Checking file if exist then append record

Question

I am creating a log file with line by line records.

1- If file does not exist, it should create file and append header row and the record 2- if it exists, check the text timeStamp in first line. If it exist then append the record otherwise add header columns and record itself

I tried both w,a and r+; nothing worked for me. Below is my code:

logFile = open('Dump.log', 'r+')
datalogFile = log.readline()
if 'Timestamp' in datalogFile:
    logFile.write('%s\t%s\t%s\t%s\t\n'%(timestamp,logread,logwrite,log_skipped_noweight))
    logFile.flush()
else:
    logFile.write('Timestamp\t#Read\t#Write\t#e\n')
    logFile.flush()
    logFile.write('%s\t%s\t%s\t%s\t\n'%(timestamp,logread,logwrite,log_skipped))
    logFile.flush()

Code fails if file don't exist

I can't see `logFile` being defined anywhere. Also, what is the error you get? — Tejas Pendse, Mar 04 '14 at 14:04
Yeah but is there no switch that create file if don't exist and append date instead of overwrite everytime? — Volatil3, Mar 04 '14 at 14:07
Possibly duplicated question (http://stackoverflow.com/questions/2967194/python-open-does-not-create-file-if-it-doesnt-exist). — wolfrevo, Mar 04 '14 at 14:15

ndpu · Accepted Answer · 2014-03-04T14:15:15.783

6

Use 'a+' mode:

logFile = open('Dump.log', 'a+')

description:

a+
Open for reading and writing. The file is created if it does not exist. The stream is positioned at the end of the file. Subsequent writes to the file will always end up at the then current end of file, irrespective of any intervening fseek(3) or similar

edited Mar 04 '14 at 14:15

answered Mar 04 '14 at 14:10

ndpu

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6
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score 1 · Answer 2 · answered Mar 04 '14 at 14:11

1

Following code would work:

import os
f = open('myfile', 'ab+') #you can use a+ if it's not binary
f.seek(0, os.SEEK_SET)
print f.readline() #print the first line
f.close()

answered Mar 04 '14 at 14:11

venpa

4,268
21
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wap26 · Answer 3 · 2014-03-04T14:10:55.757

0

Try this:

import os
if os.path.exists(my_file):
    print 'file does not exist'
    # some processing
else:
    print 'file exists'
    # some processing

edited Mar 04 '14 at 14:10

answered Mar 04 '14 at 14:05

wap26

2,180
1
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I could definitely do it but I was wondering whether some file switch can do the trick? – Volatil3 Mar 04 '14 at 14:05
@user3 I tried append switch but I need to read too before append. It is not reading at all – Volatil3 Mar 04 '14 at 14:07
if file does not exist, `open` raises an `IOError` so apart from catching this exception, there is no way – wap26 Mar 04 '14 at 14:08

score 0 · Answer 4 · answered Mar 04 '14 at 14:15

You're opening the file in r+ mode which means you assume the file exists. Also, if you intend the write on the file, you should open it with a+ mode (unashamedly stealing ndpu's explanation) Your code would become:

    logFileDetails = []
    with open("Dump.log","a+") as logFile:
        logFileDetails = logFile.readLines()
        if logFileDetails and "Timestamp" in logFileDetails:
            pass # File exists, write your stuff here
        else:
            pass # Log file doesn't exist, write timestamp here

score 0 · Answer 5 · answered Oct 11 '15 at 20:17

Checking a file existence introduces a race condition, i.e. another process can create it or delete it after the check returns false or true, respectively, creating heavy bugs. You should instead use:

if open('path\to.filename', 'a+') != '':
  stuff_if_exists
else:
  stuff_if_not_exists

Checking file if exist then append record

5 Answers5