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How to replace a line that has a specific text

I want to search for particular text and replace the line if the text is present in that line.
In this code I replace line 125, but want to replace dynamically according to the text:
file = open("config.ini", "r")
lines = file.readlines()
lines[125] = "minimum_value_gain = 0.01" + '\n'
f.writelines(lines)
f.close()
How do I make it that if a line has:
minimum_value_gain =
then replace that line with:
minimum_value_gain = 0.01

There is no reason for you to manually parse a config.ini file textually. You should use configparser to make things much simpler. This library reads the file for you, and in a way converts it to a dict so processing the data is much easier. For your task you can do something like:
import configparser
config = configparser.ConfigParser()
config.read("config.ini")
for section in config:
if config.has_option(section, "minimum_value_gain"):
config.set(section, "minimum_value_gain", "0.01")
with open("config.ini", 'w') as f:
config.write(f)

Since you are replacing complete line so if statement will do the trick for you, no need to replace text
#updated make sure one line doesn't have both values
file = open("config.ini", "r")
lines=file.readlines()
newlines = []
for line in lines:
if "minimum_value_gain" in line:
line = "minimum_value_gain = 0.01" + '\n'
if "score_threshold" in line:
line = "Values you want to add"+'\n'
newlines.append(line)
f.writelines(newlines)
f.close()

Little bit messy and not optimized but get's the job the, first readlines and inserts the next_text to the given pos(line). If the line doesn't exists Raises IndexError, else writes to the file
def replace_in_file(filename: str, search_text: str, string_to_add: str) -> None:
with open(filename, "r+") as file_to_write:
lines = file_to_write.readlines()
file_to_write.seek(0)
file_to_write.truncate()
for line in lines:
if line.startswith(search_text):
line = line.rstrip("\n") + string_to_add + "\n"
file_to_write.write(line)
replace_in_file("sdf.txt", "minimum_value_gain", " = 0.01")

You can use also the regex library of Python.
Here is an example.
It is better not to read and write in the same file, that is not good practice. Write in a different file then eventually rename it.
import re
pattern = 'minimum_value_gain'
string_to_replace = 'minimum_value_gain = 0.01\n'
file = open("config.ini", "r")
fileout = open("new_config.ini", "a")
lines=file.readlines()
newlines = [string_to_replace if re.match(pattern, line) else line for line in lines]
f.close()
fileout.writelines(lines)
fileout.close()
You can rename the file afterwards :
import os
os.remove("config.ini")
os.rename("new_config.ini", "config.ini")

Set the string you would like to look for (match_string = 'example')
Have a list output_list that is empty
Use with open(x,y) as z: (this will automatically close the file after completion)
for each line in file.readlines() - run through each line of the file
The if statement adds your replacement line if the match_string is in the line, else just the adds the line
NOTE: All variables can be any name that is not reserved (don't call something just 'list')
match_string = 'example'
output_list = []
with open("config.ini", "r") as file:
for line in file.readlines():
if match_string in line:
output_list.append('minimum_value_gain = 0.01\n')
else:
output_list.append(line)
Maybe not ideal for the first introduction to Python (or more readable) - But I would have done the problem as follows:
with open('config.ini', 'r') as in_file:
out_file = ['minimum_value_gain = 0.01\n' if 'example' in line else line for line in in_file.readlines()]

To replace a specific text in a string
a = 'My name is Zano'
b = a.replace('Zano', 'Zimmer')

Python to add a line after the exact match from the blocks of same pattern in a file

I have a file like this :
tablename-0-username=
tablename-0-password=
tablename-0-endpoint=
tablename-1-username=
tablename-1-password=
tablename-1-endpoint=
In python how to add a line after tablename-username
The keywords are like tablename-0-id=test and tablename-1-id=test1
Is there anyway in python I can create a outfile like this
tablename-0-username=
tablename-0-id=test
tablename-0-password=
tablename-0-endpoint=
tablename-1-username=
tablename-1-id=test1
tablename-1-password=
tablename-1-endpoint=
I don't want 2 files to be create, the operation should be in same file. I have created like this, but this one inserts after every other matching pattern.
find = "tablename-" + id
with open(config_file_name, "r") as in_file:
buf = in_file.readlines()
with open(config_file_name, "w") as out_file:
for line in buf:
if find in line:
line = line + pattern + "\n"
out_file.write(line)

Your problem is that, for some reason, you chose to search for a dysfunctional pattern. You have to search for something that accurately discriminates the lines yof interest from everything else. Instead, try
find = "-username="
... which uniquely identifies the lines, just as you specified in the problem description.

You can try :
find = "-endpoint"
with open(config_file_name, "r") as in_file:
buf = in_file.readlines()
with open(config_file_name, "w") as out_file:
for line in buf:
if find in line:
line = line + pattern + "\n"
out_file.write(line)
This will insert a new line (\n character) after every occurrence of the word endpoint

with open(config_file_name, "r") as in_file:
buf = in_file.readlines()
with open(config_file_name, "w") as out_file:
for line in buf:
line = line.split('=')[0]
if 'username' in line :
# You can also write a newline into the file in this block.
line = line + pattern + "\n"
out_file.write(line)
This will check for the phrase username in each line before '=' as the username can be the string "username".

