On Thu, Jun 25, 2020 at 9:02 AM Steele Farnsworth <swfarnswo...@gmail.com>
wrote:
> My point was only that, as far as I know, all the methods for built in
> container types that serve only to change what is contained return None,
> and that this was an intentional design choice, so changing it in one case
> would have to evoke a larger discussion about what those sorts of methods
> should return.
>
indeed -- and that is pretty darn baked in to Python, so I don't think it's
going to change.
Note: I"n not sure your example with setdefault is correct:
seen = {}
for i in iterable:
if seen.set_default(i, some_value) is not None:
... # do something in case of duplicates
else:
... # do something in case of first visit
A) you spelled setdefault() wrong -- no underscore
B) seen.setdefault(i, some_value) will return some_value if it's not there,
and whatever the value it is it is (in this case, starting with an empty
dict, it will be some_value always.
Running this code:
some_value = "sentinel"
iterable = [3, 2, 4, 2, 3]
seen = {}
for i in iterable:
if seen.setdefault(i, some_value) is not None:
# do something in case of duplicates
print(f'{i} was already in there')
else:
# do something in case of first visit
print(f'{i} was not already there')
results in:
In [9]: run in_set.py
3 was already in there
2 was already in there
4 was already in there
2 was already in there
3 was already in there
so not working.
But you can make it work if you reset the value:
some_value = "sentinel"
iterable = [3, 2, 4, 2, 3]
seen = {}
for i in iterable:
if seen.setdefault(i, None) is not None:
# do something in case of duplicates
print(f'{i} was already in there')
else:
# do something in case of first visit
print(f'{i} was not already there')
seen[i] = some_value
In [11]: run in_set.py
3 was not already there
2 was not already there
4 was not already there
2 was already in there
3 was already in there
But this is a bit klunky as well, not really any better than the set
version.
However, for the case at hand, adding a method similar to the
dict.setdefault() would be a reasonable thing to do. I'm not sure what to
call it, or what the API should be, but maybe:
class my_set(set):
def add_if_not_there(self, item):
if item in self:
return True
else:
self.add(item)
return False
seen = my_set()
for i in iterable:
if seen.add_if_not_there(i):
print(f'{i} was already in there')
else:
print(f'{i} was not already there')
However, while dict.setdefault does clean up and clarify otherwise somewhat
ugly code, I'm not sure this is that much better than:
for i in iterable:
if i in seen:
print(f'{i} was already in there')
else:
seen.add(i)
print(f'{i} was not already there')
But feel free to make the case :-)
Note that setdefault is in the MutableMapping ABC, so there could be some
debate about whether to add this new method to the MutableSet ABC.
-CHB
>
> I wouldn't be opposed to that discussion happening and for any changes
> that are made to happen within 3.x because I doubt that very much code that
> currently exists depends on these methods returning None or even use what
> they return at all.
>
> On Thu, Jun 25, 2020, 10:28 AM Ben Avrahami <avrahami....@gmail.com>
> wrote:
>
>> Hey all,
>> Often I've found this kind of code:
>>
>> seen = set()
>> for i in iterable:
>> if i in seen:
>> ... # do something in case of duplicates
>> else:
>> seen.add(i)
>> ... # do something in case of first visit
>>
>> This kind of code appears whenever one needs to check for duplicates in
>> case of a user-submitted iterable, or when we loop over a recursive
>> iteration that may involve cycles (graph search or the like). This code
>> could be improved if one could ensure an item is in the set, and get
>> whether it was there before in one operation. This may seem overly
>> specific, but dicts do do this:
>>
>> seen = {}
>> for i in iterable:
>> if seen.set_default(i, some_value) is not None:
>> ... # do something in case of duplicates
>> else:
>> ... # do something in case of first visit
>>
>> I think the set type would benefit greatly from its add method having a
>> return value. set.add would return True if the item was already in the set
>> prior to insertion, and False otherwise.
>>
>> Looking at the Cpython code, the set_add_entry already detects existing
>> entries, adding a return value would require no additional complexity.
>>
>> Any thoughts?
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--
Christopher Barker, PhD
Python Language Consulting
- Teaching
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