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Appending characters to each line in a txt file with python

I wrote the following python code snippet to append a lower p character to each line of a txt file:
f = open('helloworld.txt','r')
for line in f:
line+='p'
print(f.read())
f.close()
However, when I execute this python program, it returns nothing but an empty blank:
zhiwei#zhiwei-Lenovo-Rescuer-15ISK:~/Documents/1001/ass5$ python3 helloworld.py
Can anyone tell me what's wrong with my codes?

Currently, you are only reading each line and not writing to the file. reopen the file in write mode and write your full string to it, like so:
newf=""
with open('helloworld.txt','r') as f:
for line in f:
newf+=line.strip()+"p\n"
f.close()
with open('helloworld.txt','w') as f:
f.write(newf)
f.close()

well, type help(f) in shell, you can get "Character and line based layer over a BufferedIOBase object, buffer."
it's meaning：if you reading first buffer,you can get content, but again. it's empty。
so like this:
with open(oldfile, 'r') as f1, open(newfile, 'w') as f2:
newline = ''
for line in f1:
newline+=line.strip()+"p\n"
f2.write(newline)

open(filePath, openMode) takes two arguments, the first one is the path to your file, the second one is the mode it will be opened it. When you use 'r' as second argument, you are actually telling Python to open it as an only reading file.
If you want to write on it, you need to open it in writing mode, using 'w' as second argument. You can find more about how to read/write files in Python in its official documentation.
If you want to read and write at the same time, you have to open the file in both reading and writing modes. You can do this simply by using 'r+' mode.

It seems that your for loop has already read the file to the end, so f.read() return empty string.
If you just need to print the lines in the file, you could move the print into for loop just like print(line). And it is better to move the f.read() before for loop:
f = open("filename", "r")
lines = f.readlines()
for line in lines:
line += "p"
print(line)
f.close()
If you need to modify the file, you need to create another file obj and open it in mode of "w", and use f.write(line) to write the modified lines into the new file.
Besides, it is more better to use with clause in python instead of open(), it is more pythonic.
with open("filename", "r") as f:
lines = f.readlines()
for line in lines:
line += "p"
print(line)
When using with clause, you have no need to close file, this is more simple.

for each line in file write line to an individual file in python

I have a text file which needs to be separated line by line into individual text files. So if the main file contains the strings:
foo
bar
bla
I would have 3 files which could be named numerically 1.txt (containing the string "foo"), 2.txt (sontaining the string"bar") and 3.txt (containing the string "bla")
The straightforward way to do with would be to open three files for writing and writing line by line into each file. But the problem is when we have lot of lines or we do not know exactly how many there are. It seems painfully unnecessary to have to create
f1=open('main_file', 'r')
f2=open('1.txt', 'w')
f3=open('2.txt', 'w')
f4=open('3.txt', 'w')
is there a way to put a counter in this operation or a library which can handle this type of ask?

Read the lines from the file in a loop, maintaining the line number; open a file with the name derived from the line number, and write the line into the file:
f1 = open('main_file', 'r')
for i,text in enumerate(f1):
open(str(i + 1) + '.txt', 'w').write(text)

You would want something like this. Using with is the preferred way for dealing with files, since it automatically closes them for you after the with scope.
with open('main_file', 'r') as in_file:
for line_number, line in enumerate(in_file):
with open("{}.txt".format(i+1), 'w') as out_file:
out_file.write(line)

Firstly you could read the file into a list, where each element stands for a row in the file.
with open('/path/to/data','r') as f:
data = [line.strip() for line in f]
Then you could use a for loop to write into files separately.
for counter in range(len(data)):
with open('/path/to/file/'+str(counter),'w') as f:
f.write(data[counter])
Notes:
Since you're continuously opening numerous files, I highly suggest using
with open() as f:
#your operation
The advantage of using this is that you can make sure Python release the resources on time.
Details:
What's the advantage of using 'with .. as' statement in Python?

Why I can´t write the first element of a list into a text file in Python?

