[SciPy-user] help with list comprehensions
Wed Aug 1 14:01:35 CDT 2007
Stephen Yang wrote:
> Hello everyone,
> I have a question about list comprehensions. I would like to append data
> to an existing list using a list comprehension. Unfortunately, what I
> have tried does not seem to work:
> >>> y = [5, 1, 3, 5]
> >>> x = ['a', 'b']
> >>> new = [x.append(data) for data in y]
> >>> new
> [None, None, None, None]
> Can anyone help? Thanks very much in advance.
For the example you give, you shouldn't use a list comprehension at all:
If you do need a list comprehension for something more complicated (say, because
you were calling a function on each element):
x.extend([f(data) for data in y])
The reason that your code didn't do what you thought it did stems from two causes:
1) list.append() returns None.
2) You were looking at `new` which was simply the result of the list
comprehension and not `x`, the list you were trying to extend. If you had looked
at `x`, you would have seen that the data were correctly appended. However, you
shouldn't do it that way anyways.
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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