How to update existing line of file in Python?
Example: I want to update session.xml to sesion-config.xml without writing new line.
Input A.txt:
fix-config = session.xml
Expected output A.txt:
fix-config = session-config.xml
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Do you want to override a single line, or override the whole file?Ahasanul Haque– Ahasanul Haque2016-11-29 11:50:44 +00:00Commented Nov 29, 2016 at 11:50
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1Possible duplicate of Search and replace a line in a file in PythonAhasanul Haque– Ahasanul Haque2016-11-29 11:52:31 +00:00Commented Nov 29, 2016 at 11:52
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2 Answers
You can't mutate lines in a text file - you have to write an entirely new line, which, if not at the end of a file, requires rewriting all the rest.
The simplest way to do this is to store the file in a list, process it, and create a new file:
with open('A.txt') as f:
l = list(f)
with open('A.txt', 'w') as output:
for line in l:
if line.startswith('fix-config'):
output.write('fix-config = session-config.xml\n')
else:
output.write(line)
3 Comments
Ahasanul Haque
there is an exact duplicate of this question here. What do you think?
Jarvis
If you see a common question with a popular canonical duplicate, you can vote or flag the question for closure with that duplicate.
TigerhawkT3
@Jarvis - Those are really familiar words. :P
The solution @TigerhawkT3 suggested would work great for small/medium files. For extremely large files loading the entire file into memory might not be possible, and then you would want to process each line separately. Something along these lines should work:
import shutil
with open('A.txt') as input_file:
with open('temp.txt', 'w') as temp_file:
for l in input_file:
if l.startswith('fix-config'):
temp_file.write('fix-config = session-config.xml\n')
else:
temp_file.write(l)
shutil.move('temp.txt', 'A.txt')
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