Python: How to remove a comment line and write on the same file?

Question

I've a list of commented file where I need to remove the comment lines in that files(#) and need to write on that same file.

comment file:

#Hi this the comment
define host {
        use             template
        host_name       google_linux
        address         192.168.0.12
}

#commented config
#ddefine host {
#d      use             template1
#d      host_name       fb_linux
#d      address         192.168.0.13
#d}

The code I wrote to remove the comment line in a file ?

code:

>>> with open('commentfile.txt','r+') as file:
...     for line in file:
...        if not line.strip().startswith('#'):
...             print line,
...             file.write(line)
...
define host {
        use             template
        host_name       google_linux
        address         192.168.0.12
}

>>> with open('commentfile.txt','r+') as file:
...     for line in file:
...        if line.strip().startswith('#'):
...             continue
...        print line,
...        file.write(line)
...
define host {
        use             template
        host_name       google_linux
        address         192.168.0.12
}

I tried using the above two methods the print output returns correct but could not able to write on the same file again.

Output in file:

cat commentfile.txt
#Hi this the comment
    define host {
            use             template
            host_name       google_linux
            address         192.168.0.12
    }

    #commented config
    #ddefine host {
    #d      use             template1
    #d      host_name       fb_linux
    #d      address         192.168.0.13
    #d}

Expected Output:

 cat commentfile.txt
 define host {
                use             template
                host_name       google_linux
                address         192.168.0.12
        }

I've even tried Regular expression method but didn't work to write on the same file.

RE method:

for line in file:
           m = re.match(r'^([^#]*)#(.*)$', line)
           if m:
              continue

Any hint would be helpful ?

Writing to a file while you're reading from it is… tricky. Best read the entire file into memory, manipulate it, then dump it back into the file in one go. It's not that big so the memory consumed doesn't matter. — deceze
– deceze ♦, Commented Jul 29, 2016 at 12:56
If it's a small file, read the entire file into memory, close the file, do your processing and write it back out. If it's a large file, read the file, process, while writing to another file and when done, rename the temp file to the original file. — Vatine
– Vatine, Commented Jul 29, 2016 at 12:58
@deceze I'm looping over the files. But some files do have 2GB of config data too.is that okay ? if I follow your method. — Prasanna Ranganthan
– Prasanna Ranganthan, Commented Jul 29, 2016 at 13:13
No, you don't want to read 2GB of data into memory (unless you're certain you can spare 2GB of RAM). See @Vatine's suggestion above. — deceze
– deceze ♦, Commented Jul 29, 2016 at 13:15
Do you really need to do this with Python? Why not just do it with sed? — PM 2Ring
– PM 2Ring, Commented Jul 29, 2016 at 13:16

OneCricketeer · Accepted Answer · 2016-07-29 13:15:15Z

0

I don't think you can write lines to a file that you are looping over, you'll need to write out to a different file, which you can move over the original file after the loop.

Or, you can read all lines into memory, close and reopen the file and write over the lines with the newly processed lines

edited Jul 29, 2016 at 13:15

answered Jul 29, 2016 at 12:58

OneCricketeer

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ynot269 · Accepted Answer · 2016-07-29 13:21:59Z

0

Some pseudocode

open file in read open a new file in write mode call it temp loop through and do some operation (delete comments, add what ever you need to add) and write to the temp file close the original file and delete it then rename your temp file to your old file so it would be like

fileVar = open(file, 'r')
tempFile = open(file, 'w')
for line in file:
     # do your operations
     #you could write at the same time 
     tempFile.write(linePostOperations)
fileVar.close()
os.remove(filePath)
tempFile.close()
os.rename(tempFileName, newFileName)

answered Jul 29, 2016 at 13:21

ynot269

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