{"id":2034,"date":"2020-12-30T09:13:38","date_gmt":"2020-12-30T09:13:38","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=2034"},"modified":"2020-12-30T09:28:29","modified_gmt":"2020-12-30T09:28:29","slug":"python-or","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/advanced-python\/python-or\/","title":{"rendered":"Python or Operator"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about the Python or<\/code> operator and how to use it effectively.<\/p>\n\n\n\n

Introduction to the Python or operator #<\/a><\/h2>\n\n\n\n

The or<\/code> operator is a logical operator<\/a>. Typically, you use the or<\/code> operator to combine two Boolean expressions and return a Boolean value<\/a>.<\/p>\n\n\n\n

The or<\/code> operator returns True<\/code> if one of the two operands is True<\/code>. And it returns False<\/code> only if both operands are False<\/code>.<\/p>\n\n\n\n

This truth table displays the result of the or<\/code> operator:<\/p>\n\n\n\n

x<\/th>y<\/th>x or y<\/th><\/tr><\/thead>
True<\/td>True<\/td>True<\/td><\/tr>
True<\/td>False<\/td>True<\/td><\/tr>
False<\/td>True<\/td>True<\/td><\/tr>
False<\/td>False<\/td>False<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

The following example shows how to use the or<\/code> operator:<\/p>\n\n\n

is_admin = False<\/span>\nis_editor = True<\/span>\ncan_edit = is_admin or<\/span> is_editor\n\nprint<\/span>(can_edit)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
True<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
The Python or operator is short-circuiting #<\/a><\/h2>\n\n\n\n
When evaluating an expression that involves the or<\/code> operator, Python can sometimes determine the result without evaluating all the operands. This is called short-circuit evaluation or lazy evaluation.<\/p>\n\n\n\n
For example:<\/p>\n\n\n
x or y<\/code><\/span><\/pre>\n\n\n
If x<\/code> is truthy, then the or<\/code> operator returns x<\/code>. Otherwise, it returns y<\/code>.<\/p>\n\n\n\n
In other words, if x<\/code> is truthy, then the or<\/code> operator doesn’t need to evaluate y<\/code>. It just returns x<\/code> immediately. This is why the evaluation is called lazy or short-circuiting evaluation.<\/p>\n\n\n\n
The or<\/code> operator only evaluates y<\/code> and returns the result of the evaluation if x<\/code> is falsy.<\/p>\n\n\n\n
In Python, every object associates with a Boolean value<\/a>. And the x<\/code> and y<\/code> can be any object. <\/p>\n\n\n\n
This opens some useful applications of the or<\/code> operator.<\/p>\n\n\n\n
Setting a default value for a variable #<\/a><\/h2>\n\n\n\n
The or<\/code> operator allows you to set a default value for a variable, for example:<\/p>\n\n\n
var_name = value or default<\/span><\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, if value<\/code> is falsy, the or<\/code> operator return the default<\/code>.<\/p>\n\n\n\n
The following example prompts you for input. If you don’t enter anything, the lang<\/code> will default to 'Python'<\/code>:<\/p>\n\n\n
lang = input('Enter your language:'<\/span>) or<\/span> 'Python'<\/span>\nprint<\/span>(lang)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
The following example defines a function get_data()<\/code> that returns a list of numbers. It uses the built-in min()<\/code> function to find the lowest element in the list:<\/p>\n\n\n
def get_data(args=None):\n    if<\/span> args:\n        return<\/span> [1<\/span>, 2<\/span>, 3<\/span>]\n    return<\/span> []\n\n\nlowest = min(get_data(args=true<\/span>))\nprint<\/span>(lowest)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
1<\/code><\/span><\/pre>\n\n\n
It returned 1 as expected. However, the get_data()<\/code> may return an empty list like this:<\/p>\n\n\n
lowest = min(get_data())\nprint<\/span>(lowest)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
It returned a ValueError<\/code>.<\/p>\n\n\n\n
To fix this, you can use the or<\/code> operator when calling the min()<\/code> function:<\/p>\n\n\n
def get_data(args=None):\n    if<\/span> args:\n        return<\/span> [1<\/span>, 2<\/span>, 3<\/span>]\n    return<\/span> []\n\n\nlowest = min(get_data() or<\/span> [0<\/span>])\nprint<\/span>(lowest)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
0<\/code><\/span><\/pre>\n\n\n
In this example, if the get_data() function returns an empty list, the or<\/code> operator will treat its result as a falsy value.<\/p>\n\n\n\n
Since the first operand is falsy, the or<\/code> operator needs to evaluate the second operand [0]<\/code>. In this case, you can specify the default minimum value in the second operand.<\/p>\n
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