A decorator in python is just syntactic sugar, remembering that Python has first class functions and classes:

def deco(callable):
    # Do something with callable
    retval = callable
    return retval

@ deco
def example1(): pass
     #...is identical to
def example1(): pass
example1 = deco(example1)
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A decorator doesn't have to return a callable -- eg you could decorate class SomeClass with @id, and get a fairly useless number bound to the name SomeClass. And in theory you could fiddle with the code in the callable that's passed, and return the identical object but you wouldn't.

One cannot @decorate a lambda, and "_" as a name is just convention.