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Cover image for Code Smell 198 - Hidden Assumptions
Maxi Contieri
Maxi Contieri

Posted on • Originally published at

Code Smell 198 - Hidden Assumptions

Software is about contracts and ambiguous contracts are a nightmare

TL;DR: Keep your code explicit



  1. Be declarative and explicit

  2. Don't oversimplify


Hidden assumptions are underlying beliefs or expectations not explicitly stated in the code.

They are still present and can impact the behavior of the software.

Various reasons can give rise to assumptions such as incomplete requirements, incorrect presumptions about the user or environment, limitations of the programming language or tools, and bad accidental decisions.

Sample Code


tenCentimeters = 10
tenInches = 10

tenCentimeters + tenInches
# 20
# this error is based on the hidden assumption of a unit (any)
# and caused the Mars Climate Orbiter failure 
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class Unit:
    def __init__(self, name, symbol): = name
        self.symbol = symbol

class Measure:
    def __init__(self, scalar, unit):
        self.scalar = scalar
        self.unit = unit

    def __str__(self):
        return f"{self.scalar} {self.unit.symbol}"

centimetersUnit = Unit("centimeters", "cm")
inchesUnit = Unit("inches", "in")

tencCentimeters = Measure(10, centimeters)
tenInches = Measure(10, inches)

tenCentimeters + tenInches
# error until we introduce a conversion factor
# in this case the conversion is constant 
# inches = centimeters / 2.54

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[X] Manual

This is a design smell


  • Coupling


Hidden assumptions can be difficult to identify and can lead to bugs, security vulnerabilities, and usability issues.

To mitigate these risks, software developers should be aware of their assumptions and biases.

Developers also need to engage with users to understand their needs and expectations.

They must test their software in various scenarios to uncover hidden assumptions and edge cases.


More Info

Mars Climate Orbiter Disaster

Measure Solution


Code Smells are my opinion.


Photo by Christian Pfeifer on Unsplash

A human organization is just as much an information system as any computer system. It is almost certainly more complex, but the same fundamental ideas apply. Things that are fundamentally difficult, like concurrency and coupling, are difficult in the real world of people, too.

Dave Farley

This article is part of the CodeSmell Series.

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