Why does ConfigParser not write to disk immediately?

Question

Given the following script:

import ConfigParser
from datetime import datetime
import time

def write_stuff():
    section = "test"
    item = "oh hey there"
    conf_filename = "test.conf"

    conf = ConfigParser.ConfigParser()
    conf.readfp(open(conf_filename, 'r', 0))

    timestamp = datetime.now().strftime("%Y-%m-%d_%H%M%S")

    conf.set(section, timestamp, item)

    with open(conf_filename, "w", 0) as conf_file:
        # time.sleep(1)
        conf.write(conf_file)

write_stuff()
write_stuff()
write_stuff()
write_stuff()

It will only write one entry to the file, as illustrated by:

$ touch test.conf
$ python tests.py  # this is what I've named the above
$ $ cat test.conf
[test]
2012-10-10_231439 = oh hey there

However, if you uncomment the time.sleep(1), all entries appear. Strangely (to me, anyway,) this even happens if you have one call to write_stuff(), and call the script in quick succession from the shell. I would think that once Python exits, whatever is going to disk would have gone to disk. What's going on?

Environment: Python 2.7.3 on Mac OS X 10.8

score 3 · Accepted Answer · answered Oct 11 '12 at 08:48

The problem here is that the key value you are using in the config file is a time stamp with a 1 sec resolution. This means, when you call write_stuff() four times in a row, the time hasn't changed, the time stamp doesn't change, and you simply overwrite the previous value, rather than adding a new value.

What you need to do is generate a unique key value each time. If you want to keep the timestamp value, something this would work:

count = 0

def write_stuff():
    global count

    section = "test" 
    item = "oh hey there" 
    conf_filename = "test.conf" 

    conf = ConfigParser.ConfigParser() 
    conf.readfp(open(conf_filename, 'r', 0)) 

    timestamp = datetime.now().strftime("%Y-%m-%d_%H%M%S")+ "_%s" % count
    count += 1

    conf.set(section, timestamp, item) 

    with open(conf_filename, "w", 0) as conf_file: 
        conf.write(conf_file)

Note, the values written to the config file won't be in any particular order.

root · Answer 2 · 2012-10-11T06:54:19.707

You are writing the same entry over and over againg, use "a" instead of "w" to append the file:

with open("test.txt", "a") as myfile:
    myfile.write("appended text")

Mybe you want to something like this, so the section gets printed once, and you can add multiple items to it:

config.add_section('Section1')
config.set('Section1', 'an_int', '15')
config.set('Section1', 'a_bool', 'true')
config.set('Section1', 'a_float', '3.1415')
config.set('Section1', 'baz', 'fun')
config.set('Section1', 'bar', 'Python')
config.set('Section1', 'foo', '%(bar)s is %(baz)s!')

# Writing our configuration file to 'example.cfg'
with open('example.cfg', 'wb') as configfile:
    config.write(configfile)

Output:

[Section1]
bar = Python
baz = fun
a_bool = true
an_int = 15
foo = %(bar)s is %(baz)s!
a_float = 3.1415

As you see, you should then do it in one write, without calling your function multiple times.

If opened in Append mode, then the config gets written to the file 4 times... not just the Item, but also 4 different sections. I don't believe that is the correct solution. — Alexi Kostibas, Oct 11 '12 at 06:26
if you use write_stuff() 4 times, then it's bound to get written 4 times... what are you trying to achieve? Edit your question with the outcome you want, at the moment it is hard to understand. — root, Oct 11 '12 at 06:28

Why does ConfigParser not write to disk immediately?

2 Answers2