Why does my number reverser function produce an infinite loop - python-3.x

Question

Whenever i try print the number reverser function i always get an infinite loop,instead of my expected output 54321. Can someone help find the problem? Thanks.
def order(num):
x=str(num)
if x==False:
return None
else:
return order(x[1:])+(x[0])
print (order(12345))

Answer 1

Welcome to your community.
There are some problems with your code:
First:
A string never will be equal to False.
for instance:
'0' is not equal to False.
'' is not equal to False.
Second:
You cannot add a String to None.
This Error will be thrown: TypeError: can only concatenate str (not "NoneType") to str
Modify your code like this:
def order(num):
x=str(num)
if x=='':
return ''
else:
return order(x[1:])+(x[0])
print (order(12345))
Tip 1: A string can be equal to '' (empty string).

Answer 2

In your function, you compare the string x with the boolean False. This is not a correct way to test whether string x is an empty string or not.
In addition, if string x is empty, then you shouldn't return None, but an empty string: reversing an empty string should logically return an empty string.
Here I present two ways to fix your function, which I implemented under the names reverse0 and reverse1. I also present a few other alternatives to achieve the same result using python features, under the names reverse2 to reverse6. Finally I present three other ways to reverse nonnegative integers, under the names reverse7 to reverse9.
def reverse0(num):
x = str(num)
if len(x) == 0:
return ''
else:
return reverse0(x[1:]) + x[0]
def reverse1(num):
x = str(num)
if not x:
return ''
else:
return reverse1(x[1:]) + x[0]
def reverse2(num):
return str(num)[::-1]
def reverse3(num):
return ''.join(reversed(str(num)))
def reverse4(num):
x = str(num)
return ''.join(x[len(x)-1-i] for i in range(len(x)))
def reverse5(num):
x = str(num)
return ''.join(x[-i] for i in range(1, len(x)+1))
def reverse6(num):
y = ''
for c in str(num):
y = c + y
return y
# reverse7 only works for nonnegative integers
def reverse7(num):
if num < 10:
return str(num)
else:
return str(num % 10) + reverse7(num // 10)
# reverse8 only works for nonnegative integers
def reverse8(num):
l = []
while num > 9:
l.append(num % 10)
num = num // 10
l.append(num)
return ''.join(str(d) for d in l)
# reverse9 only works for nonnegative integers
# reverse9 returns an int, not an str
def reverse9(num):
y = 0
while num > 0:
y = 10 * y + (num % 10)
num = num // 10
return y
Relevant documentation:
builtin reversed;
str.join;
An informal introduction to string slices such as x[::-1].