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Python Python for File Systems Navigation Checking Directory Contents

Stephen Cole
Stephen Cole
Courses Plus Student 15,809 Points

What 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
Chris Freeman
Treehouse Moderator 68,423 Points

It 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)])
# using generator expression (without square brackets)
>>> sum(x**2 for x in range(5))
# 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