Python built-in constants and python Constants

Python built-in constants and python Constants

Introduction
There are not many built-in constants in Python. There are only six constants: True, False, None, NotImplemented, Ellipsis, and _ debug __.
1. True
1. True is a constant of the bool type used to represent the True value.
>>> TrueTrue>>> type(True)<class 'bool'>
2. Any assignment to the constant True will throw a syntax error.
>>> True = 1SyntaxError: can't assign to keyword
 
Ii. False
1. False is a constant of the bool type used to represent a False value.
>>> FalseFalse>>> type(False)<class 'bool'>
2. Any assignment to the constant False will throw a syntax error.
>>> False = 0SyntaxError: can't assign to keyword
 
Iii. None
1. None indicates None, which is the unique value of NoneType.
>>> None # indicates None, NO content output >>> type (None) <class 'nonetype '>
2. Any assignment to the constant None will throw a syntax error.
>>> None = 2SyntaxError: can't assign to keyword
 
3. If a function does not have a return statement, it is equivalent to returning None.
>>> Def sayHello (): # define the function print ('hello') >>> sayHello () Hello >>> result = sayHello () hello >>> result >>> type (result) <class 'nonetype '>
 
Iv. NotImplemented
 
1. NotImplemented is a constant of the NotImplementedType.
>>> NotImplementedNotImplemented>>> type(NotImplemented)<class 'NotImplementedType'>
2. Using the bool () function for testing, we can find that NotImplemented is a true value.
>>> bool(NotImplemented)True
3. NotImplemented is not a constant in the absolute sense, because it can be assigned but does not throw a syntax error. We should not assign a value to it, otherwise it will affect the execution result of the program.
>>> bool(NotImplemented)True>>> NotImplemented = False>>> >>> bool(NotImplemented)False
4. notImplemented is often used in some binary special methods (such as _ eq _, _ lt _) as return values, indicating that there is no implementation method, however, when Python returns NotImplemented, it intelligently exchanges two parameters for another attempt.
>>> Class A (object): def _ init _ (self, name, value): self. name = name self. value = value def _ eq _ (self, other): print ('self: ', self. name, self. value) print ('other: ', other. name, other. value) return self. value = other. value # determine whether the values of the two objects are equal >>> a1 = A ('Tom ', 1) >>> a2 = A ('Jay', 1) >>> a1 = a2self: Tom 1 other: Jay 1 True
>>> class A(object):    def __init__(self,name,value):        self.name = name        self.value = value    def __eq__(self,other):        print('self:',self.name,self.value)        print('other:',other.name,other.value)        return NotImplemented>>> a1 = A('Tom',1)>>> a2 = A('Jay',1)>>> a1 == a2self: Tom 1other: Jay 1self: Jay 1other: Tom 1False
When a1 = a2 (call _ eq _ (a1, a2) and return NotImplemented, Python will automatically call _ eq _ (a2, a1 ).
 
V. Ellipsis
 
1. Ellipsis is a constant of the ellipsis type... Is equivalent.
>>> EllipsisEllipsis>>> type(Ellipsis)<class 'ellipsis'>>>> ...Ellipsis>>> ... == EllipsisTrue
2. Using the bool () function for testing, we can find that Ellipsis is a true value.
>>> bool(Ellipsis)True
3. Ellipsis is not a constant in the absolute sense, because it can be assigned, but it will not throw a syntax error. We should not assign a value to it, otherwise it will affect the execution result of the program.
>>> bool(Ellipsis)True>>> Ellipsis = False>>> bool(Ellipsis)False
4. Ellipsis is mostly used to represent the cyclic data structure.
>>> a = [1,2,3,4]>>> a.append(a)>>> a[1, 2, 3, 4, [...]]>>> a[1, 2, 3, 4, [...]]>>> len(a)5>>> a[4][1, 2, 3, 4, [...]]>>>
 
6. _ debug __
 
1. _ debug _ is a constant of the bool type.
>>> __debug__True>>> type(__debug__)<class 'bool'>
2. A syntax error is thrown when any value assignment operation is performed on constant _ debug.
>>> __debug__ = FalseSyntaxError: assignment to keyword
3. If Python is not started using the-O option, this constant is the true value; otherwise, it is a false value.