Python 实现使用空值进行赋值 None

i = 1
i = None    # int 型数据置空
s = "string"
s = None    # 字符串型数据置空
l = [1,2,3,4]
l[2] = None   # 列表中元素置空
print(i, s, l) 

a = np.arange(10, dtype=float)
a[2] = np.nan
a[3] = None
print(a)

#判断是否为整数
type(varObj) is types.IntType 

 StringType 
 FloatType 
 DictType 
 TupleType 
 ListType 
 BooleanType

class A:
  pass
class B(A):
  pass
isinstance(A(), A)  # returns True
type(A()) == A    # returns True
isinstance(B(), A)  # returns True
type(B()) == A    # returns False

 try:
   print self.fileReadOpen
 except Exception:
   pass
 else:
   del self.fileReadOpen

def isset(v): 
  try : 
   type (eval(v)) 
  except : 
   return 0 
  else : 
   return 1 

type(None)
<class 'NoneType'>

type('')
<class ''str'>

>>> None == 0     //(int)
False
>>> None == ' '   //(str)
False
>>> None == False  //(boolean)
False

>>> d = {"a":1}
>>> dd = d.copy()
>>> d is dd
False
>>> d == dd
True

>>> inf = float("inf")
>>> ninf = float("-inf")
>>> nan = float("nan")
>>> inf is inf
True
>>> ninf is ninf
True
>>> nan is nan
True
>>> inf == inf
True
>>> ninf == ninf
True
>>> nan == nan  // nan对象时本身对象nan
False
>>> inf is float("inf")
False
>>> ninf is float("-inf")
False
>>> nan is float("nan")
False
>>> inf == float("inf")
True
>>> ninf == float("-inf")
True
>>> nan == float("nan")  //前一个nan值不等于新建对象float("nan")的值
False

>>> import math
>>> math.isinf(inf)
True
>>> math.isinf(ninf)
True
>>> math.isnan(nan)
True
>>> math.isinf(float("inf"))
True
>>> math.isinf(float("-inf"))
True
>>> math.isnan(float("nan"))
True

>>> def isnan(num):
  return num != num
>>> isnan(float("nan"))
True

>>> import numpy as np
>>> 
>>> np.isnan(np.nan)
True
>>> np.isnan(float("nan"))
True
>>> np.isnan(float("inf"))
False