reference: fluent python
magic methods
- Magic methods or dunder methods, allow objects to change behaviour of methods like
len()or[]notation. For example if you declare__len__ method of a class, when you uselen(obj) of that class, the interpreter will call__len__ mehtod you defined in the class. - They are useful because when you use that class, you don’t have to memorize random method names for standart operations. it’s not .size, .length or smht, you will know that it’s len()
- Special methods are meant to be called by python interpreter, not by you
bool represantation
- Any object can be used in a boolean context, because interpreter uses
bool(obj) method and it uses__bool__ method ot the obj.if a_string or 5:
in this contenxt, python will check bool(a_string) and bool(5)
- Basically, `bool(x)` calls `x.__bool__()` and uses the result
- By default, user defined classes are truthy, unless `__bool__` or `__len__` is implemented.
why len() is not a method?
- In Python, builtin types (like list) are C structs containing a field called
ob_size, which contains the element number in the collection. Whenlen(x) is used, instead of callingx.__len__, interpreter gets the value ofx.ob_size, which is much faster than calling a method. - “No method is called for the built-in objects of CPython: the length is simply read from a field in a C struct. Getting the number of items in a collection is a common operation and must work efficiently for such basic and diverse types as str, list, memoryview, and so on.”