Python: Comprehensions

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all the following have the same behavior
1.
>>> res = []
>>> for x in 'spam':
res.append(ord(x)) # Manual results collection

2.
>>> res = list(map(ord, 'spam')) # Apply function to sequence (or other)

3
>>> res = [ord(x) for x in 'spam'] # Apply expression to sequence (or other)


Formal comprehension syntax:

[expresion for target in iterable]

more generally:

[ expression for target1 in iterable1 if condition1
for target2 in iterable2 if condition2 ...
for targetN in iterableN if conditionN ]

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关键词：python comp COM MPR ONS expression following function sequence

res = [x + y for x in [0, 1, 2] for y in [100, 200, 300]]

is same as

>>> res = []
>>> for x in [0, 1, 2]:
            for y in [100, 200, 300]:
                  res.append(x + y)

thanks!

http://tech.pro/tutorial/1554/four-tricks-for-comprehensions-in-python