Python has well-defined rules for specifying the order in which the operators in an expression are evaluated when the expression has several operators. For example, multiplication and division have a higher precedence than addition and subtraction. Precedence rules can be overridden by explicit parentheses.
Precedence Order
When two operators share an operand, the operator with the higher precedence goes first. For example, since multiplication has a higher precedence than addition, a + b * c is treated as a + (b * c), and a * b + c is treated as (a * b) + c.
Associativity
When two operators share an operand and the operators have the same precedence, then the expression is evaluated according to the associativity of the operators. For example, since the ** operator has right-to-left associativity, a ** b ** c is treated as a ** (b ** c). On the other hand, since the / operator has left-to-right associativity, a / b / c is treated as (a / b) / c.
Precedence and Associativity of Python Operators
The Python documentation on operator precedence contains a table that shows all Python operators from lowest to highest precedence, and notes their associativity. Most programmers do not memorize them all, and those that do still use parentheses for clarity.
Order of Evaluation
In Python, the left operand is always evaluated before the right operand. That also applies to function arguments.
Python uses short circuiting when evaluating expressions involving the and or or operators. When using those operators, Python does not evaluate the second operand unless it is necessary to resolve the result. That allows statements such as if (s != None) and (len(s) < 10): ... to work reliably.
Precedence Order Gone Awry
Sometimes the precedence order defined in a language do not conform with mathematical norms. For example, in Microsoft Excel, -a^b is interpreted as (-a)^b instead of -(a^b). So -1^2 is equal to 1 instead of -1, which is the values most mathematicians would expect. Microsoft acknowledges this quirk as a "design choice". One wonders whether the programmer was relying on the C precedence order in which unary operators have higher precedence than binary operators. This rule agrees with mathematical conventions for all C operators, but fails with the addition of the exponentiation operator. Once the order was established in Microsoft Excel 2.0, it could not easily be changed without breaking backward compatibility.
Exercises
Add parentheses to the following expression to make the order of evaluation more clear.
year % 4 == 0 and year % 100 != 0 or year % 400 == 0
Answer: leapyearvariety.py shows a variety of equivalent expressions, including the following reasonable alternative.
((year % 4 == 0) and (year % 100 != 0)) or (year % 400 == 0)
Copyright © 2000–2015 by Robert Sedgewick, Kevin Wayne, and Robert Dondero. All rights reserved.
FAQs
Arithmetic Operators: From highest to lowest precedence, these are ** (exponential), * (multiplication), / (division), + (addition), - (subtraction).
What is the order of expression evaluation in Python? ›
Python evaluates expressions from left to right. Notice that while evaluating an assignment, the right-hand side is evaluated before the left-hand side. Note that if you use a literal expression, such as (3 - 7) + (3 * 4) , it is not evaluated but directly compiled.
Which operator is evaluated last in an expression in Python? ›
Arithmetic operators take precedence over logical operators. Python will always evaluate the arithmetic operators first (** is highest, then multiplication/division, then addition/subtraction). Next comes the relational operators. Finally, the logical operators are done last.
Which of the following Python operators has the highest precedence? ›
Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want.
What is the correct order of operator precedence? ›
In SQL, the order of operator precedence, listed from higher to lower precedence, is * = NOT AND OR. This implies that multiplication operations are done first, then equality checking, then the NOT operator is applied, followed by 'AND', and finally 'OR'.
How to remember operator precedence in Python? ›
It's an acronym to help you remember which mathematical operators precede the others. The abbreviation is short for Parentheses Exponentiation Multiplication Division Addition Subtraction. You can remember it with “Please Excuse My Dear Aunt Susie”.
Which of the following precedence order is correct in Python answer? ›
Answer: The correct order of precedence is given by PEMDAS which means Parenthesis (), Exponential **, Multiplication *, Division /, Addition +, Subtraction -.
What is the order of execution of methods in Python? ›
Execution always begins at the first statement of the program. Statements are executed one at a time, in order, from top to bottom. Function definitions do not alter the flow of execution of the program, but remember that statements inside the function are not executed until the function is called.
How do you order results in Python? ›
Python sorted() Function
The sorted() function returns a sorted list of the specified iterable object. You can specify ascending or descending order. Strings are sorted alphabetically, and numbers are sorted numerically. Note: You cannot sort a list that contains BOTH string values AND numeric values.
What is an example of operator precedence? ›
The precedence of an operator specifies how "tightly" it binds two expressions together. For example, in the expression 1 + 5 * 3 , the answer is 16 and not 18 because the multiplication ("*") operator has a higher precedence than the addition ("+") operator. Parentheses may be used to force precedence, if necessary.
In addition to the basic arithmetic operators (+, -, *, /, %), Python also has a variety of other operators with different precedences. Here are some examples: Comparison operators (==, != , >, <, >=, <=) have lower precedence than arithmetic operators.
What is the proper operator precedence from first to last? ›
When expressions contain operators from more than one category, arithmetic operators are evaluated first, comparison operators are evaluated next, and logical operators are evaluated last.
What is Python operator precedence? ›
Operator precedence in Python simply refers to the order of operations. Operators are used to perform operations on variables and values. Python classifies its operators in the following groups: Arithmetic operators. Assignment operators.
Which operator has highest precedence in programming? ›
Operator Precedence
| Priority | Operator |
|---|
| First (highest) | ( ) (parentheses, to group expressions) |
| [ ] (brackets, to concatenate arrays) |
| Second | . (structure field dereference) |
| [ ] (brackets, to subscript an array) |
27 more rowsThe comma operator has the lowest precedence.
What is the order of precedence of the set operators? ›
All set operators have equal precedence. If a SQL statement contains multiple set operators, then Oracle Database evaluates them from the left to right unless parentheses explicitly specify another order.
What is the hierarchy of operators? ›
The order in which the arithmetic operations are executed in an expression is called 'hierarchy of operations' or 'operator precedence'. The rules governing the precedence of operators generally follow the normal rules of algebra.
What is the order of operations in Python modulo? ›
The modulo operator ( % ) shares the same level of precedence as the multiplication ( * ), division ( / ), and floor division ( // ) operators. Both the multiplication and modulo operators have the same level of precedence, so Python will evaluate them from left to right.
What is the precedence of arithmetic operators from highest to lowest? ›
Explanation: The Precedence of the operators is nothing but the priority given to each operator in an expression.
