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Stephen Cole
Courses Plus Student 15,809 PointsWhat is the for loop inside the sum function called?
In the final example, Kenneth creates a function that calculates the sum function that seems to do a list comprehension. What is it and why does it work?
1 Answer
Chris Freeman
Treehouse Moderator 68,457 PointsIt like a list comprehension, but in this case it is a generator expression. It behaves much like a list comprehension, but gives the advantage of a generator in terms of minimal memory usage.
# standard list comprehension
>>> [x**2 for x in range(5)]
[0, 1, 4, 9, 16]
# generator expression - using parens instead of square brackets
>>> (x**2 for x in range(5))
<generator object <genexpr> at 0x000000000119D8E0>
# expand generator using list
>>> list(x**2 for x in range(5))
[0, 1, 4, 9, 16]
# using list comprehension as an argument to sum()
>>> sum([x**2 for x in range(5)])
30
# using generator expression (without square brackets)
>>> sum(x**2 for x in range(5))
30
# Not in a comprehension or generator context is a syntax error
>>> x**2 for x in range(5)
File "<stdin>", line 1
x**2 for x in range(5)
^
SyntaxError: invalid syntax