Python中的None和空(“SyntaxError: Missing parentheses in call to 'print'”)

浅浅的花香味﹌ 2022-07-14 01:48 304阅读 0赞

**“SyntaxError: Missing parentheses in call to ‘print’”**

先看一个错误：

>>> a = 1
    >>> print a
      File "<stdin>", line 1
        print a
              ^
    SyntaxError: Missing parentheses in call to 'print'
    >>> print(a)
    1

这是什么鬼？

这是Python2和Python3的区别，在Python3中，不能使用print a，需要加括号。

进入正题！

*\* None和空\**

Note that the PyTypeObject for None is not directly exposed in the Python/C API. Since None is a singleton, testing for object identity (using == in C) is sufficient. There is no PyNone\_Check() function for the same reason.

首先需要明确，**Python中没有Null NULL nullptr这样的关键字：**

>>> Null
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'Null' is not defined
    >>> NULL
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'NULL' is not defined
    >>> nullptr
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'nullptr' is not defined

所以接下来要隆重介绍一下None  
**None和False不同：**

>>> None == False
    False

**None不是0：**

>>> None == 0
    False

**None不是空字符串：**

>>> None == '' False

**None和任何其他的数据类型比较永远返回False：**

>>> None == 1
    False
    >>> None == 1.2
    False

**None有自己的数据类型NoneType:**

>>> type(None)
    <class 'NoneType'>

***Python’s None is Object-Orientated***

**None是使用is还是==**

null_variable = None
    not_null_variable = 'Hello There!'
    
    # The is keyword
    if null_variable is None:
        print('null_variable is None')
    else:
        print('null_variable is not None')
    
    if not_null_variable is None:
        print('not_null_variable is None')
    else:
        print('not_null_variable is not None')
    
    
    # The == operator
    if null_variable == None:
        print('null_variable is None')
    else:
        print('null_variable is not None')
    
    if not_null_variable == None:
        print('not_null_variable is None')
    else:
        print('not_null_variable is not None')

输出：

null_variable is None
    not_null_variable is not None
    null_variable is None
    not_null_variable is not None

特俗情况，类中：

class MyClass:
        def __eq__(self, my_object):
            # We won't bother checking if my_object is actually equal
            # to this class, we'll lie and return True. This may occur
            # when there is a bug in the comparison class.
    
            return True
    
    my_class = MyClass()
    
    if my_class is None:
        print('my_class is None, using the is keyword')
    else:
        print('my_class is not None, using the is keyword')
    
    if my_class == None:
        print('my_class is None, using the == syntax')
    else:
        print('my_class is not None, using the == syntax')

输出：

my_class is not None, using the is keyword
    my_class is None, using the == syntax