Difference between dict.clear() and assigning {} in Python

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.

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

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

As an illustration for the things already mentioned before:

>>> a = {1:2}
>>> id(a)
3073677212L
>>> a.clear()
>>> id(a)
3073677212L
>>> a = {}
>>> id(a)
3073675716L