[Python] Why doesnt the called function pick up the loop range variable? [closed]

Instead of getting binary numbers conversions of integers, I get 'none' as the output.

def dec_to_bin(n):
if n > 1: # recursively divide the number
dec_to_bin(n // 2)
global res
res += str(n % 2) # append the remainder (binary digit)

n = 17
print(n)
res = ''
print("dec_to_bin(17)=",dec_to_bin(17) )

for i in range(1, 10):
x = dec_to_bin(i)
print(i,": ",x)

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