Hello Python community,
Earlier this year I put forward a proposal for decorators on variables
which I felt was well received in concept, but fell flat on the syntax. One
of the responses to that thread was rather than abusing the decorator, to
use a new symbol to get the target of an assignment statement as a string
on the right hand side. I would call this compile-time reflection of
assignment targets, or simply reflection. Although initially continuing on
with my original syntax, the idea of using such a token in my own code came
up on more than one occasion this year and I wanted to see what it might
really look like.
>From that, this proposal is to add a new token to Python, currently spelled
<<<, which, at compile time, is replaced with the string representation of
the nearest assignment target, either assignment statement or assignment
expression. It does not bind with either keyword arguments or default
parameters. Neither does it work in any kind of non-assignment binding.
When encountered anywhere other than the right hand side of some
assignment, it is an unconditional SyntaxError.
Having access to the target at runtime is most often useful in meta
programming and the standard library has several factory functions that
require the user to repeat the variable name in the function call to get
expected behavior. Bellow are some examples of where I believe the
reflection token would be used if adopted.
>>> Point = namedtuple(<<<, 'x, y, z')
>>> Point
<class '__main__.Point'>
>>> UUIDType = NewType(<<<, str)
>>> UUIDType
__main__.UUIDType
>>> class Colors(Enum):
... Black = <<<
... GRAY = <<<
... WHITE = <<<
...
>>> Colors.GRAY.value
'GRAY'
>>> HOME = '$' + <<<
>>> HOME
'$HOME'
>>> if error := response.get(<<<):
... if errorcode := error.get(<<<):
... print(f"Can't continue, got {errorcode=}")
...
Can't continue, got errorcode=13
To get a feel for using this new token I have created a fork of the 3.11
alpha that implements a *very* incomplete version of this new grammar, just
enough to actually produce all of the examples above. It also passes a
small new test suite with further examples
https://github.com/ucodery/cpython/blob/reflection/Lib/test/test_reflection.py
.
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