Help with university work - reading and writing files

[MrKnd94]

Hello,

Me and a lot of my classmates are so confused in today's work.

The answer sheet was published, but that didn't help us at all.

Here's what they want:

Write a Python function named extract_temp that is given a line read from a text file and displays the one number (integer) found in the string:
'The high today will be 15 degrees' → 15.

Here's my code (helped by the answer sheet):

# -----------------------------------------------------------------------------------------------------------------------
def extract_temp(s):
    for n in range(0,len(s)):
        if s[n].isdigit():
            ss = s[n:]
            for n1 in range(0,len(ss)):
                if not ss[n1].isdigit:
                    snum = s[n:n+1]
                    num=int(snum)
                    return num
# -----------------------------------------------------------------------------------------------------------------------

input_file = open('ExtractTemp.txt','r')

line = input_file.readline()

number = extract_temp(line)

print(line.strip(), "-", number)

The output:

The high today will be 15 degrees - None

Apparently, we just want the number "15" outputted, with None I think. I'm not quite sure.

Any help would be greatly appreciated.

Thank you!