List assignment/creation in Python showing unexpected behaviour

Question

I was playing around with a block of code during which I stumbled upon some unexpected behaviour of Python.

I defined a list as follows:

a = [1, 2, 3]

Made another list with a loop:

res = (item for item in a if a.count(item) > 0)
print(list(res)) # [1, 2, 3] -- as expected -- Consider this is line 1

Then later I changed the initial value of first list:

a = [7, 2, 9]

I was expecting no change in res as the changes I made to a were after res was created, but to my surprise, the value of res was changed.

print(list(res)) # prints [] -- Consider this as line 2

What's more shocking is that the result of print depends on print statement on line 1, if I comment out print statement at line1 then line 2 prints [2]

Can someone please explain me what's going on here?

Reassigning `a` has nothing to do with it. You'll see the same effect if you simply do `print(list(res))` twice in a row. — Barmar, May 29 '20 at 22:17
Seriously consider studying python's protocols and generators — Pynchia, May 29 '20 at 22:19
As an aside, `(item for item in a if a.count(item) > 0)` is an inefficient algorithm for this, it is quadratic time when you can do it in linear time — juanpa.arrivillaga, May 29 '20 at 23:24

score 4 · Accepted Answer · answered May 29 '20 at 22:16

4

res = (item for item in a if a.count(x) > 0) creates a generator that is exhausted by calling list(res).

When you call the next list(res) it returns empty list - []

consider creating a list first, for example res = [item for item in a if a.count(x) > 0].

answered May 29 '20 at 22:16

Andrej Kesely

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Hey @andrej, It would be really kind of you if you could give my answer a read and post a comment on that if you think its right or wrong. – Yudhishthir Singh May 29 '20 at 22:38
@YudhishthirSingh Yes, your answer is OK - the generator can be iterated over only once. There's more info about it for example here: https://stackoverflow.com/questions/47789/generator-expressions-vs-list-comprehension – Andrej Kesely May 29 '20 at 22:44

score 0 · Answer 2 · answered May 29 '20 at 22:34

Okay, I think I get it, I was not creating another list with that loop and I was creating something called generator.

Which can be converted to list only once, that is why it prints [] if I give print statement at line 1.

And it modifies itself according to value of a until it is exhausted.

List assignment/creation in Python showing unexpected behaviour

2 Answers2