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Standard object protocols in Python

15 Feb 2016

To give your objects a more baked-in feel, you can use python’s standard object protocol functions so that native operators start to operate on your object.

By implementing the following items on your custom objects, infix operators start to work executing your custom code as per defined.

General

By overriding __bool__ in your objects, you can define how your object will respond in conditional scenarios. __bool__ effectively allows you to use your object as a condition in an if or while statement.

class Boolable:
  def __init__(self, n):
    self.num = n

  def __bool__(self):
    return self.num % 2 == 0

The method __call__ will allow your object to openly accept function calls:

class Callable:
  def __call__(self, *args, *kwargs):
    # implementation here

c = Callable()
c()

Array

The following overrides allow you to make your objects appear like containers (arrays, etc.):

Operation Method Description
Length __len__ Allows the len function to operate on the object
Iterator __iter__ Gets an object to start the iteration process
Next __next__ Gets an object to continue the iteration process
Reverse __reversed__ Reverses the internal sequence

Dictionary

The following overrides allow you to make your object respond like a dict:

Operation Method Description
Set item __setitem__ Sets an item in the dictionary
Get item __getitem__ Retrieves an item from the dictionary
Delete item __delitem__ Removes an item from the dictionary

Mathematic

The following table lists out all of the methods that you can override on a class that will give you access to arithmetic operators.

Operation Operator LHS RHS Inline
Addition + __add__ __radd__ __iadd__
Subtraction - __sub__ __rsub__ __isub__
Multiplication * __mul__ __rmul__ __imul__
True Division / __truediv__ __rtruediv__ __itruediv__
Floor Division // __floordiv__ __rfloordiv__ __ifloordiv__
Modulo % __mod__ __rmod__ __imod__
Division and Modulo divmod __divmod__ __rdivmod__  
Exponentiation ** __pow__ __rpow__ __ipow__
Shift left << __lshift__ __rlshift__ __ilshift__
Shift right >> __rshift__ __rrshift__ __irshift__
Bitwise AND & __and__ __rand__ __iand__
Bitwise OR | __or__ __ror__ __ior__
Bitwise XOR ^ __xor__ __rxor__ __ixor__
Bitwise NOT ~ __invert__    
Function Method
Floor __floor__
Ceil __ceil__
Round __round__
Negate __neg__
Positive __pos__
Absolute __abs__

Comparison

The following table lists all of the comparison operators

Operation Operator Method
Equals == __eq__
Not Equal != __ne__
Greater than, equal to >= __gte__
Greater than > __gt__
Lesser than, equal to <= __lte__
Lesser than < __lt__

Type conversions

Type Method Description
int __int__  
float __float__  
complex __complex__  
index __index__ Allows python to use your object as an array index

Context

The following override allow your objects to measure contexts:

Operation Method Description
Enter __enter__ Measures when a context enters
Exit __exit__ Measures when a context exits

These functions are useful when your object is supplied to a with statement.

class ContextMeasurement:
  def __enter__(self):
    print("Entering context")
      
  def __exit__(self, exc_class, exc_instance, traceback):
    print("Exiting context")
        
with ContextMeasurement():
  print("Inside the context right now")

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