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Cover image for Code Smell 119 - Stairs Code
Maxi Contieri
Maxi Contieri

Posted on • Originally published at

Code Smell 119 - Stairs Code

Nested boolean conditions express a business rule. Not an IF

TL;DR: Avoid checking for boolean expressions and returning an explicit boolean.


  • Declarativeness

  • Ninja Code

  • Readability

  • Arrow Code


  1. Return a boolean business formula value.


When dealing with boolean formulas, it is more readable to show a business boolean formula than a stair of boolean checks followed by returning an explicit true/false;

Sample Code


def is_platypus(self):
    if self.is_mammal():
        if self.has_fur():
            if self.has_beak():
                if self.has_tail():
                    if self.can_swim():
                        return True
    return False

# This is also wrong since it is polluted with IFs and not readable by a biologist
def is_platypus(self):
    if not self.is_mammal():
        return False
    if not self.has_fur():
        return False
    if not self.has_beak():
        return False
    if not self.has_tail():
        return False
    if not self.can_swim():
        return False 
    return True
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def is_platypus(self):
    return self.is_mammal() &&  self.has_fur() && self.has_beak() && self.has_tail() && self.can_swim()

#We can even group conditions according to animal taxonomies
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[X] Automatic

Based on syntax trees, we can safely refactor the code removing the explicit boolean value.


  • Boolean


Beware of returning booleans.

After the return, you will need an If statement which is also a code smell.


More Info


Photo by Jukan Tateisi on Unsplash

Thanks again to Nico K. for this suggestion.

The real hero of programming is the one who writes negative code.

Douglas McIlroy

This article is part of the CodeSmell Series.

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