On 2011-05-31, at 24:35 , Dan Stromberg wrote: > > On Mon, May 30, 2011 at 5:28 PM, Henry Olders <henry.old...@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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