in Python, How to join a list of tuples into one list?

Question

Following on my previous question How to group list items into tuple?

If I have a list of tuples, for example

a = [(1,3),(5,4)]

How can I unpack the tuples and reformat it into one single list

b = [1,3,5,4]

I think this also has to do with the iter function, but I really don't know how to do this. Please enlighten me.

I had hoped that there was a "splat" operator I could use for this, but unfortunately it isn't permitted specifically for comprehensions :( https://www.python.org/dev/peps/pep-0448/#id10 — robert, Feb 13 '20 at 09:54
Use sum as in this very [good answer](https://stackoverflow.com/a/952946/13168369) — kilodalton, Oct 19 '21 at 17:07

score 67 · Accepted Answer · answered Mar 07 '13 at 10:51

67

b = [i for sub in a for i in sub]

That will do the trick.

answered Mar 07 '13 at 10:51

Volatility

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oh yeah, that was easy, what was I thinking... – LWZ Mar 07 '13 at 10:55
1

Nice one. I had to think about it for a second - _"where did sub come from at the end?"_ – bjd2385 Nov 08 '16 at 21:05
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Why/how does this work? – ruief Nov 10 '17 at 23:12
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Please explain how that works, I am also confused by the double loop – Asara Nov 29 '17 at 12:30
4

@ruief @Asara the double loop works exactly as if you wrote it out as a nested for-loop: `for sub in a` picks out each sublist (or in this case, each tuple) of `a`, and then `for i in sub` picks out each element in each sublist, which is put together to form the resultant list. Hope that makes it a bit clearer. – Volatility Nov 29 '17 at 20:49
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Didn't know you could double loop in a list comp, nice. Answer works and is concise so have upvoted... but damn that syntax is hard to read. – otocan Sep 14 '21 at 13:54

score 14 · Answer 2 · answered Mar 07 '13 at 10:53

14

In [11]: list(itertools.chain(*a))
Out[11]: [1, 3, 5, 4]

If you just need to iterate over 1, 3, 5, 4, you can get rid of the list() call.

answered Mar 07 '13 at 10:53

NPE

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What's the point of `itertools.chain.from_iterable` then if you can just do that? – Volatility Mar 07 '13 at 10:57
2

@Volatility: Lazy evaluation? – NPE Mar 07 '13 at 10:58

score 3 · Answer 3 · answered Mar 07 '13 at 10:53

3

Just iterate over the list a and unpack the tuples:

l = []
for x,y in a:
   l.append(x)
   l.append(y)

answered Mar 07 '13 at 10:53

Schuh

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score 3 · Answer 4 · 2013-03-07T11:05:31.987

3

Another way:

a = [(1,3),(5,4)]
b = []

for i in a:
    for j in i:
        b.append(j)

print b

This will only handle the tuples inside the list (a) tho. You need to add if-else statements if you want to parse in loose variables too, like;

a = [(1,3),(5,4), 23, [21, 22], {'somevalue'}]
b = []

for i in a:
    if type(i) == (tuple) or type(i) == (list) or type(i) == (set):
        for j in i:
            b.append(j)
    else:
        b.append(i)

print b

edited Mar 07 '13 at 11:05

answered Mar 07 '13 at 10:54

Use `isinstance(i, collections.Iterable)` instead of the type checking (assuming `collections` has already been imported) – Volatility Mar 07 '13 at 11:15

score 1 · Answer 5 · answered Mar 07 '13 at 10:51

1

import itertools
b = [i for i in itertools.chain(*[(1,3),(5,4)])]

answered Mar 07 '13 at 10:51

danodonovan

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in Python, How to join a list of tuples into one list?

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