What is meant by operator precedence and associativity in python?

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When dealing with operators in Python we have to know about the concept of Python operator precedence and associativity as these determine the priorities of the operator otherwise, we’ll see unexpected outputs.

Operator Precedence: This is used in an expression with more than one operator with different precedence to determine which operation to perform first.

Example: Solve 

10 + 20 * 30

10 + 20 * 30 is calculated as 10 + (20 * 30)
and not as (10 + 20) * 30

Code:

Python3

expr = 10 + 20 * 30

print(expr)

Output:

610

Example: Now, let’s see an example on logical ‘or‘ & logical ‘and‘  operator.  ‘if‘ block is executed even if the age is 0. Because precedence of logical ‘and‘ is greater than the logical ‘or‘.

Python3

name = "Alex"

age = 0

if name == "Alex" or name == "John" and age >= 2 :

    print("Hello! Welcome.")

else :

    print("Good Bye!!")

Output:

Hello! Welcome.

Hence, To run the ‘else‘ block we can use parenthesis( ) as their precedence is highest among all the operators.

Python3

name = "Alex"

age = 0

if ( name == "Alex" or name == "John" ) and age >= 2 :

  print("Hello! Welcome.")

else :

  print("Good Bye!!")

Output:

Good Bye!!

Operator Associativity: If an expression contains two or more operators with the same precedence then Operator Associativity is used to determine. It can either be Left to Right or from Right to Left.

Example: ‘*’ and ‘/’ have the same precedence and their associativity is Left to Right, so the expression “100 / 10 * 10” is treated as “(100 / 10) * 10”.

Code:

Python3

print(100 / 10 * 10)

print(5 - 2 + 3)

print(5 - (2 + 3))

print(2 ** 3 ** 2)

Output:

100
6
0
512

Operators Precedence and Associativity are two main characteristics of operators that determine the evaluation order of sub-expressions in absence of brackets.

Example: Solve 

100 + 200 / 10 - 3 * 10

100 + 200 / 10 - 3 * 10 is calculated as 100 + (200 / 10) - (3 * 10)
and not as (100 + 200) / (10 - 3) * 10

Code:

Python3

expression  = 100 + 200 / 10 - 3 * 10

print(expression )

Output:

90.0

Please see the following precedence and associativity table for reference. This table lists all operators from the highest precedence to the lowest precedence.