Is there any case where len(someObj) does not call someObj's __len__ function?
Question:
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__().
Answers:
According to Mark Pilgrim, it looks like no. len(someObj) is the same as someObj.__len__();
Cheers!
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.
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.
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().)
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
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__().
According to Mark Pilgrim, it looks like no. len(someObj) is the same as someObj.__len__();
Cheers!
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.
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.
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().)
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
floatTypeError: '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