Functions related to dundermethods¶

Working with dundermethods can be tricky, because when python searches for a dundermethod, it doesn’t use the same mechanism as it does for normal attribute access. Normal attribute access like obj.attr will look for attr in the instance’s namespace (i.e. obj.__dict__) and the class’s namespace (i.e. type(obj).__dict__ plus the __dict__ of every parent class). Dundermethods, on the other hand, are only searched for in the class namespace - not the instance namespace. Defining a dundermethod in the instance namespace won’t work:

class Demo:
    pass

obj = Demo()
obj.__len__ = lambda self: 0

print(len(obj))  # throws TypeError: object of type 'Demo' has no len()

And neither will defining a dundermethod in a metaclass:

class DemoMeta(type):
    def __len__(cls):
        return 0

class Demo(metaclass=DemoMeta):
    pass

obj = Demo()

print(len(obj))  # throws TypeError: object of type 'Demo' has no len()

So if you wanted to implement your own len function, you wouldn’t have an easy way of accessing the relevant __len__ method - obj.__len__ would be incorrect because it would search the instance namespace, and type(obj).__len__ would be incorrect because it would search the metaclass namespace. That’s where these functions come in - get_bound_dundermethod(obj) or get_class_dundermethod(type(obj)) would do the work for you.

introspection.DUNDERMETHOD_NAMES = {'__abs__', '__add__', '__aenter__', '__aexit__', '__aiter__', '__and__', '__anext__', '__await__', '__bool__', '__bytes__', '__call__', '__ceil__', '__class_getitem__', '__complex__', '__contains__', '__del__', '__delattr__', '__delete__', '__delitem__', '__delslice__', '__dir__', '__div__', '__divmod__', '__enter__', '__eq__', '__exit__', '__float__', '__floor__', '__floordiv__', '__format__', '__fspath__', '__ge__', '__get__', '__getattr__', '__getattribute__', '__getitem__', '__getnewargs__', '__getslice__', '__gt__', '__hash__', '__iadd__', '__iand__', '__ifloordiv__', '__ilshift__', '__imatmul__', '__imod__', '__imul__', '__index__', '__init__', '__init_subclass__', '__instancecheck__', '__int__', '__invert__', '__ior__', '__ipow__', '__irshift__', '__isub__', '__iter__', '__itruediv__', '__ixor__', '__le__', '__len__', '__length_hint__', '__lshift__', '__lt__', '__matmul__', '__mod__', '__mul__', '__ne__', '__neg__', '__new__', '__next__', '__or__', '__pos__', '__pow__', '__prepare__', '__radd__', '__rand__', '__rdiv__', '__rdivmod__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__rfloordiv__', '__rlshift__', '__rmatmul__', '__rmod__', '__rmul__', '__ror__', '__round__', '__rpow__', '__rrshift__', '__rshift__', '__rsub__', '__rtruediv__', '__rxor__', '__set__', '__set_name__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__sub__', '__subclasscheck__', '__subclasses__', '__truediv__', '__trunc__', '__xor__'}¶

A set containing the names of all dundermethods available in python 3.9.

introspection.AUGMENTED_ASSIGNMENT_DUNDERMETHOD_NAMES = {'__iadd__', '__iand__', '__ifloordiv__', '__ilshift__', '__imatmul__', '__imod__', '__imul__', '__ior__', '__ipow__', '__irshift__', '__isub__', '__itruediv__', '__ixor__'}¶

A set containing the names of all augmented assignment dundermethods available in python 3.9.

New in version 1.1.

introspection.iter_class_dundermethods(cls, bound=None)¶

Yields all dundermethods implemented by the given class as (method_name, method) tuples.

(For the purpose of this function, “implemented” simply means “exists”. Even if the method’s value is None or anything else, it will still be yielded.)

If multiple classes in the MRO implement the same dundermethod, both methods will be yielded. Methods implemented by subclasses will always be yielded before methods implemented by parent classes.

You can cause the iteration to stop early by passing in a class as the upper bound. The MRO will only be iterated up to the bound, excluding the bound class itself. This is useful for excluding dundermethods implemented in object.

Parameters
  • cls – The class whose dundermethods to yield

  • bound – Where to stop iterating through the class’s MRO

Returns

An iterator yielding (method_name, method) tuples

Raises

TypeError – If cls is not a class

introspection.class_implements_dundermethod(cls, method_name, bound=None)¶

Checks whether the given class implements a certain dundermethod.

The method is considered implemented if any of the classes in the MRO have an entry for method_name in their __dict__. The only exception is that __hash__ methods are considered not implemented if their value is None.

Note that object implements various dundermethods, including some unexpected ones like __lt__. Remember to pass in bound=object if you wish to exclude these.

Parameters
  • cls – A class

  • method_name – The name of a dundermethod

  • bound – Where to stop searching through the class’s MRO

Returns

A boolean indicating whether the class implements that dundermethod

Raises

TypeError – If cls is not a class

introspection.class_implements_any_dundermethod(cls, methods, bound=None)¶

Checks whether the given class implements at least one of the given dundermethods.

Parameters
  • cls – A class

  • methods – The names of a bunch of dundermethods

  • bound – Where to stop searching through the class’s MRO

Returns

A boolean indicating whether the class implements any of those dundermethods

Raises

TypeError – If cls is not a class

introspection.class_implements_dundermethods(cls, methods, bound=None)¶

Checks whether the given class implements all given dundermethods.

Parameters
  • cls – A class

  • methods – The names of a bunch of dundermethods

  • bound – Where to stop searching through the class’s MRO

Returns

A boolean indicating whether the class implements all those dundermethods

Raises

TypeError – If cls is not a class

introspection.collect_class_dundermethods(cls, bound=None)¶

Generates a dict of the form {method_name: method} containing all dundermethods implemented by the given class.

If multiple classes in the MRO implement the same dundermethod, only the first implementation is included in the result.

Parameters
  • cls – The class whose dundermethods to collect

  • bound – Where to stop iterating through the class’s MRO

Returns

A {method_name: method} dict

Raises

TypeError – If cls is not a class

introspection.get_class_dundermethod(cls, method_name, bound=None)¶

Retrieves a class’s implementation of the given dundermethod.

Parameters
  • cls – A class

  • method_name – The name of a dundermethod

  • bound – Where to stop searching through the class’s MRO

Returns

The function object for the given method_name

Raises
  • TypeError – If cls is not a class

  • AttributeError – If cls does not implement that dundermethod

introspection.get_bound_dundermethod(instance, method_name, bound=None)¶

Retrieves an instance’s implementation of the given dundermethod.

New in version 1.1.

Parameters
  • instance – Any object

  • method_name – The name of a dundermethod

  • bound – Where to stop searching through the class’s MRO

Returns

A bound method for the given method_name

Raises

AttributeError – If instance does not implement that dundermethod

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