[Tim]
> I expect it would do less harm to introduce a compile-time warning for
> locals that are never referenced (such as the "a" in "set").
[Guido]
> Another warning that would be quite useful (and trap similar cases)
> would be "local variable used before set".
Java elevated that last one to a compile-time error, via its "definite
assignment" rules: you not only have to make sure a local is bound before
reference, you have to make it *obvious* to the compiler that it's bound
before reference. I think this is a Good Thing, because with intense
training, people can learn to think like a compiler too <wink>.
Seriously, in several of the cases where gcc warned about "maybe used before
set" in the Python implementation, the warnings were bogus but it was
non-trivial to deduce that. Such code is very brittle under modification,
and the definite assignment rules make that path to error a non-starter.
Example:
def f(N):
if N > 0:
for i in range(N):
if i == 0:
j = 42
else:
f2(i)
elif N <= 0:
j = 24
return j
It's a Crime Against Humanity to make the code reader *deduce* that j is
always bound by the time "return" is executed.
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