Why is the dict literal syntax preferred over the dict constructor?

Question

Why is the Python dict constructor slower than the using literal syntax?

After hot debate with my colleague, I did some comparison and got the following statistics:

python2.7 -m timeit "d = dict(x=1, y=2, z=3)"
1000000 loops, best of 3: 0.47 usec per loop

python2.7 -m timeit "d = {'x': 1, 'y': 2, 'z': 3}"
10000000 loops, best of 3: 0.162 usec per loop

What is the reason the constructor is slower? And in what situations, if any, would it be faster?

Answer 1

The constructor is slower because it creates the object by calling the dict() function, whereas the compiler turns the dict literal into BUILD_MAP bytecode, saving the function call.