list.__len__() or len(list)

[Terry Reedy]

"Nikhil" <mnikhil at gmail.com> wrote in message 
news:g0ev6o$utb$1 at registered.motzarella.org...
| Then why to have __len__() internal method at all when the built-in
| len() is faster?

Nearly all syntax constructions and builtin functions are implemented by 
calling one or another of the __special__ methods.  This is what makes 
Python code so generic and plugging your own classes into the system so 
easy.

For example, collection[key] = value is implemented by calling 
collection.__setitem__(key, value).  When you define a class with that 
method, that syntax will work with its instances just the same as for 
builtin classes.

Similarly, a+b is implemented by calling a.__add__(b).  So if a class 
defines __add__, you can 'add' its instances (whatever 'add' means for that 
class) with '+'.

(The fact that an *implementation* may followup the 'as if' rule and 
optimize operations for certain classes by combining steps does not negate 
the *language* rules given above.)

tjr