AND/OR operator between numbers in Python

Question

How do and/or operators work between numbers (or even strings)?

5 and 7  # this returns 7
7 and 5  # this returns 5
21 and 4 # this returns 4
4 and 21 # this returns 21

How does the and operator work in such cases?

And [the documentation](https://docs.python.org/3/reference/expressions.html#boolean-operations) doesn't help you? — Some programmer dude, Dec 07 '17 at 08:34
It evaluates to the first "false" operand. If both operands are "true", it evaluates to the last operand. — Tom Karzes, Dec 07 '17 at 08:36
@Someprogrammerdude Thanks! I missed searching the documentation. — Youssef13, Dec 07 '17 at 08:40

score 4 · Accepted Answer · answered Dec 07 '17 at 08:41

AND/OR :
5 and 7 #7
It check 5 first and found it is 'True' so it check for second also and return last one.

0 and 5 # 0
it check for first and found is zero so whatever will be next value it is always zero so it return 0

2 or 3 and 4 #2
first value is 2 ('True') and found OR So value is always true so return 2

(2 or 3) and 4 #4
now return 4.

T C Molenaar · Answer 2 · 2017-12-07T09:06:07.047

2

It returns the last value which is True (if there is any True value).

For an and python needs to check every value, so if they are all true then it returns the last one, if one of them is False, then it returns that value. If all of them are False it returns the first one because Python doesn't need to check the second one as well.

For an or python checks first the first one until he gets True sometime, so if the first value is True it returns this value, if the first one is False but the second one is True, it returns the second one. If they are all False it returns the last one.

The values which are equivalent to False: "",0,False,0j, 0.0, [], {}, set().

edited Dec 07 '17 at 09:06

answered Dec 07 '17 at 08:56

T C Molenaar

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also `0j`, `0.0`, `[]`, `{}`, `set()`, and any class explicitly returning False when used as bool. – viraptor Dec 07 '17 at 09:04
I will add them, but since @Yoessef13 only asked for numbers and strings I had only these – T C Molenaar Dec 07 '17 at 09:05

Guillaume · Answer 3 · 2017-12-07T08:38:01.797

1

When and is used with integers, it will give you the last value if all value are non-zero, or zero if there's at least one.

1 and 2 and 3 #3
1 and 0 and 3 #0

edited Dec 07 '17 at 08:38

answered Dec 07 '17 at 08:36

Guillaume

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Incorrect. Try `"" and 6`. You believe it is zero. It is not. – Tom Karzes Dec 07 '17 at 08:36
heh... OP asked about integers :) – Guillaume Dec 07 '17 at 08:37
The question mentioned integers as a specific case, but was more general than that: "My question is how and/or operators work between numbers ( or even strings )". `""` is a string. – Tom Karzes Dec 07 '17 at 08:39

score 0 · Answer 4 · answered Dec 07 '17 at 08:51

Let's disassemble!

>>> import dis
>>> def check_and(a, b):
...   return a and b
... 
>>> dis.dis(check_and)
  2           0 LOAD_FAST                0 (a)
              3 JUMP_IF_FALSE_OR_POP     9
              6 LOAD_FAST                1 (b)
        >>    9 RETURN_VALUE

Now it's pretty straightforward. 1. Load a to stack (It becomes TOS - Top of stack value. 2. JUMP_IF_FALSE_OR_POP tries to evaluate boolean value of TOS (Top Of Stack). If it's true, remove this value from stack, if it's not, set bytecode counter (in our case it would junmp to line 9. 3. Push b to stack. 4. Return.

I can't really tell why the evaluation order is different in the other case but as you can see the second evaluated value will be returned if first one is True.

Same happens to or and other operators. try to disassemble them yourself.

This comes handy if you want to make a quick check. Instead of writing:

def check(value)
  if value:
    return value
  else:
    return "No value"

You can write:

def check(value):
  return value or "No value"

AND/OR operator between numbers in Python

4 Answers4