UPDATE (previous answer was not correct, this is the working code from @konstantin):
...
self.ser = serial.Serial(comDev, 115200, timeout=10)
...
#Function that continue to read from Serial port until 'readUntil'
#sequence of symbols appears
def CheckReadUntil(self, readUntil):
outputCharacters = []
while 1:
ch = self.ser.read()
if len(ch) == 0:
break
outputCharacters += ch
if outputCharacters[-len(readUntil):]==readUntil:
break
outputLines = ''.join(outputCharacters)
return outputLines