Python: modify part of a string inside a .cfg file

I want to access a file (C:\Programmer\Test.txt
), find a string inside that file beginning with 'SS' and replace everything after that on the same line with a new string 'C:\Test\Flash'
The code below prints out the line I want to modify but I can't seem to find a suitable function that will replace everything after the 'SS' with the new string.
import re
for line in open('C:\Programmer\Build\Test.txt'):
if line.startswith('SS'):
print(line)
storedline = line
print(storedline)

You can do
file_path = 'C:\Programmer\Build\Test.txt'
new_line_content = 'C:\Test\Flash'
output = []
with open(file_path, 'r') as infile:
line = infile.readline()
while line:
if line[0:2] == 'SS':
output.append('SS{}\n'.format(new_line_content))
else:
output.append(line)
line = infile.readline()
with open(file_path, 'w') as outfile:
outfile.write(''.join(output))
Note that here the detection of the line(s) if line[0:2] == 'SS' is based on interpreting literally your requirement 'find a string inside that file beginning with 'SS''

take a line from a file and change it by what is the beginning of the line

So I want to look in the file something and when I read a line that begins with # I want to print something in the front of the line and in the end of it.
file = open("something.py","r")
infile = file.readlines()
for line in infile:
if line.find("#") !=-1:
index = line.find("#")
print("BLABLABLA",the line that begins with #,"BLABLABLA", end="")

How about this:
# Open file for reading
with open("infile.txt", "r") as infile:
# Open another file for writing
with open("outfile.txt", "w") as outfile:
# Read each line from the input file
for line in infile.readlines():
# Check if line begins with keyword "Hello"
if line.startswith("Hello"):
# If it does, prepend and append something to the line
line = "Prepend! >>> " + line.strip() + " <<< Append!"
# Finally, write the line to outfile
outfile.write(line + "\n")
Input:
Hello, this is a line.
Hello, this is another line.
Goodbye, these were all the lines
Output:
Prepend! >>> Hello, this is a line. <<< Append!
Prepend! >>> Hello, this is another line. <<< Append!
Goodbye, these were all the lines

Simple & short. The last two lines will change depending on your flag and what you want to actually do with the line read.
with open('input.txt', 'r') as f:
for line in f:
if line.startswith('#'):
print('!!!' + line.strip() + '!!!')

replace line if found or append - python

I have text that is key-value pairs separated by '='. I would like to replace the line if the key matches. if not, i would like to append it at the bottom. I've tried several ways, including:
def split_command_key_and_value(command):
if '=' in command:
command2 = command.split('=')
return command2
def test(command, path):
command2 = split_command_key_and_value(command)
pattern = command2[0]
myfile = open(path,'r') # open file handle for read
# use r'', you don't need to replace '\' with '/'
result = open(path, 'w') # open file handle for write
for line in myfile:
line = line.strip() # it's always a good behave to strip what you read from files
if pattern in line:
line = command # if match, replace line
result.write(line) # write every line
myfile.close() # don't forget to close file handle
result.close()
I know the above is just to replace text, but it deletes the text in the file, and I can't see why. Could someone point me in the right direction?
Thanks
Update:
I'm almost there, but some of my lines have similar keys, so mutiple lines are matching when only 1 should. I've tried to incorporate a regex boundary in my loop with no luck. My code is below. Does anyone have a suggestion?
There is some text in the file that isn't key-value, so I would like to skip that.
def modify(self, name, value):
comb = name + ' ' + '=' + ' ' + value + '\n'
with open('/file/', 'w') as tmpstream:
with open('/file/', 'r') as stream:
for line in stream:
if setting_name in line:
tmpstream.write(comb)
else:
tmpstream.write(line)
I think I got it. See code below.
def modify(self, name, value):
comb = name + ' ' + '=' + ' ' + value + '\n'
mylist = []
with open('/file/', 'w') as tmpstream:
with open('/file/', 'r') as stream:
for line in stream:
a = line.split()
b = re.compile('\\b'+name+'\\b')
if len(a) > 0:
if b.search(a[0]):
tmpstream.write(comb)
else:
tmpstream.write(line)
I spoke too soon. It stops at the key-value I provide. So, it only writes one line, and doesn't write the lines that don't match.
def modify(name, value):
comb = name + ' ' + '=' + ' ' + value + '\n'
mylist = []
with open('/file1', 'w') as tmpstream:
with open('/file2', 'r') as stream:
for line in stream:
a = line.split()
b = re.compile('\\b'+name+'\\b')
if len(a) > 0:
if b.search(a[0]):
tmpstream.write(comb)
else:
tmpstream.write(line)
Can anyone see the issue?

Because when you open file for writing
result = open(path, 'w') # open file handle for write
you just erase it content. Try to write in different file and after all work done replace old file with new one. Or read all data into memory and then process it and write to file.
with open(path) as f:
data = f.read()
with open(path, 'w') as f:
for l in data:
# make job here

first of all you are reading an writing the same file ...
you could first read it all and the write line by line
with open(path,'r') as f:
myfile = f.read() # read everything in the variable "myfile"
result = open(path, 'w') # open file handle for write
for line in myfile.splitlines(): # process the original file content 1 line at a time
# as before

I strongly recommend reading python's documentation on how to read and write files.
If you open an existing file in write-mode open(path, 'w'), its content will be erased:
mode can be (...) 'w' for only writing (an existing file with the same name will be erased)
To replace a line in python you can have a look at this: Search and replace a line in a file in Python
Here is one the solutions provided there adapted to your context (tested for python3):
from tempfile import mkstemp
from shutil import move
from os import close
def test(filepath, command):
# Split command into key/value
key, _ = command.split('=')
matched_key = False
# Create a temporary file
fh, tmp_absolute_path = mkstemp()
with open(tmp_absolute_path, 'w') as tmp_stream:
with open(filepath, 'r') as stream:
for line in stream:
if key in line:
matched_key = True
tmp_stream.write(command + '\n')
else:
tmp_stream.write(line)
if not matched_key:
tmp_stream.write(command + '\n')
close(fh)
move(tmp_absolute_path, filepath)
Note that with the code above every line that matches key (key=blob or blob=key) will be replaced.