python - 如何检查对象是否已在python中初始化？

Question

如何检查一个对象是否已经在 python 中初始化？我在多个地方有 my_tac = tac(EDL_port) ，在它已经初始化的情况下它会抛出如下所示的错误..我想添加检查 my_tac 是否已初始化，该怎么做？代码:-

#From another file
my_tac = tac(EDL_port)

#alpha.py
class tac:
   ## tac class constructor.
   #
   #  @param self
   #  @param timeout
   #  @param baud_rate
   def __init__(self, port=None, timeout=0.3, baud_rate=115200):
      if port is not None:
         self.port = port
         self.ser = serial.Serial(self.port, baud_rate, timeout=timeout)
         return
      else:
         (tac_ports,spider_ports) = tac_probe_for_devices()
         print "TAC ports..."
   ...................... 

错误:-

11/5/2016 8:54:05 PM: my_tac = tac(EDL_port)
11/5/2016 8:54:05 PM: File "C:\CST_QSPR\third_party_bin\BuildLoaderScripts\Android\android_dl\alpaca.py", line 90, in __init__
11/5/2016 8:54:05 PM: self.ser = serial.Serial(self.port, baud_rate, timeout=timeout)
11/5/2016 8:54:05 PM: File "C:\Python27\lib\site-packages\serial\serialwin32.py", line 31, in __init__
11/5/2016 8:54:05 PM: ERROR:adb -s C2687475 remount failed
11/5/2016 8:54:05 PM: super(Serial, self).__init__(*args, **kwargs)
11/5/2016 8:54:05 PM: File "C:\Python27\lib\site-packages\serial\serialutil.py", line 236, in __init__
11/5/2016 8:54:05 PM: self.open()
11/5/2016 8:54:05 PM: File "C:\Python27\lib\site-packages\serial\serialwin32.py", line 62, in open
11/5/2016 8:54:05 PM: raise SerialException("could not open port {!r}: {!r}".format(self.portstr, ctypes.WinError()))
11/5/2016 8:54:05 PM: serial.serialutil.SerialException: could not open port 'COM8': WindowsError(5, 'Access is denied.')

Answer 1

OP 请求它，OP 接收它。这是单例模式的示例，仅分解为一个类。

class Test :
    count = 0
    def __init__(self, value) :
        if self.__class__.count > 0 :
            raise Exception
        else :
            self.__class__.count += 1
            self.value = value          #for the sake of the example


t1 = Test(12)      #instantiates the object
t2 = Test(27)      #throws an error

使用 Python 3.4.3 测试。