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global statements outside functions/methods should raise SyntaxError

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ezio.melotti at gmail

Nov 15, 2009, 11:55 AM

Post #1 of 6 (435 views)
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global statements outside functions/methods should raise SyntaxError

Python currently accepts global statements at the top level:
>>> global foo
>>>

Beside being a meaningless operation, this might lead unexperienced
user to make mistakes like:
>>> foo = 5
>>> global foo # make foo global
>>> def func():
... print foo # access the global foo
...
>>> func()
5

"global foo" should raise a SyntaxError, similarly to what already
happens with "return":
>>> return foo
File "<stdin>", line 1
SyntaxError: 'return' outside function

I opened an issue on the tracker (http://bugs.python.org/issue7329)
and Benjamin suggested to discuss this here.
The test he mentioned is in test_global.py:

def test4(self):
prog_text_4 = """\
global x
x = 2
"""
# this should work
compile(prog_text_4, "<test string>", "exec")

It just says that "it should work" but it doesn't say /why/.

Any thoughts?

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fuzzyman at voidspace

Nov 15, 2009, 11:59 AM

Post #2 of 6 (411 views)
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Re: global statements outside functions/methods should raise SyntaxError [In reply to]

Ezio Melotti wrote:
> Python currently accepts global statements at the top level:
>>>> global foo
>>>>
>
> Beside being a meaningless operation, this might lead unexperienced
> user to make mistakes like:
>>>> foo = 5
>>>> global foo # make foo global
>>>> def func():
> ... print foo # access the global foo
> ...
>>>> func()
> 5
>>>> # it works!
>
> "global foo" should raise a SyntaxError, similarly to what already
> happens with "return":
>>>> return foo
> File "<stdin>", line 1
> SyntaxError: 'return' outside function
>
>
> I opened an issue on the tracker (http://bugs.python.org/issue7329)
> and Benjamin suggested to discuss this here.
> The test he mentioned is in test_global.py:
>
> def test4(self):
> prog_text_4 = """\
> global x
> x = 2
> """
> # this should work
> compile(prog_text_4, "<test string>", "exec")
>
> It just says that "it should work" but it doesn't say /why/.

Well, personally I think it would be a good thing if this raised an
exception during bytecode compilation - but it would fall under the
moratorium and have to wait a few years.

On the other hand it should be easy to get PyLint to include a checker
for this and they are very responsive to feature requests.

All the best,

Michael

>
> Any thoughts?
>
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>

--
http://www.ironpythoninaction.com/
http://www.voidspace.org.uk/blog

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benjamin at python

Nov 15, 2009, 12:08 PM

Post #3 of 6 (411 views)
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Re: global statements outside functions/methods should raise SyntaxError [In reply to]

2009/11/15 Michael Foord <fuzzyman [at] voidspace>:
> Well, personally I think it would be a good thing if this raised an
> exception during bytecode compilation - but it would fall under the
> moratorium and have to wait a few years.

It could probably be considered a bug, though, since the global
statement in that case silently has absolutely no effect.

--
Regards,
Benjamin
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python at mrabarnett

Nov 15, 2009, 12:31 PM

Post #4 of 6 (406 views)
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Re: global statements outside functions/methods should raise SyntaxError [In reply to]

Benjamin Peterson wrote:
> 2009/11/15 Michael Foord <fuzzyman [at] voidspace>:
>> Well, personally I think it would be a good thing if this raised an
>> exception during bytecode compilation - but it would fall under the
>> moratorium and have to wait a few years.
>
> It could probably be considered a bug, though, since the global
> statement in that case silently has absolutely no effect.
>
It wouldn't be adding a new feature, but it would be changing the
behaviour of programs that currently have 'global' outside a function.

Perhaps, due to the moratorium, there should be a warning before it's
actually corrected.
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tjreedy at udel

Nov 15, 2009, 12:38 PM

Post #5 of 6 (406 views)
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Re: global statements outside functions/methods should raise SyntaxError [In reply to]

Ezio Melotti wrote:
> Python currently accepts global statements at the top level:
>
> I opened an issue on the tracker (http://bugs.python.org/issue7329)
> and Benjamin suggested to discuss this here.
> The test he mentioned is in test_global.py:
>
> def test4(self):
> prog_text_4 = """\
> global x
> x = 2
> """
> # this should work
> compile(prog_text_4, "<test string>", "exec")
>
> It just says that "it should work" but it doesn't say /why/.
>
> Any thoughts?

I make the same suggestion a couple of years ago, either this list or
Py3k list, after newby reported 'problem' on python-list expecting
module-level global to do something. Guido rejected it on the basis that
he wanted to minimized differences between module code and function code.

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guido at python

Nov 15, 2009, 12:46 PM

Post #6 of 6 (406 views)
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Re: global statements outside functions/methods should raise SyntaxError [In reply to]

On Sun, Nov 15, 2009 at 12:38 PM, Terry Reedy <tjreedy [at] udel> wrote:
> Ezio Melotti wrote:
>>
>> Python currently accepts global statements at the top level:
>>
>> I opened an issue on the tracker (http://bugs.python.org/issue7329)
>> and Benjamin suggested to discuss this here.
>> The test he mentioned is in test_global.py:
>>
>> � def test4(self):
>> � � � prog_text_4 = """\
>> global x
>> x = 2
>> """
>> � � � # this should work
>> � � � compile(prog_text_4, "<test string>", "exec")
>>
>> It just says that "it should work" but it doesn't say /why/.
>>
>> Any thoughts?
>
> I make the same suggestion a couple of years ago, either this list or Py3k
> list, after newby reported 'problem' on python-list expecting module-level
> global to do something. Guido rejected it on the basis that he wanted to
> minimized differences between module code and function code.

That example should work because you could pass it to exec()/eval()
with separate dicts for locals and globals:

$ python3.0
Python 3.0 (py3k:67506, Dec 3 2008, 10:12:04)
[GCC 4.0.1 (Apple Inc. build 5484)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> glo = {}
>>> lo = {}
>>> so = 'global x; x = 2'
>>> co = compile(so, '', 'exec')
>>> exec(co, glo, lo)
>>> glo['x']
['x']
>>> exec('x = 3', glo, lo)
>>> glo['x']
2
>>> lo['x']
3
>>>

--
--Guido van Rossum (python.org/~guido)
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