On 03/23/2014 11:36 PM, Ben Finney wrote:
Jim Byrnes <jf_byr...@comcast.net> writes:
I am reading Practical Programming - An Introduction to Computer
Science Using Python 3. They give this example:
abs(-3)
3
-3 .__abs__()
3
That's a poor example, in my opinion. It's not good for an introductory
text to show calling dunder methods like that on an integer literal.
Perhaps you could communicate with the maintainer of that material, to
point out the problem with their example. Hopefully they will remove the
example from an introductory text.
To be fair to the authors this was in a section taking about how the
double underscore was special to Python and showing a little about what
was going on behind the scenes. They stressed that programmers would
almost never use them in code. I plan on sending them a message
pointing out the results they show are incorrect.
Python 3.3.5 (default, Mar 12 2014, 02:09:17)
[GCC 4.6.3] on linux
abs(-3)
3
-3 .__abs__()
-3
Yes, I get the same result as you.
The reason is that the expression is being evaluated as::
-( (3) .__abs__() )
That is:
* Create the integer object 3
* Get the result of that object's ‘__abs__’ method
* Negate (invert the sign) of the value
(If you really care, see the end of this message[0] for a demonstration
that this is exactly what happens.)
Presumably the expression should be different, as shown in your next
example::
If I use a variable it works.
x = -3
x.__abs__()
3
Yes, this is a better way to do it. But why are we calling the ‘__abs__’
function directly at all? That is, after all, the point of the ‘abs’
built-in function: to call the correct method on the object::
I am not really using it. I was just trying to verify the result shown
in an example.
>>> x = -3
>>> abs(x)
3
I am curious as to what is happening. Is the book wrong? I checked
it's errata and nothing is mentioned.
I think that this is both an erratum, and a demonstration that the
example is a bad idea for the book entirely. Can you contact the
maintainer of that work to let them know?
Thanks for the confirmation and the explanation of what was happening,
much appreciated.
Regards, Jim
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