"Python" python-built-in constants

Introduction

Python has a few built-in constants, only 6, True, False, None, notimplemented, ellipsis, __debug__

One. True

1.True is the truth constant used by the bool type to represent

>>> truetrue>>> Type (True) <class ' bool ' >

2. Any assignment to the constant true will throw a syntax error

>>> True = 1  File "<stdin>", line 1syntaxerror:can ' t assign to keyword

Two. False

1.False is a constant of type bool that represents false values

>>> falsefalse>>> Type (False) <class ' bool ' >

2. Any assignment to the constant false will throw a syntax error

>>> False = 0  File "<stdin>", line 1syntaxerror:can ' t assign to keyword

Three. None

1.None means no, he's the only value of Nonetype.

>>> none>>> Type (None) <class ' Nonetype ' >

2. Any assignment to the constant none will throw a syntax error

>>> None = 1  File "<stdin>", line 1syntaxerror:can ' t assign to keyword

3. For a function, if there is no return statement, it is equivalent to returning none

>>> def sayhello ():  #定义函数 ...     

Four. notimplemented

1.NotImplemented is a constant of type Notimplementedtype

>>> notimplementednotimplemented>>> Type (notimplemented) <class ' Notimplementedtype ' >

2. Testing with the bool () function reveals that notimplemented is a true truth

>>> bool (notimplemented) True

3.NotImplemented is not a constant in absolute sense, because he can be assigned without throwing a syntax error, and we should not assign it, or it will affect the execution of the program.

>>> bool (notimplemented) true>>> notimplemented = False

4.NotImplemented is used for some binary special methods (such as: __eq__,__it__, etc.) as the return value, indicating that there is no implementation method, and Python will be smart when the results return notimplemented 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  #判断两个对象的value值是否相等a1 = A (' Tome ', 1) a2 = A (' Jim ', 1) print (a1 = A2) #输出 #self:tome 1#other:jim 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 NotImplementeda1 = A (' Tome ', 1) a2 = A (' Jim ', 1) print (a1 = = A2) #输出 # self:tome # other:jim # self:jim # other:to Me # False

When you execute a1 = = A2 (that is, call __eq__ (A1,A2)), when you return to notimplemented, Python automatically switches the parameters to call __eq__ again (A2,A1)

Five. Ellipsis

1.Ellipsis is a constant of type ellipsistype, it and ... is equivalent to

>>> ellipsisellipsis>>> Type (ellipsis) <class ' ellipsis ' >>>> ... ellipsis>>>. = = Ellipsistrue

2. Testing with the bool () function reveals that ellipsis is a true truth

>>> bool (ellipsis) True

3. Ellipsis is not a constant in absolute sense, because he can be assigned but does not throw a grammatical error, and 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 multiple data structures to represent loops

>>> a = [1,2,3,4]>>> a.append (a) >>> a[1, 2, 3, 4, [...] >>> Len (a) 5>>> a[4][1, 2, 3, 4, [...] >>> a[4][4][1, 2, 3, 4, [...]

Six. __debug__

1.__DEBUG__ is a constant of type bool

>>> __debug__true>>> Type (__debug__) <class ' bool ' >

2. Any assignment to the constant __debug__ will throw a syntax error

>>> __debug__ = False  File "<stdin>", line 1syntaxerror:assignment to keyword

3. If Python is not started with the-o option, this constant is true, otherwise it is a false value.

"Python" python-built-in constants