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 <__main__.XXXX object at 0xAABBCCDD>

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))

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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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