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Python Language Argument passing and mutability


Example

First, some terminology:

  • argument (actual parameter): the actual variable being passed to a function;
  • parameter (formal parameter): the receiving variable that is used in a function.

In Python, arguments are passed by assignment (as opposed to other languages, where arguments can be passed by value/reference/pointer).

  • Mutating a parameter will mutate the argument (if the argument's type is mutable).

    def foo(x):        # here x is the parameter
        x[0] = 9       # This mutates the list labelled by both x and y
        print(x)
    
    y = [4, 5, 6]
    foo(y)             # call foo with y as argument
    # Out: [9, 5, 6]   # list labelled by x has been mutated
    print(y)           
    # Out: [9, 5, 6]   # list labelled by y has been mutated too
    
  • Reassigning the parameter won’t reassign the argument.

    def foo(x):        # here x is the parameter, when we call foo(y) we assign y to x
        x[0] = 9       # This mutates the list labelled by both x and y
        x = [1, 2, 3]  # x is now labeling a different list (y is unaffected)
        x[2] = 8       # This mutates x's list, not y's list
      
    y = [4, 5, 6]      # y is the argument, x is the parameter
    foo(y)             # Pretend that we wrote "x = y", then go to line 1
    y
    # Out: [9, 5, 6]
    

In Python, we don’t really assign values to variables, instead we bind (i.e. assign, attach) variables (considered as names) to objects.

  • Immutable: Integers, strings, tuples, and so on. All operations make copies.
  • Mutable: Lists, dictionaries, sets, and so on. Operations may or may not mutate.
x = [3, 1, 9]
y = x
x.append(5)    # Mutates the list labelled by x and y, both x and y are bound to [3, 1, 9]
x.sort()       # Mutates the list labelled by x and y (in-place sorting)
x = x + [4]    # Does not mutate the list (makes a copy for x only, not y)
z = x          # z is x ([1, 3, 9, 4])
x += [6]       # Mutates the list labelled by both x and z (uses the extend function).
x = sorted(x)  # Does not mutate the list (makes a copy for x only).
x
# Out: [1, 3, 4, 5, 6, 9]
y
# Out: [1, 3, 5, 9]
z
# Out: [1, 3, 5, 9, 4, 6]