The expression (sum(row) for row in M) creates what’s called a generator. This generator will evaluate the expression (sum(row)) once for each row in M. However, the generator doesn’t do anything yet, we’ve just set it up.
The statement next(G) actually runs the generator on M. So, if you run next(G) once, you’ll get the sum of the first row. If you run it again, you’ll get the sum of the second row, and so on.
>>> M = [[1,2,3],
... [4,5,6],
... [7,8,9]]
>>>
>>> G = (sum(row) for row in M) # create a generator of row sums
>>> next(G) # Run the iteration protocol
6
>>> next(G)
15
>>> next(G)
24
See also:
- Documentation on generators
- Documentation on yield expressions (with some info about generators)