len(d) number of elements, i.e. (key, value)-pairs
>>> print(numbers)
{'three': 'drei', 'two': 'zwei', 'one': 'eins'}
>>> print(len(numbers))
3
del d['k'] delete the k'th element
k in d True, if there is a key k in d
>>> "one" in numbers
True
>>> "two" in numbers
False
>>>
k not in d True, if there is no key k in d
>>> "one" not in numbers
False