Python update a key in dict if it doesn't exist

Question

I want to insert a key-value pair into dict if key not in dict.keys(). Basically I could do it with:

if key not in d.keys():
    d[key] = value

Answer 1

Use dict.setdefault():

>>> d = {1: 'one'}
>>> d.setdefault(1, '1')
'one'
>>> d    # d has not changed because the key already existed
{1: 'one'}
>>> d.setdefault(2, 'two')
'two'
>>> d
{1: 'one', 2: 'two'}