Question

I have to build a function for a grocery list from a .txt file, which looks like this:

milk
cheese

bread
hotdog buns

chicken
tuna
burgers

and so on. From the list above, my grocery list should look like [['milk', 'cheese'], ['bread', 'hotdog buns'], ['chicken', 'tuna', 'burgers']], so a list of lists in which items are separated when there is a space between them in the text file.

I have to use .readline(), and I can’t use .readlines(), .read(), or the for loop. My code right now creates an empty list:

def grocery_list(foods):
    L = open(foods, 'r')
    food = []
    sublist = []
    while L.readline() != '':
        if L.readline() != '\n':
            sublist.append(L.readline().rstrip('\n'))
        elif L.readline() == '\n':
            food.append(sublist)
            sublist = []
    return food

I don’t know where it goes wrong so it returns a fully empty list. I’m also not sure about the '' and '\n' part; the example test file I’m using, when opened in the shell, looks like this:

milk\n
cheese\n
\n
...
''
''

but does .rstrip() or the whole != '' make sense for every list? Or am I just not even on the right track?

Answer 1

One problem is that you aren’t adding the final sublist to the result. As @Xymostech mentioned, you need to capture the result of each call to readline() as the next call will be different. Here is how I would modify your code.

def grocery_list(foods):
    with open(foods, 'r') as L:        
        food = []            
        sublist = []            

        while True:
            line = L.readline()
            if len(line) == 0:
                break

            #remove the trailing \n or \r
            line = line.rstrip()

            if len(line) == 0:
                food.append(sublist)
                sublist = []                    
            else:
                sublist.append(line)
        if len(sublist) > 0:
            food.append(sublist)

        return food

Pay attention to the use of the with statement. This ensures that the file is closed after it is no longer needed.