python：防止创建空文件

Question

For a bulk task I create a couple of instances of the ProgressLog object which will each create an empty log-file no matter if there actually will be any errors. what is the best way to prevent this?

class ProgressLog(object):
    """Write a message to log + progress indicator.

    """
    total = 0

    def __init__(self, name):
        source_path, file_name = os.path.split(name)
        self.name = file_name
        self.source_path = source_path
        self.log_dir_name = r'log'
        self.make_log_dir(self.source_path, self.log_dir_name)
        self.reset()
        log_file = self._logfilename()
        try:
            self.f = open(log_file, 'w')
            print('\n***logging errors to {0}***\n'.format(log_file))
        except IOError, err:
            msg = 'Cannot open logfile {0}. Traceback is: {1}'.format(
                log_file, err)
            raise msg

    def _logfilename(self):
        ## hms_ddmmyyyy format
        log_name = r'{1}_{0}{2}_errors.csv'.format(
                                  time.strftime("%I%M%S"),
                                  time.strftime("%d%m%Y"),
                                  self.name)
        return os.path.join(self.source_path, self.log_dir_name, log_name)

Answer 1

There is no "magical" way to do it, you simply need to refactor the code to open the log file only on first actual call to log.

To achieve this, extract the part of __init__ that opens the log file into a separate _open_log method. In __init__ initialize self.f to None . Then, your actual logging method can begin with:

if self.f is None:
    self._open_log()