Swap syntax does not work about bidict HOT 4 CLOSED

You're right to doubt that this behavior can be "fixed". The current behavior is actually consistent with the relevant section of the https://docs.python.org/3/reference/simple_stmts.html docs:

Although the definition of assignment implies that overlaps between the left-hand side and the right-hand side are ‘simultaneous’ (for example a, b = b, a swaps two variables), overlaps within the collection of assigned-to variables occur left-to-right, sometimes resulting in confusion. For instance, the following program prints [0, 2]:

x = [0, 1]
i = 0
i, x[i] = 1, 2         # i is updated, then x[i] is updated
print(x)

So with b = bidict({1:1, 2:2}), b[1], b[2] = b[2], b[1] is by definition equivalent to

b[1] = b[2]
b[2] = b[1]

which correctly raises KeyAndValueDuplicationError on the first assignment (before it even gets to the second).

Would you like to submit a PR to the https://bidict.readthedocs.io/en/main/addendum.html#caveats docs? I think that's a good suggestion. Thanks!

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Thank you for your fast meaningful reply, and for your work in bidict.

To be precise, the statement b[1], b[2] = b[2], b[1] is by definition equivalent to

_temporary_internal_tuple = b[2], b[1]
b[1] = _temporary_internal_tuple[0]
b[2] = _temporary_internal_tuple[1]

rather than

b[1] = b[2]
b[2] = b[1]

The distinction is important because when using a regular python dict b[1], b[2] = b[2], b[1] results in b being equal to {1:2, 2:1} while

b[1] = b[2]
b[2] = b[1]

results in b being equal to {1:2, 2:2}.

A casual user might expect the swap to work for bidict just as it does in a regular dict so to document this difference between dict and bidict I propose writing a new section at the end of Addendum -> Caveats with the following content:

Simultanous Assignment

bidict behaves differently than dict with respect to simultaneous assignment. For example, for a regular dict e.g. b = {1:1, 2:2} we can swap two values with b[1], b[2] = b[2], b[1]. which results inb being equal to {1:2, 2:1} but if b = bidict({1:1, 2:2}) then b[1], b[2] = b[2], b[1] results in a KeyAndValueDuplicationError: (1, 2) and to swap the values one must run:

val2 = b.pop(2)
b[1],b[2] = val2, b[1]

This limitation is a direct consequence of the fact that the statement b[1], b[2] = b[2], b[1] is by definition equivalent to

_temporary_internal_tuple = b[2], b[1]
b[1] = _temporary_internal_tuple[0]
b[2] = _temporary_internal_tuple[1]

as described in https://docs.python.org/3/reference/simple_stmts.html docs:

If you agree I can submit a PR for it. Thank you again.

from bidict.

from bidict.

Documented in 22b7914.

Live at https://bidict.rtfd.io/addendum.html#simultaneous-assignment

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