How can a Python subclass control what data is stored in an immutable instance?

For this, at first understand what is __new__() method in Python. When subclassing an immutable type, override the __new__() method instead of the __init__() method.

The __new__ method gets called when an object is created whereas the __init__ method will be called to initialize the object. These are magic methods.

The magic methods that allow us to do some pretty neat tricks in object-oriented programming. They are also called Dunder methods. These methods are identified by two underscores (__) used as prefix and suffix.

Display the magic methods inherited by int class

Example

Using the fir(), we can print the magic methods −

print(dir(int))

Output

['__abs__', '__add__', '__and__', '__bool__', '__ceil__', '__class__', '__delattr__',
 '__dir__', '__divmod__', '__doc__', '__eq__', '__float__', '__floor__', '__floordiv__', '__format__', '__ge__', 
'__getattribute__', '__getnewargs__', '__gt__', '__hash__', '__index__', '__init__', '__init_subclass__', '__int__'
, '__invert__', '__le__', '__lshift__', '__lt__', '__mod__', '__mul__', '__ne__', '__neg__', '__new__', '__or__',
 '__pos__', '__pow__', '__radd__', '__rand__', '__rdivmod__', '__reduce__', '__reduce_ex__', '__repr__', 
'__rfloordiv__', '__rlshift__', '__rmod__', '__rmul__', '__ror__', '__round__', '__rpow__', '__rrshift__', 
'__rshift__', '__rsub__', '__rtruediv__', '__rxor__', '__setattr__', '__sizeof__', '__str__', '__sub__', 
'__subclasshook__', '__truediv__', '__trunc__', '__xor__', 'as_integer_ratio', 'bit_length', 'conjugate',
 'denominator', 'from_bytes', 'imag', 'numerator', 'real', 'to_bytes']

The __init__()

Example

Let us see an example of the __init__() magic method to instantiate object −

class String:
   # The magic method to initiate object
   def __init__(self, string):
      self.string = string

# Driver Code
if __name__ == '__main__':
   # object creation
   myStr = String('Demo')
   # print object location
   print(myStr)

Output

<__main__.String object at 0x7f34c97799d0>

Subclassing an immutable type

Example

Now, let’s see how can a Python subclass control what data is stored in an immutable instance.

from datetime import date

class FirstOfMonthDate(date):
   "Always choose the first day of the month"
   def __new__(cls, year, month, day):
      return super().__new__(cls, year, month, 1)

class NamedInt(int):
   "Allow text names for some numbers"
   xlat = {'zero': 0, 'one': 1, 'ten': 10, 'fifteen': 15}
   def __new__(cls, value):
      value = cls.xlat.get(value, value)
      return super().__new__(cls, value)

class TitleStr(str):
   "Convert str to name suitable for a URL path"
   def __new__(cls, s):
      s = s.lower().replace(' ', '-')
      s = ''.join([c for c in s if c.isalnum() or c == '-'])
      return super().__new__(cls, s)

# Calling
print(FirstOfMonthDate(2022, 9, 8))
print(NamedInt('fifteen'))
print(NamedInt(18))

# Create a URL path
print(TitleStr('course for beginners'))

Output

2022-09-01
15
18
course-for-beginners