Sorry!  It should be repr(a) inside the print.  Here is the fixed version:

    class X(Decimal):
        def __init__(self, a):
            print('__init__:', repr(a))
    X.from_float(42.5)   # __init__: Decimal('42.5')

    X.from_float(42)     # with _pydecimal: __init__: 42
                         # with _decimal:   __init__: Decimal('42')