Python dictionary clear

Question

In python, is there a difference between calling clear() and assigning {} to a dictionary? If yes, what is it? Example:

d = {"stuff":"things"}
d.clear()   #this way
d = {}      #vs this way

Answer 1

If you have another variable also referring to the same dictionary, there is a big difference:

>>> d = {"stuff": "things"}
>>> d2 = d
>>> d = {}
>>> d2
{'stuff': 'things'}
>>> d = {"stuff": "things"}
>>> d2 = d
>>> d.clear()
>>> d2
{}

This is because assigning d = {} creates a new, empty dictionary and assigns it to the d variable. This leaves d2 pointing at the old dictionary with items still in it. However, d.clear() clears the same dictionary that d and d2 both point at.

Answer 2

d = {} will create a new instance for d but all other references will still point to the old contents. .clear() will reset the contents, but all references to the same instance will still be correct.

Answer 3

In addition to the differences mentioned in other answers, there also is a speed difference. d = {} is over twice as fast:

python -m timeit -s "d = {}" "for i in xrange(500000): d.clear()"
10 loops, best of 3: 127 msec per loop

python -m timeit -s "d = {}" "for i in xrange(500000): d = {}"
10 loops, best of 3: 53.6 msec per loop