What logic is followed when there are multiple "and/or" statements in a line of code?

Question

In the following piece of code,

def makes_twenty(a,b):
     return a+b==20 or a==20 and b==20

Why does this input return False[makes_twenty(20,2)] while the following input returns True[makes_twenty(20,0)]?

Shouldn't the second input return False as b==20 is not being satisfied? What logic/concept am I missing here?

A good term to know here is `operator precedence`. See also the docs: https://docs.python.org/3/reference/expressions.html#operator-precedence — kutschkem, Oct 12 '22 at 11:03

score 2 · Accepted Answer · answered Oct 12 '22 at 10:58

You can always ask Python to print out how it parses your code using the ast (abstract syntax tree) module:

$ echo 'a+b==20 or a==20 and b==20' | python3 -m ast
Module(
   body=[
      Expr(
         value=BoolOp(
            op=Or(),
            values=[
               Compare(
                  left=BinOp(
                     left=Name(id='a', ctx=Load()),
                     op=Add(),
                     right=Name(id='b', ctx=Load())),
                  ops=[
                     Eq()],
                  comparators=[
                     Constant(value=20)]),
               BoolOp(
                  op=And(),
                  values=[
                     Compare(
                        left=Name(id='a', ctx=Load()),
                        ops=[
                           Eq()],
                        comparators=[
                           Constant(value=20)]),
                     Compare(
                        left=Name(id='b', ctx=Load()),
                        ops=[
                           Eq()],
                        comparators=[
                           Constant(value=20)])])]))],
   type_ignores=[])

The tree is a bit verbose, but you can see the topmost operation is OR, followed by a nested AND, i.e. Python parses your expression as

(a+b==20) or (a==20 and b==20)

(And, of course, the precedence order is well documented).

score 0 · Answer 2 · answered Oct 12 '22 at 10:58

0

makes_twenty(20,0) returns true because the first condition is true; This is the execution order: a+b==20 or (a==20 and b==20)

answered Oct 12 '22 at 10:58

Cristiano Schiaffella

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What logic is followed when there are multiple "and/or" statements in a line of code?

2 Answers2