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How to write in a dat file in python

I have this content in a dat file I can access easily, it's not at the beggining of the file but in the middle. I insert only the part of the file that I need to modify.
{
....,
",>=,",
",>=,",
.......
}
Instead of a line with ",>=,", I wish I could insert a custom string like for example
"M,<=,5", from python code, as I would have to do this on many files/many times.
I can read the file through this script, but I don't understand how to find the line I want to change in the python code and how to overwrite in it the string of my interest.
prefixed = [filename for filename in os.listdir('.') if filename.startswith("CRY")] #NQ, DIV, ecc..
for i in range(len(prefixed)):
with open(prefixed[i], 'r') as file:
lines = file.readlines()
print(lines)

prefixed = [filename for filename in os.listdir('.') if filename.startswith("CRY")] #NQ, DIV, ecc..
for i in range(len(prefixed)):
# Read lines
file = open(prefixed[i], 'r')
file_content = file.readlines()
file.close()
# Treatment
for pos, line in enumerate(file_content):
if ",>=," in line:
file_content[pos] = line.replace(",>=,", "myCustomString")
# Write lines
file = open(prefixed[i], 'w')
file.writelines(file_content)
file.close()
To modify only the first element:
prefixed = [filename for filename in os.listdir('.') if filename.startswith("CRY")] #NQ, DIV, ecc..
for i in range(len(prefixed)):
# Read lines
file = open(prefixed[i], 'r')
file_content = file.readlines()
file.close()
# Treatment
for pos, line in enumerate(file_content):
if ",>=," in line:
file_content[pos] = line.replace(",>=,", "myCustomString")
# Add break to quit loop after first replacement
break
# Write lines
file = open(prefixed[i], 'w')
file.writelines(file_content)
file.close()

Creating new files through loop using python3

I have a Dataset, which has 5 folders, in which each folder has 100 .txt files. Below code you can see that I am looping through every file, and removing certain words from those files using my StopWords.txt file.
After I remove the words I am appending the output in one file(filteredtext.txt). But I want to have these output exactly as my Dataset (5 folders which has 100 .txt file).
This is my code.
import re
import os
#insert stopwords files
stopwordfile = open("StopWords.txt", encoding='utf-8')
# Use this to read file content as a stream:
readstopword = stopwordfile.read()
stop_words = readstopword.split()
#file path to dataset
for path, _, files in os.walk("sinhala-set1"):
for file_name in files:
filepath = os.path.join(path, file_name)
print(f"Checking --> {filepath}")
file1 = open(filepath, encoding='utf-8')
# Use this to read file content as a stream:
line = file1.read()
words = line.split()
for r in words:
if not r in stop_words:
appendFile = open('filteredtext.txt','a', encoding='utf-8')
appendFile.write(" "+r)
appendFile.close()

You are appending the file because you are opening the same .txt file with appending mode appendFile = open('filteredtext.txt','a', encoding='utf-8') If you want a separate file for each loop, open a different file like this:
output_file = open('output_' + file_name), 'w', encoding='utf-8')

How to delete all characters after a "==" in each line of a file and update the file, using python script?

I have a huge list of python packages that had been installed saved with the version numbers in the file "foo.txt", I want to delete the "==" and whatever after that in each lines, and save the file.
example text in the file:
autopep8==1.5.3
beautifulsoup4==4.8.2
bleach==3.1.4
bumpversion==0.5.3
... etc

Use this line of code if you want to create a new file with updated data, if you want to just read use the data then make changes accordingly.
def read_write_file(input_path, output_path):
with open(input_path, "r") as input_file:
content = input_file.readlines()
with open(output_path, 'w') as output_file:
for line in content:
line = line[:line.find('==')]
output_file.write(line + '\n')

Use the below code.
read that file and store it in list named pkg_list.
output_lst = []
with open("input.txt", "r") as f:
input_data = f.readlines()
output_lst = [name.split("==")[0] for name in input_data]
with open("input.txt", "w") as f:
for pkg in output_lst:
f.write("{}\n".format(pkg))

How to add for loop in python?

