Python - Deconstructing a list and making it lower case using .lower()

Question

Hi I am learning Python and for an assignment, I was asked to deconstruct a list and making it lower case using .lower() method. I am confused on how to change items in my list to lowercase using .lower and would like some help.

Here is my code, I was given this structure and had to fill out the missing areas:

Deconstructing the list

def destructure(lst):
  result=[]    
  for i in lst:
    result.extend(i)
  print(result)

Changing items to lowercase

???

Function

lc=[['ONE','TWO','THREE'],['FOUR','FIVE','SIX']]
destructure(lc)

Expected Output

['ONE', 'TWO', 'THREE', 'FOUR', 'FIVE', 'SIX']
['one', 'two', 'three', 'four', 'five', 'six']

I tried using:

lower = [x.lower() for x in lst]
print(lower)

But this wouldn't work.

Answer 1

Remember that strings are immutable. Thus, when you call str.lower() or str.upper() you'll need to [re]assign the returned value.

It appears that you want to flatten the list (of lists) and produce both upper- and lower-case versions.

You could do this:

def destructure(lst, func):
    rv = []
    for e in lst:
        rv.extend([func(s) for s in e])
    return rv

lc = [['ONE','TWO','THREE'],['FOUR','FIVE','SIX']]

print(destructure(lc, str.upper))
print(destructure(lc, str.lower))

Output:

['ONE', 'TWO', 'THREE', 'FOUR', 'FIVE', 'SIX']
['one', 'two', 'three', 'four', 'five', 'six']