Perhaps I am just dumb

Hamish Lawson hamish_lawson at yahoo.co.uk
Mon Feb 11 15:47:04 CET 2002


wooks wrote:

> Further if I get magic methods right, they are special functions which
> if present enable the object to apply a unique interpretation of a
> python operator.

> So + has standard behaviour but will  adopt whatever is specified in
> the __add__ function if one is present for the class.

If an object doesn't have __add__ defined (by itself, its class, or an
inherited class), then it won't have an interpretation *at all* for +.
There's no 'standard behaviour' that is defaulted to. This holds true
for the built-in types too: 4.0 + 3 has an interpretation only because
the float type has __add__ predefined for it.

Thus

    4.0 + 3

effectively results in a call to

    (4.0).__add__(3)

The second form could actually be directly called that way in Python
2.2 (prior to 2.2, magic methods for built-in types weren't exposed to
the programmer); the brackets around the 4.0 are necessary only to
disambiguate the number from the following dot.

So there is no standard behaviour for + in the sense of some default
action that gets performed in the absense of a defined __add__ method.
However the __add__ method is already defined for most built-in types.

But the operator symbols provide more than just a prettier notation.
When the operand types are different the interpreter will inspect the
operands and will decide which operand to ask to handle the operation.

Thus 

    4 + 3.0

will effectively result in a call to

    (3.0).__radd__(4)

rather than

    (4).__add__(3.0)

since integers don't know how to add floats.


Hamish Lawson



More information about the Python-list mailing list