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pythonmagic-methods
David Locke
David Locke

Reputation: 18084

Is there any case where len(someObj) does not call someObj's __len__ function?

Is there any case where len(someObj) does not call someObj's __len__ function?

I recently replaced the former with the latter in a (sucessful) effort to speed up some code. I want to make sure there's not some edge case somewhere where len(someObj) is not the same as someObj.__len__().

Upvotes: 15

Views: 2272

Answers (5)

wjandrea
wjandrea

Reputation: 33127

There are cases where len(someObj) is not the same as someObj.__len__() since len() validates __len__()'s return value. Here are the possible errors in Python 3.6.9:

  • Too low, i.e. less than 0

    ValueError: __len__() should return >= 0

  • Too high, i.e. greater than sys.maxsize (CPython-specific, per the docs)

    OverflowError: cannot fit 'int' into an index-sized integer

  • An invalid type, e.g float

    TypeError: 'float' object cannot be interpreted as an integer

  • Missing, e.g. len(object)

    TypeError: object of type 'type' has no len()

    I mention this because object.__len__() raises a different exception, AttributeError.

It's also worth noting that range(sys.maxsize+1) is valid, but its __len__() raises an exception:

OverflowError: Python int too large to convert to C ssize_t

Upvotes: 3

Benjamin Peterson
Benjamin Peterson

Reputation: 20550

If __len__ returns a length over sys.maxsize, len() will raise an exception. This isn't true of calling __len__ directly. (In fact you could return any object from __len__ which won't be caught unless it goes through len().)

Upvotes: 18

Andrew Jaffe
Andrew Jaffe

Reputation: 27107

I think the answer is that it will always work -- according to the Python docs:

__len__(self):

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.

Upvotes: 3

Crescent Fresh
Crescent Fresh

Reputation: 116998

What kind of speedup did you see? I cannot imagine it was noticeable was it?

From http://mail.python.org/pipermail/python-list/2002-May/147079.html

in certain situations there is no difference, but using len() is preferred for a couple reasons.

first, it's not recommended to go calling the __methods__ yourself, they are meant to be used by other parts of python.

len() will work on any type of sequence object (lists, tuples, and all). __len__ will only work on class instances with a __len__ method.

len() will return a more appropriate exception on objects without length.

Upvotes: 12

brettkelly
brettkelly

Reputation: 28235

According to Mark Pilgrim, it looks like no. len(someObj) is the same as someObj.__len__();

Cheers!

Upvotes: -4

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