scope of function parameters (take two)

[Henry Olders]

On 2011-05-31, at 24:35 , Dan Stromberg wrote:

> 
> On Mon, May 30, 2011 at 5:28 PM, Henry Olders <henry.olders at mcgill.ca> wrote:
> 
> Be careful not to conflate global scoping or global lifetime, with mutability or pure, side-effect-free functions (callables).  It sounds like what you want is immutability and/or freedom from side effects, which is found most often in (pure) functional languages - which is not what Python is, nor does it attempt to be so.

I think you're right, I've been conflating scoping with side effects caused by passing mutable objects. 
> 
> In Python, and in many other languages, if you pass a scalar (or more generally, an object of an immutable type) to a function, and then change the scalar in that function, you only change the scalar within that function, not within the caller.
> 
> However, if you pass an aggregate type like a list (array) or dictionary (hash table), then the formal argument itself that you've passed is still only changeable within that function, however what it points off at _is_ changeable via that formal argument.  This is of course because otherwise passing a 1 gigabyte dictionary to a function would either have to copy the whole dictionary, or implement some sort of Copy-on-Write semantics, or make it somehow readonly.
> 
> If you need side effect-free functions in Python, you'd probably best copy your aggregates, EG:
> 
> import copy
> 
> def main():
>       a = ['a list','with','three elements']
>       print a
>       print fnc1(a)
>       print a
> 
> def fnc1(b):
>       b = copy.deepcopy(b)
>       return fnc2(b)
> 
> def fnc2(c):
>       c = copy.deepcopy(c)
>       c[1] = 'having'
>       return c

Clearly, making a copy within the function eliminates the possibility of the side effects caused by passing in mutable objects. Would having the compiler/interpreter do this automatically make python so much different? What if it were optional, like nested scopes were in python 2.1 until they became standard in 2.2?

Henry
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