Question

Input

[['1','2','3'],['a','b','c'],['6','7','8'],['e','f','g']]

Output should be:

1, 2, 3
a, b, c
6, 7, 8
e, f, g

Code:

def print_row(los):
    print ', '.join(los)


def print_table(los):
    lose = []
    if los  == []:
        return
    else:
        return print_row(los[0]) + print_table(los[1:]) 

Currently:

print_table([['1','2','3'],['a','b','c'],['6','7','8'],['e','f','g']])

Gives:

1, 2, 3
a, b, c
6, 7, 8
e, f, g

TypeError: unsupported operand type(s) for +: 'NoneType' and 'NoneType'

How do I fix this?

Answer 1

Your function returns None (an empty return statement). Return an empty string instead:

if los == []:
    return ''

You also have a lose definition there that is never used, and you can test for the empty list with not los. Last but not least, since return exits the function early, the else: statement is optional.

Next, print_row() returns None as well, since you don’t return anything from it. Simply discard it’s return value:

def print_table(los):
    if not los:
        return ''
    print_row(los[0])
    return print_table(los[1:]) 

Note that you never use the return value of print_table anyway, so you may as well not return anything; simply test if there is anything to print:

def print_table(los):
    if los:
        print_row(los[0])
        print_table(los[1:]) 

Now you can just inline the print_row function (which is just a one-liner):

def print_table(los):
    if los:
        print ', '.join(los[0])
        print_table(los[1:]) 

This is still recursive; Python isn’t that strong at recursion, but it can do looping very well:

def print_table(los):
    for sublist in los:
        print ', '.join(sublist)

which can be reduced to a join on newlines too:

def print_table(los):
    print '\n'.join(', '.join(sublist) for sublist in los)