CODEWITHSUNDEEP
python
Juanjo Conti
Juanjo Conti

Reputation: 30013

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

Upvotes: 3

Views: 12214

Answers (4)

Beni Cherniavsky-Paskin
Beni Cherniavsky-Paskin

Reputation: 10039

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.

Upvotes: 3

ironfroggy
ironfroggy

Reputation: 8119

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.

Upvotes: 4

Doug T.
Doug T.

Reputation: 65599

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.

Upvotes: 3

Pepijn
Pepijn

Reputation: 4253

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

Upvotes: 3

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