Python len() built-in function

From the Python 3 documentation 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).

Example

Return the the number of items of an object:

>>> len('hello')
# 5

>>> len(['cat', 3, 'dog'])
# 3

Test of emptiness

Test of emptiness Test of emptiness of strings, lists, dictionaries, etc., should not use len, but prefer direct boolean evaluation. 
>>> a = [1, 2, 3]

# bad
>>> if len(a) > 0:  # evaluates to True
...     print("the list is not empty!")
...
# the list is not empty!

# good
>>> if a: # evaluates to True
...     print("the list is not empty!")
...
# the list is not empty!

Python abs() built-in function Python aiter() built-in function Python all() built-in function Python any() built-in function Python ascii() built-in function Python bin() built-in function Python bool() built-in function Python breakpoint() built-in function Python bytearray() built-in function Python bytes() built-in function Python callable() built-in function Python chr() built-in function Python classmethod() built-in function Python compile() built-in function Python complex() built-in function Python delattr() built-in function Python dict() built-in function Python dir() built-in function Python divmod() built-in function Python enumerate() built-in function Python eval() built-in function Python exec() built-in function Python filter() built-in function Python float() built-in function Python format() built-in function Python frozenset() built-in function Python getattr() built-in function Python globals() built-in function Python hasattr() built-in function Python hash() built-in function Python help() built-in function Python hex() built-in function Python id() built-in function Python __import__() built-in function Python input() built-in function Python int() built-in function Python isinstance() built-in function Python issubclass() built-in function Python iter() built-in function Python len() built-in function Python list() built-in function Python locals() built-in function Python map() built-in function Python max() built-in function Python memoryview() built-in function Python min() built-in function Python next() built-in function Python object() built-in function Python oct() built-in function Python open() built-in function Python ord() built-in function Python pow() built-in function Python print() built-in function Python property() built-in function Python range() built-in function Python repr() built-in function Python reversed() built-in function Python round() built-in function Python set() built-in function Python setattr() built-in function Python slice() built-in function Python sorted() built-in function Python staticmethod() built-in function Python str() built-in function Python sum() built-in function Python super() built-in function Python tuple() built-in function Python type() built-in function Python vars() built-in function Python zip() built-in function