When trying to use a function call to modify the value of a variable in the calling function, I observed the following difference in behavior when passing a list and set as function parameters.
#Passing list as parameter
def fun(a):
a = a.append(13)
a = [12]
print(a)
fun(a)
print(a)
#Output
[12]
[12, 13]
#Passing set as a parameter
def fun(a):
a = a.union({13})
a = {12}
print(a)
fun(a)
print(a)
#Output
{12}
{12}
My question is, why is the changes from the function carried over to the calling function when using a list but not when using set even though they are both of mutable data types?
The operations you are comparing are not equivalent.
alist.append alters the list at alist (and returns None).
aset.union creates a new set but doesn't modify the original.
If you did aset.add(13) that would be the set equivalent of alist.append(13) (adding a new element to an existing collection).
If you did alist = alist + [13] that would be the list equivalent of aset = aset.union({13}) (creating a new collection containing extra elements).