Python replace text value regular expression

Question

Is it possible to just replace a specific text's value in a file python? The below code replaces the string however I could'nt find a way to replace only the project_id or project_name's value.

import re 

def replace():
    with open("/Users/test/file.txt", "r") as sources:
        lines = sources.readlines()
    with open("/Users/test/file.txt", "w") as sources:
        for line in lines:
            sources.write(re.sub(r"Settings", 'Project_settings', line))

    
replace()

file.txt

Settings
######################
project_id = "468324678997"
project_name = "test"

output:

Project_settings
######################
project_id = "468324678997"
project_name = "test"

I would like to replace project_name's value to "abc"

Desired output: file.txt

Settings
######################
project_id = "468324678997"
project_name = "abc"

Can you add your desired output? Do you want to replace the word "test" in your example file? — Iain Shelvington, Aug 06 '21 at 03:52
What happened when you tried running the code, and how is that different from your desired result? — Karl Knechtel, Aug 06 '21 at 04:00
In your own words, where you have `re.sub(r'^# project_name', 'abc', line)`, what *exactly* do you think that means? What source are you using to learn how to use regular expressions, and what happened when you tried following a tutorial? — Karl Knechtel, Aug 06 '21 at 04:01
Updated the question, i'm using re.sub to replace the string and it works. however i'm having issues to replace only the variable value using regex. — nad87563, Aug 06 '21 at 04:26
Perhaps that's because there is no Python variable named "project_name." That's a piece of text that you are writing to a file, not the name of a variable in a Python script. — Paul Cornelius, Aug 06 '21 at 04:28
Why does your expected output have `Settings` and your code has `Project_settings`? — OneCricketeer, Aug 06 '21 at 04:38
Also, what do you expect to happen if you had a line like `project_name = "Settings Test"`? — OneCricketeer, Aug 06 '21 at 04:39
Settings is not going to occur anywhere in the file. Why does your expected output have Settings and your code has Project_settings - its just an example code for replacing a specific string in a file. — nad87563, Aug 06 '21 at 04:52

score 2 · Accepted Answer · answered Aug 06 '21 at 04:44

I suggest using a dictionary of config overrides, not regex

This way, you are not accidentally replacing all matching text

def replace(file, overrides):
    with open(file, "r") as sources:
        lines = sources.readlines()
    with open(file, "w") as sources:
        # skip the header rows
        next(lines)
        next(lines)
        for line in lines:
            config_key = line.split(' = ')[0]
            if config_key in overrides:
                sources.write('{} = {}\n'.format(config_key, overrides[config_key]))

overrides = {
  'project_name': 'abc'
}
replace("/Users/test/file.txt", overrides)

There are likely better Python libraries that can read property files and allow you to override specific values

IMO, that isnt really needed because if the headers are removed you can just `import config.py` — OneCricketeer, Aug 06 '21 at 05:20
So, we agree in that point: once recognized the _major pattern_ of given file-format (and ignoring headers) you can just use existing libs to parse & modify. — hc_dev, Aug 06 '21 at 11:31

hc_dev · Answer 2 · 2021-08-06T11:40:25.757

Automate the text-editor's regex-replace

My recipe for replacing each matching line in an arbitrary formatted text-file.

As arguments to re.sub you could use a pair of regular-expressions: pattern-to-match and replacement, like regex-tuple (r'^Settings', r'Project_settings').

The data structure to store these could be a list of tuples (pairs), like replacements.

import re

replacements = [
    ('^Settings', 'Project_settings'),
    (r'^project_name = "(.*)"', r'project_name = "abc"'), # simply replaces with fixed value
    (r'^project_id = "(.*)"', r'project_id = "P-\1"')  # copies in the found group 1 at \1
]


def replace():
    lines = []
    with open("project_properties.cfg", "r") as config:
        for line in config.readlines():
            for pair in replacements:  # apply each replacement that matches line
                line = re.sub(pair[0], pair[1], line)  # substitute found pattern with replacement
            lines.append(line)  # add each read line to the buffer (if changed or not)

    with open("project_properties_.cfg", "w") as config: # Note: used a new filename
        config.writelines(lines)


replace()

Will result in desired output (plus inserting-replacement at project_id):

Project_settings
######################
project_id = "P-468324678997"
project_name = "abc"

Parsing and Updating with existing libs

As OneCricketeer's answer suggests:

When you manage to ignore the header, you will recognize strong similarity (i.e. key = value representation) to at least two common config-file formats:

This would allow you to research and find existing libraries, like Python's built-in configparser:

to modify an INI-file
or properties-file

Python replace text value regular expression

2 Answers2

Automate the text-editor's regex-replace

Parsing and Updating with existing libs