[Solved] Question to understand solution of python challenge

Question

I just had the following code in a python challenge:

What ist the output of this code?
x = [1, 2, 3]
def func(x):
a = 42
x[1] = 42
x = a
func(x)
print(x)

I assumed the output to be 42, because the function says x = a and a is 42. But the correct answer was [1, 42, 3]. Where was my mistake?

Answer 1

Alexander ,

x has the same id from defining the list in main frame, up to and including x[1]=42 in the function. this means that x in main frame and in function are pointing to the same memory location / object.

when x = a in function is executed, this is changjng. x in function will get a new id, but x in main frame will keep its original id. this is the reason why the result is [1, 42, 3]

you can run the code to see this:
https://code.sololearn.com/cIK11r8TuVBe/?ref=app

Answer 2

Oh sorry didn't notice that line.
Okay, I am not sure, but I think,
"You can modify the value of list inside the function but you cannot change the whole list"
when you give x[1]=54
it changes the value inside list . But when you write x=a it changes the whole list it doesn't work for the list declared outside the function.It only changes the value of local variable x(declared as a parameter) inside the function.So when you print(x) it prints [1, 42, 3].

Answer 3

I added some variable identification printouts in the code. You'll see different value returned from id(x) inside func(), which means the <x> as function argument and the <x> assigned value 42 (as local variable) are two different thing.

x = [ 1, 2, 3 ]
def func( x ):
print( 'inside func() id(x) = ', id( x ) )
a = 42
x[ 1 ] = 42
x = a
print( 'inside func() id(x) = ', id( x ) )
print( 'outside func() id(x) = ', id( x ) )
func( x )
print( x )

Answer 4

Thanks, I got this point. But I struggle with this line of code within the function:

x = a

Why will the list not be overwritten by the value of a?

Answer 5

Thank all of you! The hint of lokal and global variables helped me much :-) I looked up some more details and this was also helpful for me to go deeper: https://www.datacamp.com/community/tutorials/scope-of-variables-JUMP_LINK__&&__python__&&__JUMP_LINK

I also learned, that I can ask for the IDs of the variables to verify the different objects with the id() function :-)