Mutating a python incoming object by reference

Question

I have a legit situation like this where I need to implement a method that changes the incoming object itself (convert list into dict)

def f(l): # f is a function that receives a list as input
    pass # Implement this


x = ['Hello', 'World']
f(x)
print(x) # x is now a dict {'hello':'world'}

How will you implement the above f() ?

The key thing is upon calling f() the incoming object itself must be mutated into different type.

Note: Genuine answers welcome instead of questioning the situation.

Answer 1

Python does not have a way to do what you want. You cannot change the binding of a variable in the calling context from within a function.

Of course, "cannot" has to be qualified: Python has many introspection capabilities, even letting you examine the calling stack, and the globals and locals in calling frames. But I strongly recommend against going that way.

I know you said you didn't want people to question the need for this, but I can't help thinking there is a better way to solve the problem.

Answer 2

I dont think this is possible as you can modify a mutable when you pass it and have it reflect the changes in the global scope but you cannot change what it references.

def f(x):

    z = dict()
    for i in range(0,len(x),2):
        z[x[i]] = x[i+1]

    # x = z
    x.append(1)
    return x


x=[1,2]
z=f(x)
print(z) #[1, 2, 1]
print(x) #[1, 2, 1]

But

def f(x):

    z = dict()
    for i in range(0,len(x),2):
        z[x[i]] = x[i+1]

    x = z
    return x


x=[1,2]
z=f(x)
print(z) #{1: 2}
print(x) #[1, 2]

Python allows you to modify the global scope of mutable variables but you cannot change the reference itself.