CODEWITHSUNDEEP
python
pm007
pm007

Reputation: 383

python if - else operator precedence

I came across a very nasty example of the if - else operator in Python where the code looked good but the result was completely different than expected. Here is a short version:

1 + 4 if None else 3  # returns 3

Looking at the Operator Precedence table in the documentation, it seems that if - else has almost the lowest precedence.

Is there anything special with if - else that treats everything on the left side of the if as one expression?

Upvotes: 2

Views: 3063

Answers (4)

Sujit Dhamale
Sujit Dhamale

Reputation: 1446

This is example of conditional expression in python.

It will execute block between IF and ELSE

  1. if block is return non zero or TRUE then it will execute left side of IF statement
  2. if block is Zero or FALSE or NONE then it will execute right side of ELSE statement

1 + 4 if None else 3 In above line NONE menace FALSE will return so it will execute right side of ELSE block which is 3

Please see below code to understand conditional expression in python

a,b=10,1
v="A is less tahn B" if a < b else "A is greater than B"
print(v)

output --> A is greater than B

Upvotes: -1

glglgl
glglgl

Reputation: 91159

Well, None is always false, so that you always get 3.

1 + 4 would be the result if you used a true value instead of None:

1 + 4 if True else 3

gives 5.

That's exactly because of low precedence: it is the same as

(1 + 4) if ... else 3

as - you know - the precedence of if ... else is lower than that of +.

Upvotes: 6

Pi Marillion
Pi Marillion

Reputation: 4674

The difference is in the order, not the precedence. Both the if...else operator in Python and the ?...: operator in c have extremely low precedence (lower than + and -, anyway).

Python: result_if_true if conditional else result_if_false

c: conditional ? result_if_true : result_if_false

Consider the order of your example:

print 1 + 4 if None else 3

is the same as:

print (1 + 4) if (None) else (3)

which translates to c as:

cout << (0) ? (1 + 4) : (3);

where None is translated to 0 since None is a falsey in Python.

To get your c into Python...

#include <iostream>
using namespace std;

int main() {
    int x = (1 + 0) ? (4) : (3);
    cout << x; // prints 4
    return 0;
}

Translates to:

x = 4 if 1 + 0 else 3
print x

Upvotes: 2

MrAlexBailey
MrAlexBailey

Reputation: 5289

Perhaps you are looking for:

print 1 + (4 if None else 3)

Which prints 4

Upvotes: 0

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