[Python-ideas] Trial balloon: adding variable type declarations in support of PEP 484

[Alexander Belopolsky]

On Thu, Aug 4, 2016 at 12:15 PM, Guido van Rossum <gvanrossum at gmail.com> wrote:
> It will do exactly the same as
>
> a = None
> def f():
>     if False: a = ...
>     a
>
> This raises UnboundLocalError (a subclass of NameError).

That's what I expected.  Moreover, it looks like there is a precedent
for such behavior:

$ python3 -O
>>> a = None
>>> def f():
...   if __debug__: a = ...
...   a
...
>>> f()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in f
UnboundLocalError: local variable 'a' referenced before assignment

>>> dis(f)
  3           0 LOAD_FAST                0 (a)
              3 POP_TOP
              4 LOAD_CONST               0 (None)
              7 RETURN_VALUE

(Note that the if statement is optimized away.)

BTW, a: int looks like an "incomplete" assignment to me.  It's a
statement that under-specifies a: gives it a type, but not a value.
Visually, ':' is a shorter version of '=' and a half of the venerable
Pascal's ':='.  It all makes sense to me, but your milage may vary if
your first language was JavaScript rather than ALGOL. :-)