[Python-Dev] PEP 572 semantics: all capabilities of the assignment statement

Tim Peters tim.peters at gmail.com
Thu Jul 5 00:06:37 EDT 2018

[Ivan Pozdeev]

> Victor Stinner in "Assignment expression and coding style: the while
> True case" and others have brought to attention
> that the AE as currently written doesn't support all the capabilities of
> the assignment statement, namely:
> * tuple unpacking
> * augmented assignment
> (I titled the letter "all capabilities" 'cuz I may've missed something.)
> Assignment statements (and `for` headers) also allow arbitrarily complex
applications of subscripting and attribute references.  Like

    for a[b].cde[f, g(h, i, j)].k in range(10):

> Should it?

Already been considered and rejected, so no.

Victor already showed why, but didn't quite realize it ;-)

while True:
    name, token = _getname(g)
    if not name:

Allowing to unpack the two names in an AE wouldn't really help, because
there's still no satisfying way then to _reference_ just the `name` part in
the `while` test:

    while ((name, token) := _getname(g)) ... and now what??

You could use "a trick", relying on that a 2-tuple is always truthy:

    while ((name, token) := _getname(g)) and name:

Using a trick sucks.  Not relying on a trick is ugly and inefficient:

    while [((name, token) := _getname(g)), name][-1]:
    while [((name, token) := _getname(g)), name].pop():

where the thing to be tested is the second list element (e.g., `name ==
"not done yet"`).

So no plausible use cases were ever presented, and the idea was dropped.

If we _also_ added something akin to C's comma expressions (evaluate a
sequence of expressions and throw away all the results except for the
last), then a reasonable spelling akin to the last one above could be used,
but without the strained cruft to muck with a result list (or tuple).

At which point my rule for AE would likely never trigger ("if it's not
obviously better, don't use it").
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