write and Replace particular line in file

Question

I want to replace value of key(i.e db_host, addons_path) with $$$$.

Input text file contains the following:

#         


        

Answer 1

What about something like this ?

def replace_string(s):
    s = s.replace('db_host','$$$$')
    return s
with open('test.txt','r') as fo:
    line = fo.read() # you could use strip() or split() additionally
new_string = replace_string(line)

Answer 2

This is much simpler achieved with UNIX tools.

Nevertheless here's my solution:

bash-4.3$ cat - > test.txt
#Test.txt#
addons_path=/bin/root
admin_passwd = abctest
auto_reload = False
csv_internal_sep = ,
db_host = 90.0.0.1
bash-4.3$ python
Python 2.7.6 (default, Apr 28 2014, 00:50:45) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> with open("test.txt", "r") as f:
...     pairs = (line.split("=", 1) for line in f if not line.startswith("#"))
...     d = dict([(k.strip(), v.strip()) for (k, v) in pairs])
... 
>>> d["db_host"] = "$$$$"
>>> with open("out.txt", "w") as f:
...     f.write("\n".join(["{0:s}={1:s}".format(k, v) for k, v in d.items()]))
... 
>>> 
bash-4.3$ cat out.txt
db_host=$$$$
admin_passwd=abctest
auto_reload=False
csv_internal_sep=,
addons_path=/bin/rootbash-4.3$ 

1. Read the file and parse it's key/value pairs by = into a dict (ignoring comments).
2. Change the db_host key in the resulting dictionary.
3. Write out the dictionary using = as key/value separators.

Enjoy :)

Update: As a reuseable set of functions:

def read_config(filename):
    with open(filename, "r") as f:
        pairs = (line.split("=", 1) for line in f if not line.startswith("#"))
        return dict([(k.strip(), v.strip()) for (k, v) in pairs])


def write_config(d, filename):
    with open(filename, "w") as f:
        f.write("\n".join(["{0:s}={1:s}".format(k, v) for k, v in d.items()]))

Answer 3

If you want to do that with Python, you can use the following function:

def replace_in_file(filename, key, new_value):
    f = open(filename, "r")
    lines = f.readlines()
    f.close()
    for i, line in enumerate(lines):
        if line.split('=')[0].strip(' \n') == key:
            lines[i] = key + ' = ' + new_value + '\n'
    f = open(filename, "w")
    f.write("".join(lines))
    f.close()

replace_in_file("file.txt", 'db_host', "7777")

Answer 4

use fileinput module.

import fileinput

for line in fileinput.input('data.txt',backup='.bak',inplace=1):
    print line.rstrip().replace('Python','Perl') 
    #or print line.replace('Python','Perl'),