Question

Usually I access dict keys using keys() method:

d = {'a':1, 'b':2, 'c':3}

for k in d.keys(): print k

But sometimes I see this code:

for k in d: print k

Is this code correct? safe?

Answer 1

Although this was already mentioned, I wanted to add some exact numbers to these discussion. So I compared:

def iter_dtest(dtest):
    for i in dtest:
        pass

and

def list_dtest(dtest):
    for i in dtest.keys():
        pass

A dictionary with 1 000 000 items was used (float keys) and I used timeit with 100 repetitions. These are the results:

Python 2.7.1:
iter_dtest: 3.92487884435s
list_dtest: 6.24848171448s

Python 3.2.1:
iter_dtest: 3.4850587113842555s
list_dtest: 3.535072302413432s

Obviously calling dtest.keys() has some downsides in Python 2.x