How does len work?

Question

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 = o.__len__()
    if isinstance(l, int):
        return l
    original_len(o)

Do not want to do this. Why do you think it is remotely a good idea? — ironfroggy, Jan 31 '10 at 15:11
I agree with ironfroggy. What are you actually trying to accomplish? How does your int subclass differ from int? — mzz, Jan 31 '10 at 15:14

score 4 · Answer 1 · answered Jan 31 '10 at 15:11

4

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.

answered Jan 31 '10 at 15:11

ironfroggy

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score 3 · Accepted Answer · answered Jan 31 '10 at 14:20

3

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

answered Jan 31 '10 at 14:20

Pepijn

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Could be any problem if I override len? – Juanjo Conti Jan 31 '10 at 14:25
4

@Juanjo Conti: You confuse other developers who read your code and it makes your code more difficult to reuse. – Felix Kling Jan 31 '10 at 14:32

score 3 · Answer 3 · answered Jan 31 '10 at 14:20

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.

Beni Cherniavsky-Paskin · Answer 4 · 2010-01-31T16:01:07.773

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()

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.

How does len work?

4 Answers4