List comprehension in Python outputs None for function string evaluation

Question

Trying to pass a list in Python through a function I have defined, however when I run the code I get 'None' as the output.

For reproducibility I'll use a simple example in which I want to make all words in a string uppercase.

def upcase(x):
    print(x.upper())

upcase("word")

This of course gives:

WORD

But now, if I attempt to apply my function to an entire string, as in the following:

randoms = ['chicken', 'window', 'carpet', 'lampshade', 'crosswalk']

randoms_upcased = [upcase(x) for x in randoms]
print(randoms)
print(randoms_upcased)

I get:

CHICKEN
WINDOW
CARPET
LAMPSHADE
CROSSWALK
['chicken', 'window', 'carpet', 'lampshade', 'crosswalk']
[None, None, None, None, None]

Whereas the output I desire is this.

['chicken', 'window', 'carpet', 'lampshade', 'crosswalk']
['CHICKEN', 'WINDOW', 'CARPET', 'LAMPSHADE', 'CROSSWALK']

Is this possible? Any help gratefully receieved.

Answer 1

Your upcase function prints out the result, but it doesn't return anything. Since you aren't specifying a return value, it defaults to returning None . You'll want to add an explicit return:

def upcase(x):
    x = x.upper()
    print(x)
    return x

Answer 2

I think it is because the function upcase(s) is made to just print the string s in uppercase but it does not return anything. It should work if you return the uppercase string from the function.

Answer 3

You need to return the string you are printing.

This should work:

def upcase(x):
    return (x.upper())

Answer 4

First store result in another list( z ) then return ( z ) in the function:

def upcase(x):
    z = ([y.upper() for y in x])
    return z
l1 = ['chicken', 'window', 'carpet', 'lampshade', 'crosswalk']
upcase(l1)

output:

['CHICKEN', 'WINDOW', 'CARPET', 'LAMPSHADE', 'CROSSWALK']