Python: replace line in file starting with a pattern

Question

In a text file there is this line

initial_mass = unknown_int

I want to replace the line with

initial_mass = known_int

As the names suggest, I won't know the initial value but I want to replace whatever it is with a known value. Using Python, how do I search for a line that starts with 'initial_mass', then replace the entire line with 'initial_mass = known_int'?

I am thinking along the lines of

import fileinput
    for line in fileinput.FileInput(filename, inplace=True):
    line=line.replace(old, new)
    print(line),

How can I set old to the line containing initial_mass = uknown_int? I have looked at startswith() but I don't know how to use that to get what I want. I've also tried regex and find it utterly confusing.

The file is not particularly large, but I will need to iterate over this process many times.

I am unfamiliar with sed/awk. I also find sed utterly confusing. Could you write with commentary the code you are thinking would achieve the same purpose? — astromonerd
– astromonerd, Commented Sep 9, 2014 at 16:23

unutbu · Accepted Answer · 2014-09-09 16:35:59Z

22

You don't need to know what old is; just redefine the entire line:

import sys
import fileinput
for line in fileinput.input([filename], inplace=True):
    if line.strip().startswith('initial_mass = '):
        line = 'initial_mass = 123\n'
    sys.stdout.write(line)

edited Sep 9, 2014 at 16:35

answered Sep 9, 2014 at 16:06

unutbu

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5 Comments

astromonerd Over a year ago

This is not working for me. The code compiles but no changes are made in the file.

astromonerd Over a year ago

To note, the unknown and known values are part of the string, and not typed numbers. I see the \n represents numbers. I tried removing it and replacing it with \s for string, but the file remains unchanged.

unutbu Over a year ago

I tested the code on a sample file; it works for me. I don't see a plausible reason why it should not be working for you (assuming the obvious stuff like the filename is correct -- which is probably true since you do not report an OSError, and assuming the lines actually do begin with 'initial_mass = '.) The \n represents a new line character. It should not be changed to \s.

astromonerd Over a year ago

Do white spaces count? EDIT: yeah they do!

unutbu Over a year ago

Whitespace is important. If the lines begin with spaces, line.startswith('initial_mass = ') will be False. That would explain the problem. You could use line.strip().startswith('initial_mass = ') instead, however.

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