How to check if there's a tuple inside a list?

Question

Why does this:

seq = [(1, 2), (3, 4), (5, 6)]
print(() in seq)

return False? How can I check if there's a tuple, or even a generic sequence, inside a sequence with no specific values, as in this answer.

Will the list be arbitrarily nested? Or just flat as in your example? — Brad Solomon, Oct 29 '18 at 12:58
What you are checking is existence of an empty tuple in the list. You can check type instead. — Sid, Oct 29 '18 at 12:59
Why did you expect that to work? Really. I don't understand what made you think that might work. It doesn't seem to have just been a wild guess, as you seem legitimately confused about the fact it doesn't work. What was the thought process that led you to think it might do what you want? If you find yourself confused about this sort of thing, you should ask *yourself* that question and have an answer for it before posting. If you do, you'll often find that there's something you thought was true but aren't actually sure, and if you go check, then you will probably not even need to post. — jpmc26, Oct 29 '18 at 21:17

timgeb · Accepted Answer · 2018-10-29T13:05:30.273

28

() is an empty tuple. seq does not contain an empty tuple.

You want

>>> seq = [(1, 2), (3, 4), (5, 6)]
>>> any(isinstance(x, tuple) for x in seq)
True

For a generic sequence you can use

>>> from collections import abc
>>> any(isinstance(x, abc.Sequence) for x in seq)
True

However, lots of objects are informally treated as sequences but neither implement the full protocol abc.Sequence defines nor register as a virtual subclass of Sequence.

Read this excellent answer for additional information.

You can find a question about detecting sequences here.

edited Oct 29 '18 at 13:05

answered Oct 29 '18 at 12:59

timgeb

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I believe `abc.Iterable` is also useful here – cat Oct 29 '18 at 14:03
2

@cat Unfortunately, not really. I wrote in detail about why checking against `abc.Iterable` is unreliable [here](https://stackoverflow.com/a/36407550/3620003). – timgeb Oct 29 '18 at 14:10
Curious: does `any()` operate greedily, stopping at first `True`-thy occurrence? – Brad Solomon Oct 30 '18 at 02:20
@BradSolomon yes, [`any`](https://docs.python.org/3/library/functions.html#any) is lazy. – timgeb Oct 30 '18 at 06:12

score 3 · Answer 2 · edited Oct 29 '18 at 13:19

3

What you are checking is the existence of an empty tuple in the list.

You can check the type instead.

def has_tuple(seq):    
    for i in seq:
        if isinstance(i, tuple):
            return True
    return False

edited Oct 29 '18 at 13:19

timgeb

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answered Oct 29 '18 at 13:01

Sid

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@timgeb I had deliberately provided pseudocode to encourage OP to implement it himself/herself. – Sid Oct 29 '18 at 15:50
I'm sorry if you did not like the edit. Feel free to roll it back. – timgeb Oct 29 '18 at 15:54

How to check if there's a tuple inside a list?

2 Answers2