[Tutor] Globals?

[Kent Johnson]

I think the idea is that x in the enclosing scope should be changed when 
_ is called. To closure purists :-) a language doesn't have closures if 
it can't do this.

Python will bind the *values* of variables in an enclosing scope into a 
closure, which is good enough for many uses :-)

Personally I haven't found much need for 'true' closures. orbitz, can 
you say why this is such a pain? How would you use a true closure?

A workaround is to store the variable in a list, but it's a bit ugly:

x = [3]
def fn(x):
     def _():
        x[0] += 1
        return x[0]
     return _

Kent

Lloyd Kvam wrote:
> It DOES work in current versions of Python, exactly as you coded it.
> 
> In older Pythons (e.g. 1.52) you would have had to specifying the
> enclosing variables explicitly
> 	def _(x=x):
> 
> A lot of old critiques are still present on the web.  wikipedia has a
> terrific article describing Python that is current (at least 2.3.3).
> 
> http://en.wikipedia.org/wiki/Python_programming_language
> 
> 
> 
> On Sat, 2004-11-13 at 14:07, orbitz wrote:
> 
>>In my opinion, this behavior really sucks too. Like when it comes to 
>>closures.  As far as I know, Python does not *really* support closures, 
>>like you would get with something like lisp. Correct me if I'm wrong.  
>>This means code like:
>>
>>def fn(x):
>>    def _():
>>       x += 1
>>       return x
>>    return _
>>
>>Will not work, which can be a pain in the ass.
>>
>>
>>Kent Johnson wrote:
>>
>>
>>>Liam,
>>>
>>>When you make any assignment to a variable inside a function, Python 
>>>assumes that the variable is local to the function. Then any use of 
>>>the variable before it's first assignment is an error.
>>>
>>>To force a variable in a function to be global, put a 'global' 
>>>statement in the function. You need to add
>>>  global badConnectCycle
>>>to your function getReturns
>>>
>>>If you don't make any assignment to a variable, then the global 
>>>(module) namespace is searched. That is why badUserList works fine - 
>>>you never assign it, you just access the list methods.
>>>
>>>Kent
>>>
>>>Liam Clarke wrote:
>>>
>>>
>>>>Hi all,
>>>>Having trouble with something, it is 3:30am in the morning, so this
>>>>may be a dumb problem, if so, I apologise.
>>>>
>>>>In my prog, there's two variables created right at the get go -
>>>>
>>>>import imaplib
>>>>import email.Parser
>>>>import os
>>>>import os.path
>>>>import datetime
>>>>import md5
>>>>from pause import *
>>>>
>>>>badUserList=[]
>>>>badConnectCycle=0
>>>>
>>>>as I want them to be global within this module, so that another module
>>>>can pick them out easily.
>>>>
>>>>Now, I just added badConnectCycle, badUserList has been there awhile,
>>>>and it's used in
>>>>
>>>>the function connectToImap which is called by getReturns which is
>>>>called by main(), and my other module can get it no problem, so
>>>>badUserList is fine.
>>>>
>>>>badConnectCycle... is giving me errors -
>>>>
>>>>badConnectCycle is used in getReturns, as so -
>>>>if session == "NoConnect" :             badConnectCycle += 1
>>>>            continue
>>>>
>>>>
>>>>function getReturns ends as follows -
>>>>if badConnectCycle == len(user) and badConnectCycle > 0: return 
>>>>("NoConnect","")
>>>>if badUserList and not sender: return ('NoLogin',"")
>>>>if matchindex[0] and not sender: return ('NoNew', '')
>>>>if not sender: return ("NoMatch","")
>>>>return (sender, msgdata)
>>>>
>>>>and it's at that line
>>>>
>>>> if badConnectCycle == len(user) and badConnectCycle > 0:
>>>>
>>>>that I get this error:
>>>>
>>>>UnboundLocalError: local variable 'badConnectCycle' referenced before
>>>>assignment.
>>>>
>>>>Which is confusing me because badUserList is used within a function
>>>>called by getReturns, and I've had no problem with it.
>>>>
>>>>Help anyone? Much appreciated if you can.
>>>>
>>>>Regards,
>>>>
>>>>Liam Clarke
>>>
>>>_______________________________________________
>>>Tutor maillist  -  Tutor at python.org
>>>http://mail.python.org/mailman/listinfo/tutor
>>>
>>
>>_______________________________________________
>>Tutor maillist  -  Tutor at python.org
>>http://mail.python.org/mailman/listinfo/tutor