How to understand python function rewriting

Add corresponding methods in the custom class so that the instances created by the custom class can perform built-in function operations like built-in objects. This is function rewriting.

Object to string function: repr(obj), returns an expression string that can represent this object, usually eval(repr(obj)) == obj (this string is usually interpreted and executed by python used to run the programmer), str(obj) returns a string through the given object (this string is usually read by humans).

Rewriting method of object to string function:

Rewriting method of repr(obj) function:

def __repr__(self):

str(obj) function rewriting method:

def __str__(self):

Instructions:

1. The str(obj) function first searches, obj. __str__() method, call this method and return the result

2. If there is no obj.__str__() method, return the result of the obj.__repr__() method and return

3. If The obj.__repr__ method does not exist, then calling the __repr__ instance method of the object class displays a string in the format of 8b988a0d974fd8aab2b69bf9b2cf8564

Example:

# 此示例示意通过重写 repr 和 str方法改变转为字符串的规则
class MyNumber:
    def __init__(self, value):
        '构造函数,初始化MyNumber对象'
        self.data = value
 
    def __str__(self):
        '''转换为普通人识别的字符串'''
        # print("__str__方法被调用!")
        return "自定义数字类型对象: %d" % self.data
 
    def __repr__(self):
        '''转换为eval能够识别的字符串'''
        return 'MyNumber(%d)' % self.data
 
 
n1 = MyNumber(100)
n2 = MyNumber(200)
print('repr(n1) ====>', repr(n1))
print('str(n2)  ====>', str(n2))

Related recommendations: "Python Video Tutorial"

How to rewrite other built-in functions:

__abs__ abs(obj) function

__len__ len(obj) function (must return an integer)

__reversed__ reversed(obj) function (must return an iterable object

__round__ round(obj) function

Example:

# 此示例示意abs 函数的重写
class MyInteger:
 
    def __init__(self, v):
        self.data = v
 
    def __repr__(self):
        return 'MyInteger(%d)' % self.data
 
    def __abs__(self):
        v = abs(self.data)
        return MyInteger(v)  # 用v创建另一个MyInteger对象
 
    def __len__(self):
        return 10000
 
I1 = MyInteger(-10)
print('I1 =', I1)
 
I2 = abs(I1)
print("I2 =", I2)
 
print('len(I2)=', len(I2))  # 10000

Rewriting of numerical conversion function:

__complex__ complex(obj) function

__int__ int(obj) function

__float__ float(obj) function

__bool__ bool(obj) function (see Boolean test function overloading below)

Example:

# 此示例示意数据转换构造函数的重写方法
class MyNumber:
 
    def __init__(self, value):
        self.data = value
 
    def __repr__(self):
        return 'MyNumber(%d)' % self.data
 
    def __int__(self):
        'int函数的重载'
        return self.data
 
n1 = MyNumber(100)
x = int(n1)
print(n1)
 
print(bool(n1))  # True
n2 = MyNumber(0)
print(bool(n2))  # True

Boolean test function rewrite:

Format:

__bool__

Function:

Used for bool(obj) function value

Used in the truth value expression of if statement

Used for the truth value of while statement

description in the expression:

1. When there is a __bool__(self) method in the custom class, the return value of this method is used as bool(obj ) return value

2. When there is no __bool__(self) method, bool(x) returns whether the return value of the __len__(self) method is zero to test the Boolean value

3. When there is no __len__(self) method, True will be returned directly

Example:

# 此示例示意bool(x) 函数的重写
class MyList:
    '自定义类型的列表，用来保存数据，内部用一个列表来存储数据'
 
    def __init__(self, iterable=()):
        self.data = [x for x in iterable]
 
    def __repr__(self):
        return 'MyList(%s)' % self.data
 
    def __len__(self):
        '''返回长度'''
        print("__len__方法被调用")
        return len(self.data)
 
    def __bool__(self):
        print("__bool__方法调用")
        for x in self.data:
            if not x:
                return False
        return True
        # return False  # <<=== 所有对象都为False
 
myl = MyList([0, -1, 2, -3])
# myl = MyList()
print(myl)
print(bool(myl))
if myl:
    print("myl为真值")
else:
    print('myl为假值')

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