What is the logic behind the output of my python code?

In your code

data = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]]

subsequently data[0], that is passed to the function as m, is

[[1, 2], [3, 4]]

then

v = m[0][0]

is

1

Then your loop

for row in m:
    for element in row:
        if v < element: v = element

Here you're iterating over [[1, 2], [3, 4]] , and then over each element in nested list [1, 2] and then [3, 4], and trying to compare each number in the list to 1, so v becomes 2, then 3 and then 4, so upon the code prints

4

Just running the code print

4

Line by Line :

data = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]]

We have a list of list of list (depth 3)

so : fum(data[0]) = fum([[1, 2], [3, 4]])

v = data[0][0][0] = 1

for row in data[0] means we iterate in the elements of [[1, 2], [3, 4]], so first element is [1,2] second element is [3,4]

for element in row means we will iterate in the elements of row so : element = 1, then 2, 3, 4

if v < element: v = element

Just means v takes the value of the max value of (v, element)

So this functions takes the first element of the list data and returns the max value of the n lists of m elements contained by the first element of data.

First, let's go over what's being passed to fun():

print(data[0])  # This outputs the first item in 'data[]'
# Output: [[1,2], [3,4]]

Now let's break down the first line of the function. We have v=m[0][0].

print(m)
# [[1,2], [3, 4]] (What we passed to the function)

print(m[0])
# [1,2] (The first item in the list)

print(m[0][0])
# 1 (The first item of the the first item in the original list)

So this line is just getting the very first number in the nested list.

Next, the for loops:

for row in m:
    print(row)

# [1,2] (iter 1)
# [3,4] (iter 2)

This is just cycling through all the list items in the list that was passed to the function.

Then, for each of those cases, we're iterating through each number:

for row in m:
   for element in row:
       print(element)
# 1
# 2
# 3
# 4

Lastly, the if statement is comparing the current element (1, 2, 3, and then 4) to the current value of v (which starts off as 1). If v is less than the current element, we replace v with the current element.

So, for example, in the first iteration of the loop, we compare:

if v < element (if 1 < 1)

Which is false, so we move on to the next loop, where we compare:

if 1 < 2

This is True, so we execute the next line which states v = element (or v = 2)

So on the next loop, we are now comparing:

if 2 < 3

Which is True, so v will again be assigned the value of element

At the end of both for loops, we end up returning the largest value of m, in this case: 4