On 5/29/2011 7:59 AM, Mel wrote:
Henry Olders wrote:
I just spent a considerable amount of time and effort debugging a program.
The made-up code snippet below illustrates the problem I encountered:
def main():
a = ['a list','with','three elements']
print a
print fnc1(a)
print a
def fnc1(b):
return fnc2(b)
def fnc2(c):
c[1] = 'having'
return c
This is the output:
['a list', 'with', 'three elements']
['a list', 'having', 'three elements']
['a list', 'having', 'three elements']
I had expected the third print statement to give the same output as the
first, but variable a had been changed by changing variable c in fnc2.
It seems that in Python, a variable inside a function is global unless
it's assigned. This rule has apparently been adopted in order to reduce
clutter by not having to have global declarations all over the place.
I would have thought that a function parameter would automatically be
considered local to the function.
Function *parameters* are names, the first *local names* of the function.
It doesn't make sense to me to pass a global to a function as a parameter.
You are right, in a way;-). Global *names* are just names. When you call
a function, you pass *objects* as *arguments*. Of course, you may refer
to the object by a global name to pass it, or you can pass a string
object that contains a global name.
It doesn't look like a question of local or global. fnc2 is passed a
container object and replaces item 1 in that container. You see the results
when fnc2 prints the object it knows as `c`, and you see again when main
prints the object it knows as `a`. Python doesn't pass parameters by
handing around copies that can be thought of as local or global. Python
passes parameters by binding objects to names in the callee's namespace. In
your program the list known as `a` in main is identically the same list as
the one known as `c` in fnc2, and what happens happens.
Right. Python has one unnamed 'objectspace'. It has many, many
namespaces: builtins, globals for each module, locals for each function
and class, and attributes for some instances. Each name and each
collection slot is associated with one object. Each object can have
multiple associations, as in the example above.
--
Terry Jan Reedy
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