Is the if statement condition mutually-exclusive to multiple variables?

Question

Let's say that I have three lists and want to add all elements that are integers to a list named int_list:

test1 = [1, 2, 3, "b", 6]
test2 = [1, "foo", "bar", 7]
test3 = ["more stuff", 1, 4, 99]
int_list = []

I know that I can do the following code to append all integers to a new list:

for elem1, elem2, elem3 in zip(test1, test2, test3):
    if elem1 is int:
        int_list.append(elem1)
    if elem2 is int:
        int_list.append(elem2)
    if elem3 is int:
        int_list.append(elem3)

Is there anyway that I can merge the if statements into one conditional statement? Or make it less code? Is there a more pythonic way to do this? I tried doing the following code, but it would include elements that were not integers:

for elem1, elem2, elem3 in zip(test1, test2, test3):
        if (elem1 is int, elem2 is int, elem3 is int):
            int_list.append(elem1)
            int_list.append(elem2)
            int_list.append(elem3)

This will **not append** all integers per se, since `zip` will stop from the moment one of the lists has finished... — Willem Van Onsem, Mar 24 '17 at 17:18

score 2 · Accepted Answer · edited May 23 '17 at 11:46

2

There are some problems with your code:

zip constructs tuples of elements as long as all lists have elements, if one of the lists is exhausted, zip stops;
elem1 is int does not do type-checking, it does reference equality checking, you can use isinstance(..) for that; and
as @Jean-FrançoisFabre says, the if (...) will always succeed since you test it with a tuple with one or more elements: regardless of the values of that tuple the truthiness is True.

Why not first join them thogether? You can for instance use a tuple of the lists, like:

for sublist in (test1,test2,test3):
    for x in sublist:
        if isinstance(x,int):
            int_list.append(x)

Or you can use list comprehension:

int_list = [x for sublist in (test1,test2,test3)
              for x in sublist if isinstance(x,int)]

edited May 23 '17 at 11:46

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answered Mar 24 '17 at 17:21

Willem Van Onsem

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There's no generator in your "use a generator" example. – user2357112 Mar 24 '17 at 17:22
@user2357112: yeah initially I wanted to use a generator to chain them together, but halfway I changed my mind. – Willem Van Onsem Mar 24 '17 at 17:24
also `(elem1 is int, elem2 is int, elem3 is int)` is always `True` because it's a non-empty tuple. – Jean-François Fabre Mar 24 '17 at 17:24

score 2 · Answer 2 · edited May 23 '17 at 11:46

2

You use zip, but you want itertools.chain(). Additionally you should use isinstance to check class membership.

from itertools import chain

for elem in chain(test1, test2, test3):
    if isinstance(elem, int):
        int_list.append(elem)

edited May 23 '17 at 11:46

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answered Mar 24 '17 at 17:22

Pierce Darragh

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Is the if statement condition mutually-exclusive to multiple variables?

2 Answers2