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Python: list, tuple, list comprehension and generator 

Dr. Xi
Joined:
04/09/2007
Posts:
565

November 15, 2009 21:44:23
Python lists are enclosed in square brackets. When a list of objects are enclosed in parentheses it is a tuple (an immutable list). List comprehensions are enclosed in square brackets. But when you change the square brackets to parentheses, you get a generator not a tuple! 
>>> l = [1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> t = (1, 2, 3, 4, 5, 6, 7, 8, 9)
>>> type(l)
<type 'list'>
>>> type(t)
<type 'tuple'>
>>> l2 = [i for i in l if i % 2 != 0]
>>> l2
[1, 3, 5, 7, 9]
>>> g1 = (i for i in l if i % 2 != 0)
>>> type(g1)
<type 'generator'>
>>> g2 = (i for i in t if i % 2 != 0)
>>> type(g2)
<type 'generator'>
>>> g1 == g2
False
>>> sum(g2)
25
>>> sum(g1)
25

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