5

As I learn Python I have encountered some different styles. I am wondering what the difference between using "else" is as opposed to just putting code outside of the "if" statement. To further explain my question, here are two blocks of code below. 

x = 5
if x == 5:
    return True
else:
    return False

I understand that this is returning False if x != 5, but how does this code below contrast to the code above? Is it the exact same thing, or is there a slight difference? Is there a benefit of using one over the other?

x = 5
if x == 5:
    return True
return False

2 Answers2

6

In your code, there isn't any difference, because the if part ends with a return if the condition is true, and the code will exit anyway. And if the condition is false, the else branch will be executed, also ending with a return, so the else is not required.

It's a matter of style, but it can be argued that the second option is preferred, IMHO is simpler to read and with less rightward drift - in fact some languages/compilers will flag it with a warning, because the else would be unnecessary.

The key point here is that when both branches of a conditional end with a return then the else is not mandatory. But if that's not the case, then you need to use else, otherwise you'll end up executing code that was not intended. For example, here you can not remove the else:

if n > 10:
  n = 1
else:
  n *= 10
Óscar López
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  • So then is there any reason to use else at all? –  Jun 17 '17 at 19:23
  • In this case no, but if you weren't _returning_ from both branches, then using `else` would be mandatory. – Óscar López Jun 17 '17 at 19:23
  • Thank you, I just noticed that if you try using a string without "else" it will print the string and then "None". I'm assuming that leaving out "else" would be only boolean specific then. –  Jun 17 '17 at 19:31
  • It's not "boolean specific", but I'd have to see an example to understand what you did ;) – Óscar López Jun 17 '17 at 19:33
  • If I make an "if" statement and replace the "return True/False" with "print('hi')/bye" then it would print hi/bye and then "None" in addition –  Jun 17 '17 at 19:35
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    I guess that's because you're printing the value returned by the function, but my answer still applies - it you don't end with a return on both branches, then you'll get different (often erroneous) results if you omit the `else`. – Óscar López Jun 17 '17 at 19:39
5

There is a very slight difference, but it is one you wouldn't actually care about. Given these two functions:

def f1():
    x = 5
    if x == 5:
        return True
    else:
        return False

def f2():
    x = 5
    if x == 5:
        return True
    return False

look at the byte code resulting from each:

>>> dis.dis(f1)
  4           0 LOAD_CONST               1 (5)
              2 STORE_FAST               0 (x)

  5           4 LOAD_FAST                0 (x)
              6 LOAD_CONST               1 (5)
              8 COMPARE_OP               2 (==)
             10 POP_JUMP_IF_FALSE       16

  6          12 LOAD_CONST               2 (True)
             14 RETURN_VALUE

  8     >>   16 LOAD_CONST               3 (False)
             18 RETURN_VALUE
             20 LOAD_CONST               0 (None)
             22 RETURN_VALUE
>>> dis.dis(f2)
 11           0 LOAD_CONST               1 (5)
              2 STORE_FAST               0 (x)

 12           4 LOAD_FAST                0 (x)
              6 LOAD_CONST               1 (5)
              8 COMPARE_OP               2 (==)
             10 POP_JUMP_IF_FALSE       16

 13          12 LOAD_CONST               2 (True)
             14 RETURN_VALUE

 14     >>   16 LOAD_CONST               3 (False)
             18 RETURN_VALUE

For the first function, Python still generates a pair of instructions for the unreachable implied return None.

chepner
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