Python: Replace a string as variable in a file

Question

If I have a file contents like this:

old_string
-old_string

I want to change only "old_string" to be "+new_string" so the result looks like

+new_string
-old_string

my code gives this result:

+new_string
-+new_string

This is my code :

    with open(filename) as f:

    s = f.read()

    if old_string not in s:

        return False

with open(filename, 'w') as f:

    s = s.replace(old_string, new_string)

    f.write(s)

    return True

i Tried regex, but it won't work since I pass the regex as a variables, this is what I did so far:

    with open (filename, 'r' ) as f:

       content = f.read()

content_new = re.sub('(\%old_string)', r'\new_string'%(old_string,new_string), content, flags = re.M)

You can probably write your own implementation of replace that checks if the previous char is a '-'. — Greg Schmit
– Greg Schmit, Commented Feb 18, 2017 at 15:07
@Adam - Would really appreciate it if you marked the answer that helped you solve your problem as the correct answer :-) — wandering-geek
– wandering-geek, Commented Feb 18, 2017 at 15:26

wandering-geek · Accepted Answer · 2017-02-18 15:14:33Z

You could just ignore the lines which have a hyphen("-") at the beginning, and replace the rest.

The script below is a little different from yours. I have put in comments to help you understand. This should be easy to understand.

filename ="some_file"
output_filename = "some_other_file"

old_string = "old_string"
new_string = "+new_string"

input_file_handle = open(filename,"r") # File being opened in read mode
output_file_handle = open(output_filename, "w") # File being opened in write mode

# Read in input file line by line
for line in input_file_handle:

    # Write to output file and move on to next line
    if old_string not in line:
        output_file_handle.write(line+"\n")
        continue

    # This line contains the old_string. We check if it starts with "-". 
    # If it does, write original line and move on to next line
    if line.startswith("-"):
        output_file_handle.write(line+"\n")
        continue


    # At this stage we are absolutely sure we want to replace this line's contents
    # So we write the replaced version to the new file
    output_file_handle.write(new_string+"\n")


# Close both file handles
input_file_handle.close()
output_file_handle.close()

l'L'l · Accepted Answer · 2017-02-18 16:46:35Z

1

The syntax is a bit off; you'll probably want to do something more like this:

import re

test_str = ("old_string\n"
            "-old_string")

match = "old_string"
subst = "+new_string"

regex = r"^{}".format(match)

# You can manually specify the number of replacements by changing
# the 4th argument

result = re.sub(regex, subst, test_str, 0)

if result:
    print (result)

# Note: for Python 2.7 compatibility, use ur"" to prefix 
# the regex and u"" to prefix the test string and substitution.

The ^ assertion in the pattern is something I recommend using, as it indicates the string to match should start at the very beginning of the line thus won't match -old_string.

edited Feb 18, 2017 at 16:46

answered Feb 18, 2017 at 15:12

l'L'l

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Comments

Greg Schmit · Accepted Answer · 2017-04-17 22:57:45Z

1

A benefit to my solution is that it does not rely on the "do-not-replace" prefix being at the beginning of the line.

If you wanted to solve this problem without regular expressions you could write your own replace method:

replace.txt:

old_string
-old_string

old_string -old_string --old_string old_string

replace.py:

import sys
import fileinput

def replace_exclude(string, search, replace="", excluding_char='-'):
    # Does replace unless instance in search string is prefixed with excluding_char.
    if (not string) or (not search): return None
    for i in range(len(string)):
        while string[i-1] == excluding_char:
            i += 1
        if i < len(string):
            for j in range(len(search)):
                possible = True
                if not (string[i + j] == search[j]):
                    possible = False
                    break
        if possible:
            string = string[0:i] + replace + string[i+len(search):]
            i += len(replace)
    return string

filename = "replace.txt"

for line in fileinput.input([filename], inplace=True):
    sys.stdout.write(replace_exclude(line, "old_string", "+new_string"))

replace.txt after running replace.py:

+new_string
-old_string

+new_string -old_string --old_string +new_string

edited Apr 17, 2017 at 22:57

answered Feb 18, 2017 at 16:16

Greg Schmit

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4 Comments

Adam Over a year ago

Hey Greg, Would that work for python 2.7 ? I get error in print(replace_exclude(line, "old_string", "+new_string"), end='')

Greg Schmit Over a year ago

Oh, no I develop in Python 3, and I recommend you upgrade to it. If you want specific answers in the old, outdated, legacy Python 2, then you should tag your question specifically python2.x or python2.7.

Greg Schmit Over a year ago

As an aside, I should say that my claim that Python 2 is legacy should be backed by evidence: wiki.python.org/moin/Python2orPython3

Greg Schmit Over a year ago

I edited my answer to include Python 2.7 support by using sys.stdout.write instead of print. The actual replace_exclude method didn't need to change, just the method of writing out.

Piyush S. Wanare · Accepted Answer · 2017-02-18 17:17:47Z

-1

This will work for you:-

import re
with open("output.txt", "a") as myfile:

    with open('input.txt') as f:
        lines = f.readlines()
        for line in lines:
            # print str(line)
            ret = re.sub(r"(\s|^|$)old_string(\s|^|$)",r" +new_string ",line) #It will replace if line contain 'old_string' by '+new_string'
            # print ret
            myfile.write(ret+'\n')

Note:- Check output.txt

edited Feb 18, 2017 at 17:17

answered Feb 18, 2017 at 15:30

Piyush S. Wanare

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7 Comments

Adam Over a year ago

where did you specify "old_string" ?

Piyush S. Wanare Over a year ago

@Adam Input string is your old string.

Piyush S. Wanare Over a year ago

@Adam,check I have update the ans try to run this by replacing your file name.

Adam Over a year ago

That seems to be fine, but if I have old_string after a word like for example options old_string. it returns it like options+new_string maybe I didn't explain it in my question

Adam Over a year ago

Yes I tried it, it works fine if the old_string in the start of the sentence, but if it comes after a string as I explained, it mess it up.

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