Question

So let’s say I wanna make a dictionary. We’ll call it d. But there are multiple ways to initialize a dictionary in Python! For example, I could do this:

d = {'hash': 'bang', 'slash': 'dot'} 

Or I could do this:

d = dict(hash='bang', slash='dot') 

Or this, curiously:

d = dict({'hash': 'bang', 'slash': 'dot'}) 

Or this:

d = dict([['hash', 'bang'], ['slash', 'dot']]) 

And a whole other multitude of ways with the dict() function. So obviously one of the things dict() provides is flexibility in syntax and initialization. But that’s not what I’m asking about.

Say I were to make d just an empty dictionary. What goes on behind the scenes of the Python interpreter when I do d = {} versus d = dict()? Is it simply two ways to do the same thing? Does using {} have the additional call of dict()? Does one have (even negligible) more overhead than the other? While the question is really completely unimportant, it’s a curiosity I would love to have answered.

Answer 1

>>> def f(): ...     return {'a' : 1, 'b' : 2} ...  >>> def g(): ...     return dict(a=1, b=2) ...  >>> g() {'a': 1, 'b': 2} >>> f() {'a': 1, 'b': 2} >>> import dis >>> dis.dis(f)   2           0 BUILD_MAP                0               3 DUP_TOP                            4 LOAD_CONST               1 ('a')               7 LOAD_CONST               2 (1)              10 ROT_THREE                         11 STORE_SUBSCR                      12 DUP_TOP                           13 LOAD_CONST               3 ('b')              16 LOAD_CONST               4 (2)              19 ROT_THREE                         20 STORE_SUBSCR                      21 RETURN_VALUE         >>> dis.dis(g)   2           0 LOAD_GLOBAL              0 (dict)               3 LOAD_CONST               1 ('a')               6 LOAD_CONST               2 (1)               9 LOAD_CONST               3 ('b')              12 LOAD_CONST               4 (2)              15 CALL_FUNCTION          512              18 RETURN_VALUE         

dict() is apparently some C built-in. A really smart or dedicated person (not me) could look at the interpreter source and tell you more. I just wanted to show off dis.dis. 🙂