19.3: Generator expressions

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Generator expressions are similar to list comprehensions, but with parentheses instead of square brackets:

>>> g = (x**2 for x in range(5))
>>> g
<generator object <genexpr> at 0x7f4c45a786c0>

The result is a generator object that knows how to iterate through a sequence of values. But unlike a list comprehension, it does not compute the values all at once; it waits to be asked. The built-in function next gets the next value from the generator:

>>> next(g)
0
>>> next(g)
1

When you get to the end of the sequence, next raises a StopIteration exception. You can also use a for loop to iterate through the values:

>>> for val in g:
...     print(val)
4
9
16

The generator object keeps track of where it is in the sequence, so the for loop picks up where next left off. Once the generator is exhausted, it continues to raise StopIteration:

>>> next(g)
StopIteration

Generator expressions are often used with functions like sum, max, and min:

>>> sum(x**2 for x in range(5))
30