python: prevent creating empty files

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)

Why are you creating the logfile yourself and not let python handle it? docs.python.org/2/howto/logging-cookbook.html — RvdK
– RvdK, Commented Feb 26, 2015 at 10:12
As for as I understand this should be expected open(log_file, 'w') creates a file if none exists. What exactly are you trying to do? When are you expecting an exception to be thrown? — Tejas Pendse
– Tejas Pendse, Commented Feb 26, 2015 at 10:23

user4815162342 · Accepted Answer · 2015-02-26 11:51:47Z

2

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()

edited Feb 26, 2015 at 11:51

answered Feb 26, 2015 at 10:24

user4815162342

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2 Comments

Skaperen Over a year ago

explained another way: open the output (log) file only when/if there is data to be written.

user4815162342 Over a year ago

@Skaperen Thanks, I've now amended the answer to start off with an abstract similar to yours.

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