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

浅浅的花香味﹌ 2022-07-14 01:48 389阅读 0赞

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

先看一个错误：

  1. >>> a = 1
  2. >>> print a
  3.   File "<stdin>", line 1
  4.     print a
  5.           ^
  6. SyntaxError: Missing parentheses in call to 'print'
  7. >>> print(a)
  8. 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：

  1. >>> Null
  2. Traceback (most recent call last):
  3.   File "<stdin>", line 1, in <module>
  4. NameError: name 'Null' is not defined
  5. >>> NULL
  6. Traceback (most recent call last):
  7.   File "<stdin>", line 1, in <module>
  8. NameError: name 'NULL' is not defined
  9. >>> nullptr
  10. Traceback (most recent call last):
  11.   File "<stdin>", line 1, in <module>
  12. NameError: name 'nullptr' is not defined

所以接下来要隆重介绍一下None
None和False不同：

  1. >>> None == False
  2. False

None不是0：

  1. >>> None == 0
  2. False

None不是空字符串：

  1. >>> None == '' False

None和任何其他的数据类型比较永远返回False：

  1. >>> None == 1
  2. False
  3. >>> None == 1.2
  4. False

None有自己的数据类型NoneType:

  1. >>> type(None)
  2. <class 'NoneType'>

Python’s None is Object-Orientated

None是使用is还是==

  1. null_variable = None
  2. not_null_variable = 'Hello There!'
  4. # The is keyword
  5. if null_variable is None:
  6.     print('null_variable is None')
  7. else:
  8.     print('null_variable is not None')
  10. if not_null_variable is None:
  11.     print('not_null_variable is None')
  12. else:
  13.     print('not_null_variable is not None')
  16. # The == operator
  17. if null_variable == None:
  18.     print('null_variable is None')
  19. else:
  20.     print('null_variable is not None')
  22. if not_null_variable == None:
  23.     print('not_null_variable is None')
  24. else:
  25.     print('not_null_variable is not None')

输出：

  1. null_variable is None
  2. not_null_variable is not None
  3. null_variable is None
  4. not_null_variable is not None

特俗情况，类中：

  1. class MyClass:
  2.     def __eq__(self, my_object):
  3.         # We won't bother checking if my_object is actually equal
  4.         # to this class, we'll lie and return True. This may occur
  5.         # when there is a bug in the comparison class.
  7.         return True
  9. my_class = MyClass()
  11. if my_class is None:
  12.     print('my_class is None, using the is keyword')
  13. else:
  14.     print('my_class is not None, using the is keyword')
  16. if my_class == None:
  17.     print('my_class is None, using the == syntax')
  18. else:
  19.     print('my_class is not None, using the == syntax')

输出：

  1. my_class is not None, using the is keyword
  2. my_class is None, using the == syntax

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