I'm creating new files from originally existing ones in the mdp folder by changing a couple of lines in those files using python. I need to do this for 1000 files. Can anyone suggest a for loop which reads all files and changes them and creates new in one go?
This way I have to change the the number followed by 'md_' in the path and it's tedious because there are a 1000 files here.
I tried using str() but there was a 'could not read file error'
fin = open("/home/abc/xyz/mdp/md_1.mdp", "rt")
fout = open("/home/abc/xyz/middle/md_1.mdp", "wt")
for line in fin:
fout.write(line.replace('integrator = md', 'integrator
= md-vv'))
fin = open("/home/abc/xyz/middle/md_1.mdp", "rt")
fout = open("/home/abc/xyz/mdb/md_1.mdp", "wt")
for line in fin:
fout.write(line.replace('dt = 0.001', 'dt
= -0.001'))
fin.close()
fout.close()

os.listdir(path) is your friend:
import os
sourcedir = "/home/abc/xyz/mdp"
destdir = "/home/abc/xyz/middle"
for filename in os.listdir(sourcedir):
if not filename.endswith(".mdp"):
continue
source = os.path.join(sourcedir, filename)
dest = os.path.join(destdir, filename)
# with open(xxx) as varname makes sure the file(s)
# will be closed whatever happens in the 'with' block
# NB text mode is the default, and so is read mode
with open(source) as fin, open(dest, "w") as fout:
# python files are iterable... avoids reading
# the whole file in memory at once
for line in fin:
# will only work for those exact strings,
# you may want to use regexps if number of
# whitespaces vary etc
line = line.replace("dt = 0.001", "dt = -0.001")
line = line.replace(
'integrator = md',
'integrator = md-vv'
)
fout.write(line)

Assuming you want to edit all files that are located in the mdp folder you could do something like this.
import os
dir = "/home/abc/xyz/mdp/"
for filename in os.listdir(dir):
with open(dir + filename, "r+") as file:
text = file.read()
text = text.replace("dt = 0.001", "dt = -0.001")
file.seek(0)
file.write(text)
file.truncate()
This will go through every file and change it using str.replace().
If there are other files in the mdp folder that you do not want to edit, you could use and if-statement to check for the correct file name. Add something like this to encase the with open statement.
if filename.startswith("md_")

Looping multiple files into a single csv file in python

I am trying to process several files into a single, merged csv file using python. So far, I have
files = ["file1.txt", "file2.txt", "file3.txt"]
def doSomething(oldfile):
content = []
with open oldfile as file:
content = file.read().splitlines()
file.close()
return content.reverse()
with open("newfile.txt", "w") as file:
w = csv.writer(file, dialect = "excel-tab")
for i in range(0, len(files)):
w. writerows(doSomething(files[i])
file.close()
The new file is being created, but there is nothing in it. I am curious about what is going on.
Thanks!

For starters, list.reverse() reverses the list in place and doesn't return anything so you're essentially returning None from your doSomething() function. You'll actually want to split that into two lines:
content.reverse()
return content
If you want to streamline your code, here's a suggestion:
def doSomething(oldfile):
with open(oldfile, "r") as f:
return reversed(f.read().splitlines())
files = ["file1.txt", "file2.txt", "file3.txt"]
with open("newfile.txt", "wb") as file:
w = csv.writer(file, dialect = "excel-tab")
for current_file in files:
w.writerows(doSomething(current_file))

I think your program crashes for several reasons:
open(..) is a function, so you cannot write:
with open oldfile as file:
a with statement for files is used to enforce closing of a file, so file.close() is actually not necessary.
.reverse() works inplace: it returns None, you can use reversed(..) for that.
You can fix it with:
files = ["file1.txt", "file2.txt", "file3.txt"]
def doSomething(oldfile):
content = []
with open(oldfile,'r') as file:
return list(reversed(file))
with open("newfile.txt", "w") as file:
w = csv.writer(file, dialect = "excel-tab")
for oldfile in files:
w.writerows(doSomething(oldfile))
I also used a for loop over the list, instead of the indices, since that is more "pythonic". Furthermore a file is iterable over its rows. So one can use reversed(file) to obtain the lines of the file in reverse.