Python: Custom-made classes

__str__

__repr__

__iter__

__next__

__getitem__

__getattr__

Normally, when we call a method or property of a class, it will be an error if it does not exist. For example Student , define a class:

class Student (object):     def __init__ (self):         ' Michael '

Call name Properties, no problem, but call a nonexistent score property, there is a problem:

>>> s = Student ()print(s.name) Michaelprint(s.score) Traceback ( Most recent call last):  'Student'score '

The error message clearly tells us that the attribute was not found score .

To avoid this error, in addition to adding a score property, Python has another mechanism, which is to write a __getattr__() method that dynamically returns an attribute. Modify the following:

 class   Student (object):  def  __init__   =  " michael  "  def  __getattr__  " Span style= "COLOR: #000000" > (self, attr):  if  attr=="  score   " :  return " 

When a non-existent property is called, for example score , the Python interpreter attempts to invoke __getattr__(self, ‘score‘) the property, so that we have a chance to return score the value:

>>> s = Student ()>>> s.name'Michael'>>> S.score99

The return function is also perfectly possible:

class Student (object):     def __getattr__ (self, attr):         if attr=='age':            returnLambda: 25

Just call the method to change to:

>>> s.age ()25

Note that an existing attribute is called only if no attribute is found, __getattr__ for example name , it is not located in __getattr__ .

Python: Custom-made classes