to do at all. This is one of those times.

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I don't think you can, unless you write your own len. The builtin len always return an int.

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You won't be able to. At least if you want it to work with the rest of python. See the definition of len

Called to implement the built-in function len(). Should return the length of the object, an integer >= 0. Also, an object that doesn’t define a nonzero() method and whose len() method returns zero is considered to be false in a Boolean context.

Italics emphasis mine.

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As others say, don't do this. Consider how usage of this class would look:

length = len(s)     # the reader assumes `q` is an int.
length.in_yards()   # the reader is going WTF?!

Instead of violating the reader's expectations, why don't you just add a different method:

s.length_in_yards()

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P.S. Doesn't solve this question, but if you have a good reason to write custom integer-like objects, you might be interested in the __index__ special method that allows such object to be directly usable for indexing built-in sequences.