I got a text file like this
Bruce
brucechungulloa#outlook.com
I've used this to read the text file and export it to a list
with open('info.txt') as f:
info = f.readlines()
for item in info:
reportePaises = open('reportePaises.txt', 'w')
reportePaises.write("%s\n" % item)
But when I want to write the elements of the list(info) into another text file, only the info[1] is written (the mail)
How can I write the entire list onto the text file?

with open('data.csv') as f:
with open('test2.txt', 'a') as wp:
for item in f.readlines():
wp.write("%s" % item)
wp.write('\n') # adds a new line after the looping is done
That will give you:
Bruce
brucechungulloa#outlook.com
In both files.

You were having problems because every time you open a file with 'w' flag, you overwrite it on the disk. So, you created a new file every time.
You should open the second file only once, in the with statement:
with open('info.txt') as f, open('reportePaises.txt', 'w') as reportePaises:
info = f.readlines()
for item in info:
reportePaises.write(item)
As #Pynchia suggested, it's probably better not to use .readlines(), and loop directly on input file instead.
with open('info.txt') as f, open('reportePaises.txt', 'w') as reportePaises:
for item in f:
reportePaises.write(item)
This way you don't create a copy of the while file in your RAM by saving it to a list, which may cause a huge delay if the file is big (and, obviously, uses more RAM). Instead, you treat the input file as an iterator and just read next line directly from your HDD on each iteration.
You also (if I did the testing right) don't need to append '\n' to every line. The newlines are already in item. Because of that you don't need to use string formatting at all, just reportePaises.write(item).

You are opening your file in write mode every time you write to a file, effectively overwriting the previous line that you wrote. Use the append mode, a, instead.
reportePaises = open('reportePaises.txt', 'a')
Edit: Alternatively, you can open the file once and instead of looping through the lines, write the whole contents as follows:
with open('reportePaises.txt', 'w') as file:
file.write(f.read())

Try this without open output file again and again.
with open('info.txt') as f:
info = f.readlines()
with open('reportePaises.txt', 'w') as f1:
for x in info:
f1.write("%s\n" % x)
That will work.

Two problems here. One is you are opening the output file inside the loop. That means it is being opened several times. Since you also use the "w" flag that means the file is truncated to zero each time it is opened. Therefore you only get the last line written.
It would be better to open the output file once outside the loop. You could even use an outer with block.

You can simply try the below code. Your code did not work because you added the opening on file handler 'reportPaises' within the for loop. You don't need to open the file handler again and again.
Try re running your code line by line in the python shell as it is very easy to debug the bugs in the code.
The below code will work
with open('something.txt') as f:
info = f.readlines()
reportePaises = open('reportePaises.txt', 'w')
for item in info:
reportePaises.write("%s" % item)
You don't need to add a \n to the output line because when you perform readlines, the \n character is preserved in the info list file. Please look observe below.
Try below
with open('something.txt') as f:
info = f.readlines()
print info
The output you will get is
['Bruce\n', 'brucechungulloa#outlook.com']

question about splitting a large file

Hey I need to split a large file in python into smaller files that contain only specific lines. How do I do this?

You're probably going to want to do something like this:
big_file = open('big_file', 'r')
small_file1 = open('small_file1', 'w')
small_file2 = open('small_file2', 'w')
for line in big_file:
if 'Charlie' in line: small_file1.write(line)
if 'Mark' in line: small_file2.write(line)
big_file.close()
small_file1.close()
small_file2.close()
Opening a file for reading returns an object that allows you to iterate over the lines. You can then check each line (which is just a string of whatever that line contains) for whatever condition you want, then write it to the appropriate file that you opened for writing. It is worth noting that when you open a file with 'w' it will overwrite anything already written to that file. If you want to simply add to the end, you should open it with 'a', to append.
Additionally, if you expect there to be some possibility of error in your reading/writing code, and want to make sure the files are closed, you can use:
with open('big_file', 'r') as big_file:
<do stuff prone to error>

