PEP 750: Tag Strings For Writing Domain-Specific Languages

PEPs
60

The current implementation seems to capture the source variables rather than the result of the expression, which of course leads to this behaviour:

def ident(o):
    return o

items = [ident"{x}" for x in range(10)]
strs = [o.getvalue() for o in items]
strs
[9, 9, 9, 9, 9, 9, 9, 9, 9, 9]

I didn’t go too far into edge cases around comprehensions, async, and the like, but I would assume they exist based on this implementation.

Compare to this behaviour, which is what I mean by eagerly evaluating each expression:

class ident:
    def __init__(self, x):
        self.x = x
    def getvalue(self):
        return self.x

items = [ident(x) for x in range(10)]
strs = [o.getvalue() for o in items]
strs
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

(Edit again). And the most compelling example I have for doing things eagerly:

def ident(o):
    return o

with lock:
    s = ident"{calculate_under_lock()}"

s.getvalue()

How do you communicate to users that their expressions aren’t calculated when/where they think they will be? The complexity goes way up for each use, for not a lot of real benefit. Evaluating the expressions in the context they are written saves all the trouble.

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