Vitja Makarov, 25.01.2011 08:15:
> I want to write simple code to find unbound variables.
There is some code that does (part of) that already, in a transform IIRC.
It's used to detect if we need to initialise a local variable to None or
can safely set it to NULL.
> As it's hard to detect them in common case it will mark them as:
> - bound (defenitley bound)
> def foo():
> a = 1
> print a
>
> Bound variables should be initialized to NULL
>
> - unbound (defenitley unbound)
> def foo():
> print a
> a = 1
>
> Unbound variables should be initialized to Py_None.
> And user may be warned (should depend on command line flags)
>
> - don't know (not sure here)
> def foo(x):
> if x:
> a = 1
> [else: # optional
> a = 2]
> print a
>
> Algo is too stupid it don't know much about branches, so it's not sure here.
Well, it *can't* know what will happen at runtime. That's not being stupid
at all, it's just the correct way to do it.
> This ones will be initialized to Py_None.
To be correct, they'd have to get initialised to NULL and a check needs to
get generated on read access as long as we don't know for sure if it has
been initialised or not. CPython raises an exception on unbound locals, so
Cython should do the same and should do it efficiently. When we get to the
point that we safely know which variables are being initialised and for
which only the runtime behaviour can tell us if they are or not, I think
it's fine to add the little performance penalty of checking for NULL in
exactly the unsafe cases.
> Also I would like to check for unused variables and unreachable code
> (this should be removed).
Some unreachable code is getting dropped during constant folding, usually
stuff like "if False: ...", but I agree that there's always more that can
be done. Think of this:
def x():
do_some_stuff()
return # disable rest of code, e.g. during debugging
unreachable_code()
Cython shouldn't bother generating dead code here (even if the C compiler
would drop it anyway).
Knowing where a function returns and if it has trailing code with a default
None return would also be good to know. It would be a step towards
inferring the return type of functions automatically.
Stefan
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