On Saturday, October 13, 2012, Cameron Simpson wrote:
> I'm having some trouble with closures when defining a decorator.
<snip>
> However, I can't make my make_file_property function work. I've stripped
> the code down and it does this:
<snip>
> Traceback (most recent call last):
> File "foo.py", line 21, in <module>
> def f(self, foo=1):
> File "foo.py", line 4, in file_property
> return make_file_property()(func)
> File "foo.py", line 10, in made_file_property
> if attr_name is None:
> UnboundLocalError: local variable 'attr_name' referenced before
> assignment
>
> Observe above that 'unset_object' is in locals(), but not 'attr_name'.
> This surprises me.
>
> The stripped back code (missing the internals of the file property
> watcher) looks like this:
>
> import sys
>
> def file_property(func):
> return make_file_property()(func)
>
> def make_file_property(attr_name=None, unset_object=None, poll_rate=1):
> print >>sys.stderr, "make_file_property(attr_name=%r, unset_object=%r,
> poll_rate=%r): locals()=%r" % (attr_name, unset_object, poll_rate,locals())
> def made_file_property(func):
You're missing a "nonlocal" declaration here.
print >>sys.stderr, "made_file_property(func=%r): locals()=%r" %
> (func, locals())
> if attr_name is None:
> attr_name = '_' + func.__name__
You assign to it, but there's no nonlocal declaration, so Python thinks
it's a local var, hence your error.
Pardon my brevity and some lack of trimming; I'm on a smartphone and in a
rush.
- Chris
lock_name = attr_name + '_lock'
> def getprop(self):
> with getattr(self, lock_name):
> # innards removed here
> pass
> return getattr(self, attr_name, unset_object)
> return property(getprop)
> return made_file_property
>
> @file_property
> def f(self, foo=1):
> print "foo=%r" % (foo,)
>
> @make_file_property(attr_name="_blah")
> def f2(self, foo=2):
> print "foo=%r" % (foo,)
>
> Can someone explain what I'm doing wrong, or tell me this is a python
> bug?
> --
> Cameron Simpson <c...@zip.com.au <javascript:;>>
>
> Bolts get me through times of no courage better than courage gets me
> through times of no bolts!
> - Eric Hirst <e...@u.washington.edu <javascript:;>>
> --
> http://mail.python.org/mailman/listinfo/python-list
>
--
Cheers,
Chris
--
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