[issue44555] Dictionary operations are LINEAR for any dictionary (for a particular code).

[Daniel Fleischman]

Daniel Fleischman <danielfleischman at gmail.com> added the comment:

Thank you for your message!

I'm not particularly looking for an implementation, I was just surprised by
this behavior.

It can get even worse. Consider this example:

```
d = large_dict()
# remove all but one element of d, runs in quadratic time as explained above
while len(d) > 1:
    del d[next(iter(d))]

# now iterating over d takes O(d), even when d has only one item:

# the following prints one key, but takes O(N)
for key in d:
    print(key)
```

I think this example is better, since a person would expect iterating over
a singleton dictionary would be really fast, but it can be made as slow as
one wants. A "print_keys()" function would reasonably be expected to take
O(size of dictionary), but it doesn't.

Am I right to think that this is a performance bug?

On Fri, Jul 2, 2021, 8:10 PM Dennis Sweeney <report at bugs.python.org> wrote:

>
> Dennis Sweeney <sweeney.dennis650 at gmail.com> added the comment:
>
> For what it's worth, using collections.OrderedDict gives constant-time
> behavior you're looking for.
>
> ----------
> nosy: +Dennis Sweeney
>
> _______________________________________
> Python tracker <report at bugs.python.org>
> <https://bugs.python.org/issue44555>
> _______________________________________
>

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44555>
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