Multiply keys*values in a dict?

Question

a = {2: 4, 3: 2, 5: 1, 7: 1}

The keys represent prime numbers; the values represent counters. I want to calculate the number you get by iterating through the dictionary keys*values and summing the total. What is the most Pythonic way to do this?

>>> [k*v for k,v in a.items()]
[8, 6, 5, 7]

but

>>> sum(k*v for k,v in a.items())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not callable

Answer 1

This way:

sum(k*v for k,v in a.items())

or with semantic naming:

sum(p*c for p,c in primesToCounts.items())