A Python Trick: Adding a Lambda Method to a Class

It does not take much to add a lambda function to a Python class. For example, consider the following:

  >>> class A: pass
  >>> f = lambda self,x:x
  >>> setattr(A,"f",f)

This code adds the method f to class A. For example:

  >>> a=A()
  >>> a.f(1)
  1

However, the f method created this way does not have the expected Python name:

  >>> A.f.__name__
  ''

Further, defining this value is not possible; the instancemethod f does not have a __name__ attribute.

The trick is to name the lambda function before defining the class method:

  >>> class A: pass
  >>> f = lambda self,x:x
  >>> f.__name__ = "f"
  >>> setattr(A,"f",f)
  >>> f.__name__
'f'

This is simple, but it took too long to figure this out...