On 5/9/2011 9:27 AM, Stefan Behnel wrote:
Eli Bendersky, 09.05.2011 14:56:
It's a known Python gotcha (*) that the following code:
x = 5
def foo():
print(x)
x = 1
print(x)
foo()
Will throw:
UnboundLocalError: local variable 'x' referenced before assignment
On the usage of 'x' in the *first* print. Recently, while reading the
zillionth question on StackOverflow on some variation of this case, I
started thinking whether this behavior is desired or just an
implementation
artifact.
Well, basically any compiler these days can detect that a variable is
being used before assignment, or at least that this is possibly the
case, depending on prior branching.
ISTM that your suggestion is to let x refer to the outer x up to the
assignment and to the inner x from that point on. IMHO, that's much
worse than the current behaviour and potentially impractical due to
conditional assignments.
However, it's also a semantic change to reject code with unbound locals
at compile time, as the specific code in question may actually be
unreachable at runtime. This makes me think that it would be best to
discuss this on the python-ideas list first.
If nothing else, I'd like to see a discussion on this behaviour being an
implementation detail of CPython or a feature of the Python language.
Stefan
--
Terry Jan Reedy
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