TL;DR: should we make `del x` an expression that returns the value of `x`.
## Motivation
I noticed yesterday that `itertools.combinations` has an optimization for when
the returned tuple has no remaining ref-counts, and reuses it - namely, the
following code:
>>> for v in itertools.combinations([1, 2, 3], 1):
... print(id(v))
... del v # without this, the optimization can't take place
2500926199840
2500926199840
2500926199840
will print the same id three times. However, when used as a list comprehension,
the optimization can't step in, and I have no way of using the `del` keyword
>>> [id(v) for v in itertools.combinations([1, 2, 3], 1)]
[2500926200992, 2500926199072, 2500926200992]
`itertools.combinations` is not the only place to make this optimization -
parts of numpy use it too, allowing
a = (b * c) + d
to elide the temporary `b*c`. This elision can't happen with the spelling
bc = b * c
a = bc + d
My suggestion would be to make `del x` an expression, with semantics "unbind
the name `x`, and evaluate to its value".
This would allow:
>>> [id(del v) for v in itertools.combinations([1, 2, 3], 1)]
[2500926200992, 2500926200992, 2500926200992]
and
bc = b * c
a = (del bc) + d # in C++, this would be `std::move(bc) + d`
## Why a keyword
Unbinding a name is not something that a user would expect a function to do.
Functions operate on values, not names, so `move(bc)` would be surprising.
## Why `del`
`del` already has "unbind this name" semantics, and allow it to be used as an
expression would break no existing code.
`move x` might be more understandable, but adding new keywords is expensive
## Optional extension
For consistency, `x = (del foo.attr)` and `x = (del foo[i])` could also become
legal expressions, and `__delete__`, `__delattr__`, and `__delitem__` would now
have return values. Existing types would be free to continue to return `None`.
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