from datetime import datetime
strtime = '12:43:51.578'
strformat = '%H:%M:%S'
typedatetime = datetime.strptime(strtime[:-4], strformat)
milliseconds = int(strtime[-3:])
typedatetime = typedatetime.replace(microsecond=milliseconds * 1000)
print(typedatetime)
'''
run:
1900-01-01 12:43:51.578000
'''