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Correct way to write line to file?

How do I write a line to a file in modern Python? I heard that this is deprecated:

print >>f, "hi there"

Also, does "\n" work on all platforms, or should I use "\r\n" on Windows?

"\n" is not Unix-specific. When the file is opened in text mode (the default), it is translated automatically to the correct line ending for the current platform. Writing "\r\n" would produce "\r\r\n" which is wrong.
Just add the print ord(os.linesep) statement to see the ascii code (10 on most UNIX systems)
Why do you think it is deprecated?

u
user35443

This should be as simple as:

with open('somefile.txt', 'a') as the_file:
    the_file.write('Hello\n')

From The Documentation:

Do not use os.linesep as a line terminator when writing files opened in text mode (the default); use a single '\n' instead, on all platforms.

Some useful reading:

The with statement

open() 'a' is for append, or use 'w' to write with truncation

'a' is for append, or use

'w' to write with truncation

os (particularly os.linesep)


This example is better than the open/close example. Using with is a safer way to remember to close a file.
I don't have to call the_file.close() ?
@HorseSMith: I see. Hopefully my most recent edit clarifies my intent. Feel free to edit my answers if they are "rather useless and misleading".
@user3226167: That's an interesting point. But why would you open a binary file to write plain text?
M
Martin Tournoij

You should use the print() function which is available since Python 2.6+

from __future__ import print_function  # Only needed for Python 2
print("hi there", file=f)

For Python 3 you don't need the import, since the print() function is the default.

The alternative would be to use:

f = open('myfile', 'w')
f.write('hi there\n')  # python will convert \n to os.linesep
f.close()  # you can omit in most cases as the destructor will call it

Quoting from Python documentation regarding newlines:

On output, if newline is None, any '\n' characters written are translated to the system default line separator, os.linesep. If newline is '', no translation takes place. If newline is any of the other legal values, any '\n' characters written are translated to the given string.


-1 "If you want to be sure, add os.linesep to the string instead of \n" would require newline="" otherwise you'd get \r\r\n on Windows. There is no reason to futz about with os.linesep at all.
@Sorin: Your edit to add write mode is of course an improvement. However you strangely remain intransigent about os.linesep. See my answer. By the way, the documentation that you quote is for 3.x, but this part is also valid for 2.x in text mode: any '\n' characters written are translated to the system default line separator, os.linesep* ... Windows: writing os.linesep is the same as writing \r\n which contains \n which is translated to os.linesep which is \r\n so the end result is \r\r\n.
@John you were right, I corrected the os.linesep bug. Thanks.
For appending isnt it open('myfile','a') instead open('myfile','w')?
@BradRuderman That's part of the POSIX standard for what constitutes a "line" in a text file, i.e. every line in a text file must be terminated by a newline, even the last line.
d
dcoles

The python docs recommend this way:

with open('file_to_write', 'w') as f:
    f.write('file contents\n')

So this is the way I usually do it :)

Statement from docs.python.org:

It is good practice to use the 'with' keyword when dealing with file objects. This has the advantage that the file is properly closed after its suite finishes, even if an exception is raised on the way. It is also much shorter than writing equivalent try-finally blocks.


I don't like this way when I need to nest the with inside a loop. That makes me constantly open and close the file as I proceed in my loop. Maybe I am missing something here, or this is really a disadvantage in this particular scenario?
How about looping within the with?
@j7nn7k for line in fd:
J
John Machin

Regarding os.linesep:

Here is an exact unedited Python 2.7.1 interpreter session on Windows:

Python 2.7.1 (r271:86832, Nov 27 2010, 18:30:46) [MSC v.1500 32 bit (Intel)] on
win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.linesep
'\r\n'
>>> f = open('myfile','w')
>>> f.write('hi there\n')
>>> f.write('hi there' + os.linesep) # same result as previous line ?????????
>>> f.close()
>>> open('myfile', 'rb').read()
'hi there\r\nhi there\r\r\n'
>>>

On Windows:

As expected, os.linesep does NOT produce the same outcome as '\n'. There is no way that it could produce the same outcome. 'hi there' + os.linesep is equivalent to 'hi there\r\n', which is NOT equivalent to 'hi there\n'.

It's this simple: use \n which will be translated automatically to os.linesep. And it's been that simple ever since the first port of Python to Windows.

There is no point in using os.linesep on non-Windows systems, and it produces wrong results on Windows.

