CODEWITHSUNDEEP
pythonconditional-statementslazy-evaluation
Hossein Kalbasi
Hossein Kalbasi

Reputation: 1861

Python - order of conditions in IF statement with AND Operator

Is it a good practice to rely on the order of expressions when we have multiple and statements?

More precisely, I have a string (the string could be None at times) and based on its length I want some actions. Is the following a good code?

import os

mystr = os.getenv('MYSTR_ENV_VAR')

if mystr and len(mystr)>10:
    print('do sth')
else:
    print('do sth else')

Note that when mystr is None, the code len(mystr) will throw an error if executed. Therefore, I am relying on the if statement to first make sure mystr is not empty or None, and then evaluate the length. This code works and if it is safe, the logic suits my needs. Is it the correct Pythonic way of doing this?

Upvotes: 1

Views: 2244

Answers (2)

Sercan
Sercan

Reputation: 2169

Python evaluates boolean conditions lazily. So your code is safe as if statement will check your string existence first.

From docs:

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.

The expression x or y first evaluates x; if x is true, its value is returned; otherwise, y is evaluated and the resulting value is returned.

Upvotes: 3

rite2hhh
rite2hhh

Reputation: 412

Check the docs for Python 3.x, the operators support short-circuiting, i.e. if the first expression fails the if test in an and compound test, the succeeding tests aren't evaluated

For or

This is a short-circuit operator, so it only evaluates the second argument if the first one is false.

For and

This is a short-circuit operator, so it only evaluates the second argument if the first one is true.

Upvotes: 0

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