Consider the following:
items =  items.append("apple") items.append("orange") items.append("banana") # FAKE METHOD: items.amount() # Should return 3
How do I get the number of elements in the list
len() function can be used with several different types in Python - both built-in types and library types. For example:
>>> len([1,2,3]) 3
How to get the size of a list?
To find the size of a list, use the builtin function,
items =  items.append("apple") items.append("orange") items.append("banana")
Everything in Python is an object, including lists. All objects have a header of some sort in the C implementation.
Lists and other similar builtin objects with a "size" in Python, in particular, have an attribute called
ob_size, where the number of elements in the object is cached. So checking the number of objects in a list is very fast.
But if you're checking if list size is zero or not, don't use
len - instead, put the list in a boolean context - it treated as False if empty, True otherwise.
Return the length (the number of items) of an object. The argument may be a sequence (such as a string, bytes, tuple, list, or range) or a collection (such as a dictionary, set, or frozen set).
len is implemented with
__len__, from the data model docs:
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__()[in Python 2 or
__bool__()in Python 3] method and whose
__len__()method returns zero is considered to be false in a Boolean context.
And we can also see that
__len__ is a method of lists:
And in fact we see we can get this information for all of the described types:
>>> all(hasattr(cls, '__len__') for cls in (str, bytes, tuple, list, xrange, dict, set, frozenset)) True
lento test for an empty or nonempty list
To test for a specific length, of course, simply test for equality:
if len(items) == required_length: ...
But there's a special case for testing for a zero length list or the inverse. In that case, do not test for equality.
Also, do not do:
if len(items): ...
Instead, simply do:
if items: # Then we have some items, not empty! ...
if not items: # Then we have an empty list! ...
I explain why here but in short,
if items or
if not items is both more readable and more performant.