Why do operators and methods of built-in types differ

[Csaba Hoch]

Hi,

if I write the following:

    >>> 1+1
    2

it seems to be exactly equivalent to this:

    >>> (1).__add__(1)
    2

However, if I write invalid code and try to add a list to an int, the
errors will be different:

    >>> 1+[]
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: unsupported operand type(s) for +: 'int' and 'list'

    >>> (1).__add__([])
    NotImplemented

I found that operator.__add__(1, []) gives the same result as 1+[].

What is the reason behind this difference between the __add__ operator
and int.__add__?

Thank you,
Csaba