Understanding pure functions vs modifiers in object-oriented programming: Concepts and examples

[Rajeev Bagra]

Source: Created  with AI tool

In object-oriented programming, the terms “pure function” and “modifier” (or “mutator”) refer to two different ways methods interact with an object’s state.

Pure Function:

A pure function is a method that:
– Does not modify the state of the object it is called on.
– Instead, it returns a new value (or object) based on the object’s state and the parameters passed to it.
– It does not cause side effects. It relies only on the inputs and produces consistent outputs without altering anything external to the method.

Modifier (or Mutator):

A modifier is a method that changes the state of the object it is called on. Instead of returning a new object or value, it typically alters the internal attributes (fields) of the object, which can impact the future behavior of the object.

Explanation of the increment Method:

Let’s break down what the code is doing:

# inside class Time:

def increment(self, seconds):
seconds += self.time_to_int()
return int_to_time(seconds)

• This method takes a seconds argument and adds it to the current time (converted to seconds using the method time_to_int).
• It returns a new Time object, which is the result of adding the given number of seconds to the existing time.
• It does not modify the original time object. The original time stays the same, and instead, a new Time object is created and returned.

This is why it is considered a pure function—it does not alter the state of the original object; it just computes and returns a new result.

Modifier Example:

On the other hand, a modifier method would directly change the state of the object. Here’s an example:

# inside class Time:

def increment(self, seconds):
self.seconds += seconds # Directly modify the object's state

In this version, instead of returning a new Time object, the method modifies the current object’s seconds attribute. This makes it a modifier because it changes the state of the object directly.

Example to Illustrate Pure Function vs Modifier:

Suppose you have a Time class representing a time of day.

Pure Function Example:

class Time:
def __init__(self, hours, minutes):
self.hours = hours
self.minutes = minutes

def add_time(self, minutes):
# Pure function: Returns a new Time object without modifying the original
new_minutes = self.minutes + minutes
new_hours = self.hours + new_minutes // 60
return Time(new_hours % 24, new_minutes % 60)

# Usage
time1 = Time(10, 30)
time2 = time1.add_time(45)

print(time1.hours, time1.minutes) # Outputs: 10 30 (Original object remains unchanged)
print(time2.hours, time2.minutes) # Outputs: 11 15 (New object is returned)

In this example:
– add_time is a pure function because it does not change time1. It returns a new Time object (time2) without affecting the original.

Modifier Example:

class Time:
def __init__(self, hours, minutes):
self.hours = hours
self.minutes = minutes

def increment(self, minutes):
# Modifier: Changes the state of the object itself
self.minutes += minutes
self.hours += self.minutes // 60
self.minutes = self.minutes % 60
self.hours = self.hours % 24

# Usage
time1 = Time(10, 30)
time1.increment(45)

print(time1.hours, time1.minutes) # Outputs: 11 15 (Object is modified)

In this example:
– increment is a modifier because it changes the internal state of time1 by updating its hours and minutes attributes.

Summary:

• A pure function computes and returns a new value or object without changing the original object. It has no side effects.
• A modifier alters the internal state of the object on which it is called, changing its behavior and attributes going forward.

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