Changing argument value

[Bruno Desthuilliers]

flyfree a écrit :
>>>>def fooA(y):
> 
> 	y = [3,4]
> 	return y
> 
> 
>>>>def fooB(y):
> 
> 	y[0] = 3
> 	y[1] = 4
> 	return y
> 
> 
>>>>x = [1,2]
>>>>fooA(x)
> 
> [3, 4]
> 
>>>>x
> 
> [1, 2]
> 
> 
>>>>fooB(x)
> 
> [3, 4]
> 
>>>>x
> 
> [3, 4]
> ===============
> 
> From above, the original argument value of fooA is same as before
> [1,2]
> but the original argument value of fooB is changed from [1,2] to
> [3,4].
> 
> What is the difference between "y = [3,4]" and "y[0]=3  
> y[1] =4 "

In the first case, you rebind the local name y to a new list object - 
and since the name is local, rebinding it only affects the local 
namespace. In the second case, you mutate the list object (bound to the 
local name y) passed to the function. The point to remember is that in 
Python, all variables are references to objects, but names (including 
params) are local.

HTH