question on log as an instance method

[Chris Rebert]

On Sun, Oct 7, 2012 at 1:33 AM, Franck Ditter <franck at ditter.org> wrote:
> Hi ! Here is Python 3.2.3, MacOSX-Lion
>
> Question 0 : I may consider + as an hidden instance method , as
> 1+2 is equivalent to (1).__add__(2) ?

No, it's not nearly that simple. It's technically equivalent to
operator.add(1, 2) [
http://docs.python.org/library/operator.html#operator.add ], which
hints that there's additional logic involved. Some examples of the
complexities (in the general case):
* special methods are looked up on the objects' classes, ignoring
per-instance attributes; see
http://docs.python.org/reference/datamodel.html#special-method-lookup-for-new-style-classes
* trying to add objects of incompatible types raises TypeError rather
than AttributeError (which one might otherwise expect when a class
doesn't define __add__() [or similar])
* such TypeErrors are often raised directly by the interpreter itself,
rather than from within the body of some specific implementation of an
operator special method
* falling back to reflected methods (in the case of +, __radd__() [
http://docs.python.org/reference/datamodel.html#object.__radd__ ])
when the normal method fails or is not defined
* returning NotImplemented triggers fallback (or if already attempting
fallback, may cause the operation to fail)

<snip>
> Question 2 : After importing math,

Why would that be relevant? Python is not generally the sort of
language that would have the mere importation of a std lib module
significantly affect language semantics.

> why can't I consider log as
> an instance method, after all ?
>>>> (4).__log__()
> AttributeError: 'float' object has no attribute '__log__'

Because Python just simply did not choose to make "take the (natural)
logarithm of" a built-in, overloadable operation (hence, in part, why
you had to `import math` to even be able to access that calculation).
And it further didn't happen to define math.log() in terms of a .log()
or .__log__() instance method.

Basically, it's somewhat arbitrary and some historical reasons are involved.

Cheers,
Chris