How do you replace a line of text in a text file (python)

Question

I have a text file that looks like this:

unknown value 1
unknown value 2
unknown value 3
unknown value 4
unknown value 5

How can I choose a line and replace its contents with another string?

For example:

Change unknown value 1 to unknown value 0.

How can I accomplish this?

score 13 · Answer 1 · edited May 11 '16 at 11:47

13

import fileinput

for line in fileinput.input('inFile.txt', inplace=True): 
      print line.rstrip().replace('oldLine', 'newLine'),

This replaces all lines with the text 'oldLine', if you want to replace only the first one then you need to add a condition and break out of the loop.

Adding rstrip() avoids adding an extra space after each line

edited May 11 '16 at 11:47

Community

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1

answered May 18 '13 at 10:28

elyase

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You are potentially modifying multiple lines instead of a single selected one. – kampu May 18 '13 at 11:07
You are right, I added a note. – elyase May 18 '13 at 11:51
This adds a new, blank line between each line (even if there is no match) – s g May 19 '15 at 22:35
I am interested for the solution of the condition and break loop here if possible. If you can add it to the solution I will really appreciate! – Tony Jan 09 '17 at 23:22

score 11 · Answer 2 · answered May 18 '13 at 09:41

11

Try this

with open('file', 'r') as input_file, open('new_file', 'w') as output_file:
    for line in input_file:
        if line.strip() == 'to replace':
            output_file.write('new line\n')
        else:
            output_file.write(line)

answered May 18 '13 at 09:41

oleg

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How do you replace a line of text in a text file (python)

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