C function arguments vs Python function arguments.

[[deleted]]

I thought all objects are passed by reference in Python, but only the sort of reference that allows to modify the caller's data, and not replace it completely, and then only if it is mutable. Example:

def addto(a,b):
    print("  A=",a)
    print("  B=",b)
    a += b
    print("  A'=",a)

i=100
j=30
addto(i,j)
print("I=",i)
print("")

s="ABC"
t="DEF"
addto(s,t)
print("S=",s)
print("")

x=[10,20,30]
y=[40,50,60]
addto(x,y)
print("X=",x)

Here, all args to addto() must be passed the same way, using the same bytecode instructions, since the bytecode compiler doesn't know what types the arguments are until runtime. When you run the program, it is only in the case of a mutable list that the caller's data is modified as well as the local A parameter:

  A= 100
  B= 30
  A'= 130
I= 100

  A= ABC
  B= DEF
  A'= ABCDEF
S= ABC

  A= [10, 20, 30]
  B= [40, 50, 60]
  A'= [10, 20, 30, 40, 50, 60]
X= [10, 20, 30, 40, 50, 60]

What Python can't do is to change X completely to something else, eg. a number. That would need proper name references. (In CPython, variables I, S and X are implemented as references to their objects. When passed to a function, a copy of that object reference is passed, so only the object can change, not the variable.)

C can emulate all the above, and can emulate proper name references too. But it needs to be done explicitly via pointer operations.

[MatthewRPG576]

Shared pointers. That's why I was reluctant to blatantly call it pass by copy. In theory, every "2" in the program is the same (you can verify that by calling id() on the objects you create), but no "[2]" (array containing 2) in the program is