Question

Instead of using common OOP, like Java and C# do with their base class Object or object, Python uses special methods for basic behaviour of objects. Python uses __str__ which is used when the object is passed to print:

>>> class Demo:
>>>   def __str__(self):
>>>     return "representation"

>>> d = Demo()
>>> print(d)
representation

The same with len:

>>> class Ruler:
>>>   def __len__(self):
>>>     return 42

>>> r = Ruler()
>>> len(r)
42

What I would expect is something like this:

>>> class Ruler:
>>>   def len(self):
>>>     return 42

>>> r = Ruler()
>>> r.len()
42

What is the reason for using special methods indirectly instead of calling usual methods directly?

Answer 1

The reason for this is explained well in the Python documentation here:

http://docs.python.org/faq/design.html#why-does-python-use-methods-for-some-functionality-e-g-list-index-but-functions-for-other-e-g-len-list

The major reason is history. Functions
were used for those operations that
were generic for a group of types and
which were intended to work even for
objects that didn’t have methods at
all (e.g. tuples). It is also
convenient to have a function that can
readily be applied to an amorphous
collection of objects when you use the
functional features of Python (map(),
apply() et al).

In fact, implementing len(), max(),
min() as a built-in function is
actually less code than implementing
them as methods for each type. One can
quibble about individual cases but
it’s a part of Python, and it’s too
late to make such fundamental changes
now. The functions have to remain to
avoid massive code breakage.

(This was answered in the comments, but needs to be shown as a real answer for the sake of future readers.)