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Python

Posted by john larson
john larson
16,594 Points

Is the reason we use dunder functions in classes...

Looking up the various dunder functions. It looks to me like maybe the reason we use the dunder functions when making classes but not functions: In functions dunders are already there behind the scenes AND in classes we need to be explicit because we are deliniating all the information ourselves???

Alexander Davison
Alexander Davison
65,469 Points

Tagging Chris Freeman

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,426 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,426 Points

I'm sure you've heard "everything in Python is an object." This also means that everything derives from some class. Functions are no different, deriving from the class function. The "dunder" methods are built-in Python methods to the underlying class each having a specific purpose. Details on all the magic methods are in the docs.

It is possible to overwrite the magic methods on functions and classes. When new classes are defined it is common to overwrite some of the built-in methods (typically __init__) to create custom functionality but this is not required.

Using type() and dir() to inspect a trivial function, you can see the "dunder" magic methods:

# Trivial class
>>> class MyClass:
...     pass
... 
>>> dir(MyClass)
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__qualname__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__']

# Trivial function
>>> def foo():
...     pass
... 
>>> foo()
>>> type(foo)
<class 'function'>
>>> >>> dir(foo)
['__annotations__', '__call__', '__class__', '__closure__', '__code__', '__defaults__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__get__', '__getattribute__', '__globals__', '__gt__', '__hash__', '__init__', '__kwdefaults__', '__le__', '__lt__', '__module__', '__name__', '__ne__', '__new__', '__qualname__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']

The same is true for a simple integer:

>>> type(5)
<class 'int'>
>>> dir(5)
['__abs__', '__add__', '__and__', '__bool__', '__ceil__', '__class__', '__delattr__', '__dir__', '__divmod__', '__doc__', '__eq__', '__float__', '__floor__', '__floordiv__', '__format__', '__ge__', '__getattribute__', '__getnewargs__', '__gt__', '__hash__', '__index__', '__init__', '__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__', 'bit_length', 'conjugate', 'denominator', 'from_bytes', 'imag', 'numerator', 'real', 'to_bytes']
john larson
john larson
16,594 Points

Chris, that is a beautifully laid out answer. Thank you for that. re:

  • "It is possible to overwrite the magic methods on functions and classes."
  • I don't think I have seen dunders used on functions, only classes. (I get that there is a relation to what functions do and what classes do). But I think that Kenneth has only used dunders on objects like:
class trying_to_understand:
    __int__()  
#but not:
def im_still_foggy():
    do something or other

So, I think you are saying that dunders can be used on functions AND classes?

Chris Freeman
Chris Freeman
Treehouse Moderator 68,426 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,426 Points

My main point is that all objects, including functions, have built-in dunder methods. In classes, some of the dunder methods can be overridden with custom versions or can extend or modify the inherited method of the parent class method (using super()). In a normal function usage, it would be rare to modify an inherited method from the function class.