On Friday, June 26, 2020, at 04:54 -0500, Steven D'Aprano wrote:
On Thu, Jun 25, 2020 at 05:27:16PM +0300, Ben Avrahami 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.
Whereas this:
# Check for duplicates.
add element to the collection
was it already there before you just added it?
is a weird way to think about the problem.
already_there = seen.add(element)
if already_there:
# handle the duplicate case
Who thinks like that? *wink*
Anyone who practices EAFP rather than LBYL? Or is that why you're
winking?
But either way, you also have to decide whether the `add` (or
the new
method) should *unconditionally* insert the element, or only do
so if it
wasn't present. This makes a big difference:
seen = {2}
already_there = seen.add(2.0)
At this point, is `seen` the set {2} or {2.0}? Justify why one
answer is
the best answer.
The actual best answer is left as an exercise for the interested
reader, but whatever it is, it's justified by backwards
compatibility, the existing definition of "present," and the
principle of least surprise:
Python 3.8.3 (default, May 17 2020, 18:15:42)
[GCC 10.1.0] on linux
Type "help", "copyright", "credits" or "license" for more
information.
>>> seen = {2}
>>> 2.0 in seen
True
>>> seen.add(2.0)
>>> seen
{2}
Unless the new method is called set.add_with_different_semantics.
;-)
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