To find the size of a list, use the builtin function, len:
items = [] items.append["apple"] items.append["orange"] items.append["banana"]And now:
len[items]returns 3.
Explanation
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.
len[s]
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:
object.__len__[self]
Called to implement the built-in function len[]. Should return the length of the object, an integer >= 0. Also, an object that doesnt 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:
items.__len__[]returns 3.
Builtin types you can get the len [length] of
And in fact we see we can get this information for all of the described types:
Do not use len to 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! ...or
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.