Restore original file in case of an error

Question

I need to write data to a file and overwrite it if file exists. In case of an error my code should catch an exception and restore original file (if applicable)

How can I restore file? Should I read original one and put content i.e. in a list and then in case of an exception just write this list to a file?

Are there any other options? Many thanks if anyone can provide some kind of an example

Cheers

Answer 1

The code below will make a copy of the original file and set a flag indicating if it existed before or not. Do you code in the try and if it makes it to the end it will set the worked flag to True and your are go to go otherwise, it will never get there, the exception will still be raised but the finally will clean up and replace the file if it had existed in the first place.

import os
import shutil

if os.path.isfile(original_file):
    shutil.copy2(original_file, 'temp' + original_file)
    prev_existed = True
else:
    prev_existed = False

worked = False    
try:
    with open(original_file, 'w') as f:
        ## your code

    worked = True
except:
    raise        
finally:
    if not worked and prev_existed:
        shutil.copy2('temp' + original_file, original_file)

Answer 2

Any attempt to restore on error will be fragile; what if your program can't continue or encounters another error when attempting to restore the data?

A better solution would be to not replace the original file until you know that whatever you're doing succeeded. Write whatever you need to to a temporary file, then when you're ready, replace the original with the temporary.

Answer 3

1. One way to do it is the append the new data at the end of the existing file. If you catch an error , delete all that you appended. If no errors, delete everything that you had before appending so the new file will have only the appended data.

2. Create a copy of your file before starting capturing the data in a new file.