longqian9...@gmail.com wrote:
> In pyhton 3.1, I found the following code will succeed with argument 1
> to 4 and fail with argument 5 to 9. It is really strange to me. I
> suspect it may be a buy in exec() function. Does anyone have some idea
> about it? Thanks.
>
>
> t1="""
> class foo:
> def fun():
> print('foo')
> def main():
> global foo
> foo.fun()
> main()
> """
> t2="""
> class foo:
> def fun():
> print('foo')
> def main():
> foo.fun()
> main()
> """
>
> import sys
> import copy
> if sys.argv[1]=='1':
> exec(t1)
> elif sys.argv[1]=='2':
> exec(t2)
> elif sys.argv[1]=='3':
> exec(t1,{},{})
> elif sys.argv[1]=='4':
> exec(t2,globals(),locals())
> elif sys.argv[1]=='5':
> exec(t2,{},{})
> elif sys.argv[1]=='6':
> exec(t2,globals(),{})
> elif sys.argv[1]=='7':
> exec(t2,{},locals())
> elif sys.argv[1]=='8':
> exec(t2,copy.copy(globals()),locals())
> elif sys.argv[1]=='9':
> exec(t2,globals(),copy.copy(locals()))
There are only two cases that matter: identical local/global namespaces and
distinct local/global namespaces:
>>> code = """\
... x = 42 # put x into the local namespace
... def f():
... print(x) # look up x in the global namespace
... f()
... """
>>> exec(code, {}, {})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<string>", line 4, in <module>
File "<string>", line 3, in f
NameError: global name 'x' is not defined
>>> ns = {}
>>> exec(code, ns, ns)
42
Also note that
>>> globals() is locals()
True
on the module level.
Peter
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