Why is a sublist mutable in a for loop?

Question

I am a beginner in python and I find this about mutabilty quite confusing and non intuitive. Given a list:

lst = [[1, 2, 3], [4, 5, 6]]

And trying to change the list within a for-loop.

for i in lst:
    i = "test"

I understand that this does not change the list. But:

for i in lst:
    i[1] = "test"

I was surprised that referring to the sublist led to following outcome:

[[1, 'test', 3], [4, 'test', 6]]

I tried to understand with a visualizer but I don't get it. Would anybody please explain this in plain words? Thank you.

Answer 1

Assignment (which is what the = operator does) assigns a value to a name.

i is a variable name in the local namespace. At the time it runs inside the for loop, it refers to a list. By assigning a string to it, you cause the name to no longer refer to a list, but instead refer to the string. The list is unaffected -- the only thing that changes is the value associated with the name.

i[1] is a name that specifies a specific location inside one of the two lists, depending on what i is set to at the time. By assigning a string to it, you cause the name to no longer refer to the object that previously occupied that space inside the list (an integer, 2 or 5 depending on the list) and instead refer to the string. The integer is unaffected -- the only thing that changes is the value associated with the name.

So in each case, assignment is doing the same thing -- it's making a name refer to a new thing instead of the old thing it referred to. The difference is that the second case is a special name in that it refers to a property of a mutable object.

Answer 2

In the first case, you simply have a copied reference to the element.

i ---> lst[n]

In the latter case, you are dereferencing the reference, and changing data (not in a copy):

i[i] ---> lst[n][i]

Therefore assigning to i[n] will point to the actual mutable object, not a copy of it.

Answer 3

for does not make copies of each element it yields. As such, the yielded object retains all the properties of the original (since it is the original), including mutability.

Answer 4

since for loop in case of string is called in different way like for i in lst:

it means if lst is list or array of string then i is referenced to the value of x[i] directly that is why in first case result was "test " two times because length of "lst" was just 2

while in second case i[1] means i ------lst[i] i[1]--------`-lst[i][1] means first value equal lst[0][1]---[1,'test',3] second value equal lst[1][1]--[4,'test',6]