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[buran]

Read evaluation order and operator precedence

Python evaluates expressions from left to right. Notice that while evaluating an assignment, the right-hand side is evaluated before the left-hand side.

So in your example:

1. it will first evaluate 3, resulting in 3
   
2. then it will evaluate multi(3) - this will throw error if multi expects 2 arguments, see below
   
3. then it will evaluate add(result from multi(3))
   
4. then it will assign the result from item 3 to a
   
5. then, on next line, it evaluate a - first term on the right-hand side.
   
6. then it will evaluate a+3
   
7. then it will evaluate div(result from a + 3)
   
8. then it will evaluate mod(result from div(result from a + 3))
   
9. then it evaluate 12
   
10. then it will evaluate sub(12)
    
11. then it will evaluate a + result from mod(result from div(result from a + 3)) = result from sub(12)
    
12. then will assign the result to a
    

You did not confirm but, I would assume these are functions from operator module, although there is no multi, but mul.
So, first of all, your example is invalid, because all these functions expect 2 arguments. e.g. mul(3) will throw error

>>> from operator import mul
>>> mul(3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: mul expected 2 arguments, got 1