Swap syntax does not work

[jmoraleda]

In a regular dict, e.g. w = {1:1, 2:2} one can swap the values of two keys with w[1],w[2] = w[2],w[1] but this does not work if w is a bidict giving the error KeyAndValueDuplicationError: (1, 2).

If this behavior can be fixed (which I doubt) it should, but otherwise perhaps is could be documented as a limitation and a difference with a regular dict.

By the way, as a work around (in case others arrive here with the same problem), one can simply pop the offending element before the swap as follows:

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

[jab]

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!

[jmoraleda]

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.

[jab]

Thanks for the great explanation! Please do submit the PR and I’ll take a look ASAP. Thanks for your contribution!

…

On Sat, Sep 24, 2022 at 17:43 Jorge Moraleda ***@***.***> wrote: 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 newTuple = b[2], b[1] b[1] = newTuple[0] b[2] = newTuple[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 t the statement b[1], b[2] = b[2], b[1] is by definition equivalent to newTuple = b[2], b[1] b[1] = newTuple[0] b[2] = newTuple[1] as described in the relevant section of the https://docs.python.org/3/reference/simple_stmts.html docs: ------------------------------ If you agree I can submit a PR for it. Thank you again. — Reply to this email directly, view it on GitHub <#255 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAP3YFP24GJEPQWGHGAHPLV75YXPANCNFSM6AAAAAAQTOW6UE> . You are receiving this because you commented.Message ID: ***@***.***>

[jab]

Documented in 22b7914.

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