Do you mean breaking it down into subsections? Like if I had a file with chapter 1, chapter 2, and chapter 3, you want it to be broken down into separate files for each chapter?
The way I've done this is similar to Wilduck's response, but closes the input file as soon as it reads in the data and keeps all the lines read in.
data_file = open('large_file_name', 'r')
lines = data_file.readlines()
data_file.close()
outputFile = open('output_file_one', 'w')
for line in lines:
if 'SomeName' in line:
outputFile.write(line)
outputFile.close()
If you wanted to have more than one output file you could either add more loops or open more than one outputFile at a time.
I'd recommend using Wilducks response, however, as it uses less space and will take less time with larger files since the file is read only once.

How big and does it need to be done in python? If this is on unix, would split/csplit/grep suffice?

First, open the big file for reading.
Second, open all the smaller file names for writing.
Third, iterate through every line. Every iteration, check to see what kind of line it is, then write it to that file.
More info on File I/O: http://docs.python.org/tutorial/inputoutput.html

Replace a word in a file

I am new to Python programming...
I have a .txt file....... It looks like..
0,Salary,14000
0,Bonus,5000
0,gift,6000
I want to to replace the first '0' value to '1' in each line. How can I do this? Any one can help me.... With sample code..
Thanks in advance.
Nimmyliji

I know that you're asking about Python, but forgive me for suggesting that perhaps a different tool is better for the job. :) It's a one-liner via sed:
sed 's/^0,/1,/' yourtextfile.txt > output.txt
This applies the regex /^0,/ (which matches any 0, that occurs at the beginning of a line) to each line and replaces the matched text with 1, instead. The output is directed into the file output.txt specified.

inFile = open("old.txt", "r")
outFile = open("new.txt", "w")
for line in inFile:
outFile.write(",".join(["1"] + (line.split(","))[1:]))
inFile.close()
outFile.close()
If you would like something more general, take a look to Python csv module. It contains utilities for processing comma-separated values (abbreviated as csv) in files. But it can work with arbitrary delimiter, not only comma. So as you sample is obviously a csv file, you can use it as follows:
import csv
reader = csv.reader(open("old.txt"))
writer = csv.writer(open("new.txt", "w"))
writer.writerows(["1"] + line[1:] for line in reader)
To overwrite original file with new one:
import os
os.remove("old.txt")
os.rename("new.txt", "old.txt")
I think that writing to new file and then renaming it is more fault-tolerant and less likely corrupt your data than direct overwriting of source file. Imagine, that your program raised an exception while source file was already read to memory and reopened for writing. So you would lose original data and your new data wouldn't be saved because of program crash. In my case, I only lose new data while preserving original.

o=open("output.txt","w")
for line in open("file"):
s=line.split(",")
s[0]="1"
o.write(','.join(s))
o.close()
Or you can use fileinput with in place edit
import fileinput
for line in fileinput.FileInput("file",inplace=1):
s=line.split(",")
s[0]="1"
print ','.join(s)

f = open(filepath,'r')
data = f.readlines()
f.close()
edited = []
for line in data:
edited.append( '1'+line[1:] )
f = open(filepath,'w')
f.writelines(edited)
f.flush()
f.close()
Or in Python 2.5+:
with open(filepath,'r') as f:
data = f.readlines()
with open(outfilepath, 'w') as f:
for line in data:
f.write( '1' + line[1:] )
This should do it. I wouldn't recommend it for a truly big file though ;-)
What is going on (ex 1):
1: Open the file in read mode
2,3: Read all the lines into a list (each line is a separate index) and close the file.
4,5,6: Iterate over the list constructing a new list where each line has the first character replaced by a 1. The line[1:] slices the string from index 1 onward. We concatenate the 1 with the truncated list.
7,8,9: Reopen the file in write mode, write the list to the file (overwrite), flush the buffer, and close the file handle.
In Ex. 2:
I use the with statement that lets the file handle closing itself, but do essentially the same thing.