python magic method: make custom classes more like built-in types

Python’s magic methods are those predefined functions of type __XXX__ in Python.

The biggest advantage of using Python's magic methods is that python provides simple methods to make objects behave like built-in types.

__str__ function

The __str__ function is used to process the output content when printing the instance itself. If this function is not overridden, an object name and memory address are output by default.

For example:

>>> class Student(object):
...     def __init__(self,name):
...             self._name = name
...
>>> print Student()

Output:

<__main__.Student 
object at 0x0000000002A929E8>.

So how do we make the output results more readable? We can override the __str__ function. For example

>>> class Student(object):
...     def __init__(self, name):
...             self._name = name
...     def __str__(self):
...             return  "I'm a student, named %s" % self._name
...
>>> print Student("Charlie")

The output result is:

I'm a student, named Charlie.

When we apply the str() function to the object, In fact, the __str__ function of the object is called.

__repr__ Function

__repr__ also serializes objects, but __repr__ More for the python compiler. __str__ is more readable.

When we apply the repr() function to an object, what we call is actually the __repr__ function of the function.

Paired with repr() is the eval() function. The eval() function converts the serialized object back into an object. The premise is that the object implements the __repr__ function.

The above paragraph is based on my own understanding, I don’t know whether it is right or wrong.

>>> item = [1,2,3]
>>> repr(item)
'[1, 
2, 3]'
>>> other_item = eval(repr(item))
>>> other_item[1]
2

__iter__ function

We often use for...in... to iterate over lists or tuples. That is list inherits from Iterable. Iterable implements the __iter__ function.

If you want to turn a custom object into an iterable object, you must implement two methods: __iter__ and next.

__iter__ function returns an object. When iterating, the next function will be continuously called to get the next value until StopIteration is captured.

Teacher Liao Xuefeng’s tutorial writes the __next__ method, I don’t know why.

class Fib(object):
    def __init__(self):
        self.a, self.b = 0, 1
 
    def __iter__(self):
        return self
 
    def next(self):
        self.a, self.b = self.b, self.a + self.b
        if self.a > 10000:
            raise StopIteration
        return self.a
 
 
for i in Fib():
    print i

__getitem__ function

The above implements the object by implementing the __iter__ function Iterate.

#So how to implement the object to extract elements by subscript.

This is done by implementing the __getitem__ method of the object.

Let’s give one