function scope

MRAB google at mrabarnett.plus.com
Tue Feb 3 01:13:27 CET 2009


Baris Demir wrote:
 > Hi everybody,
 >
 > I am quite new to python and using it for my thesis.  Luckily I found
 > out some kind of behavior surprising to me and so unwanted in my code. I
 > could not find any explanation, so solution for the code.
 > It is simply like this:
 >
 > /*li = another_module.global_variable
 > f=simpleCut(li)
 >
 > def simpleCut(d=dict()):
 >       temp=d
 >       for i in temp.keys():
 >           if    (temp[i] == .......) :
 >              temp[i]=new_value
 > return temp
 > */
 > here /*simpleCut(d=dict())*/ is a function that changes the value of the
 > key in the d if it satisfies conditions and returns the new dictionary.
 > what i do not understand is, when i checked the original dictionary
 > /*li,*/ i also found out that, the value of that key(operated in the if
 > statement) had also been changed. so, simply, the simpleCut function
 > also changes the variables of its own argument, even there is a 'temp'
 > variable. That is a scandal for my code:)
 >
 > please tell me, if this is not possible and i am missing another thing
 > in my code, or if this kind of behavior is possible, what is the
 > philosophy of it, and how can i change it???
 >
It's called "Reference Semantics".

"d" refers to a dict. When you say "temp=d" it makes "temp" also refer
to that dict; it doesn't copy the dict. If you want a copy when you have
to do so explicitly with "temp=d.copy()".

When you pass "li" into simpleCut() you're not making a copy either!



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