What are “inheritable alternative constructors”?

I stumbled over the term “inheritable alternative constructors” in this answer: https://stackoverflow.com/a/1669524/633961

The link points to a place where classmethod gets explained.

Do other programming languages have this feature, too?

Answer

One of the things that you can do with ANY language that has class methods (or similar) is provide alternative constructors. A slightly contrived Python3 example below :

class Color():
     def __init__( self, red, green, blue):
         self._red, self._green, self._blue = red, green, blue
     
     @classmethod
     def by_name( cls_, color_name ):
        color_defs = {'white':(255,255,255), 'red':(255,0,0),
                       'green':(0,255,0),'blue':(0,0,255)}
        return cls_( *color_defs[color_name] )

with this class you can now do :

    red = Color(255,0,0) # Using the normal constructor
    # or
    red = Color.by_name('red') # Using the alternative 

In Python the ‘by_name’ method would normally be called a factory method, rather than a constructor, but it uses the normal constructor methods.

Because this ‘by_name’ method is just a classmethod, it means you subclass it, the class method is inherited too – so it can be used on any subclass: i.e. it is inheritable and extensible.

An example in Python of a subclass which extends the Color class above, and extends the constructor and the ‘by_name’

class ColorWithAlpha( Color ):
      def __init__(self, red, green, blue, alpha=1.0):
           super().__init__(red,green,blue)
           self._alpha = alpha
      
      @classmethod
      def by_name( cls_, color_name, alpha):
          inst = super().by_name(color_name)
          inst._alpha = alpha
          return inst

red_alpha = ColorWithAlpha(255,0,0,0.5)
red2_alpha = ColorWithAlpha.by_name('red',0.5)

Other languages have similar alternative constructors (for instance C++ allows multiple constructors based on the arguments types), and these methods are all inheritable (i.e. subclasses can use them too (or extend them as necessary). I can’t speak of other languages, but I am sure other OOP languages will have similar constructors/factory method capabilities.