DO NOT USE os.linesep!


great example -- curious if you're an ipython user? nice functions for formatting sessions
I'm not entirely sure what you are trying to tell us here. os.linesep will return the line term character (or string) as defined by the operating system. Windows uses \r\n for line endings by default. However, a single \n is recognised. Using \n is going to give a fully portable OUTPUT but os.linesep is not wrong on windows.
@Gusdor: The point is that if you explicitly use os.linesep in Windows in text mode, the outcome is \r\r\n which is wrong. "Windows uses ..." is meaningless. The C runtime library (and hence Python) translate \n to \r\n on output in text mode. Other software may behave differently. It is NOT the case that all software running on Windows recognises a lone \n as a line separator when reading in text mode. Python does. Microsoft's Notepad text editor doesn't.
Arguably somebody else will be reading it, not you, with some mickey-mouse software that will barf about the extra \r ...
@Gusdor are you coming to python from a different language, where using '\n' results in output of '\n' on window, rather than '\r\n' -- so it lacks the '\r' expected by dumb text editors? As John says, that isn't how Python behaves -- '\n' is automatically replaced by '\r\n', if that is what os.linesep says to do. Hence, explicitly saying os.linesep is "wrong" here. Its like Department of Redundancy Department. Yes you can do it. No, you don't want to.
H
Hyperboreus

I do not think there is a "correct" way.

I would use:

with open ('myfile', 'a') as f: f.write ('hi there\n')

In memoriam Tim Toady.


But the OP might want to write additional stuff to the file. Here the file will be closed when the with goes out of scope.
Erm, yeah. That is the idea of using with. If you want to keep the file open, just call open at the beginning and call close when you are done...
@mtrw. True. OP was appending.
As far as python is concerned is RIP Tim Toady - and very very very rightfully so
The zen of perl would be a bit of an oxymoron.
K
Keith

In Python 3 it is a function, but in Python 2 you can add this to the top of the source file:

from __future__ import print_function

Then you do

print("hi there", file=f)

R
Robin Keskisarkka

If you are writing a lot of data and speed is a concern you should probably go with f.write(...). I did a quick speed comparison and it was considerably faster than print(..., file=f) when performing a large number of writes.

import time    

start = start = time.time()
with open("test.txt", 'w') as f:
    for i in range(10000000):
        # print('This is a speed test', file=f)
        # f.write('This is a speed test\n')
end = time.time()
print(end - start)

On average write finished in 2.45s on my machine, whereas print took about 4 times as long (9.76s). That being said, in most real-world scenarios this will not be an issue.

If you choose to go with print(..., file=f) you will probably find that you'll want to suppress the newline from time to time, or replace it with something else. This can be done by setting the optional end parameter, e.g.;

with open("test", 'w') as f:
    print('Foo1,', file=f, end='')
    print('Foo2,', file=f, end='')
    print('Foo3', file=f)

Whichever way you choose I'd suggest using with since it makes the code much easier to read.

Update: This difference in performance is explained by the fact that write is highly buffered and returns before any writes to disk actually take place (see this answer), whereas print (probably) uses line buffering. A simple test for this would be to check performance for long writes as well, where the disadvantages (in terms of speed) for line buffering would be less pronounced.

start = start = time.time()
long_line = 'This is a speed test' * 100
with open("test.txt", 'w') as f:
    for i in range(1000000):
        # print(long_line, file=f)
        # f.write(long_line + '\n')
end = time.time()

print(end - start, "s")

The performance difference now becomes much less pronounced, with an average time of 2.20s for write and 3.10s for print. If you need to concatenate a bunch of strings to get this loooong line performance will suffer, so use-cases where print would be more efficient are a bit rare.


There is no need to call print() multiple times for each line, you can input each line as a positional argument and specify a newline delimiter using print(line1, line2, ... sep="\n"). This makes only one write call.
C
Community

Since 3.5 you can also use the pathlib for that purpose:

Path.write_text(data, encoding=None, errors=None) Open the file pointed to in text mode, write data to it, and close the file:

import pathlib

pathlib.Path('textfile.txt').write_text('content')

a
alper

When you said Line it means some serialized characters which are ended to '\n' characters. Line should be last at some point so we should consider '\n' at the end of each line. Here is solution:

with open('YOURFILE.txt', 'a') as the_file:
    the_file.write("Hello")

in append mode after each write the cursor move to new line, if you want to use w mode you should add \n characters at the end of the write() function:

the_file.write("Hello\n")

"in append mode after each write the cursor move to new line" – no it's not.
k
kmario23

One can also use the io module as in:

import io
my_string = "hi there"

with io.open("output_file.txt", mode='w', encoding='utf-8') as f:
    f.write(my_string)

Z
Zenul_Abidin

If you want to avoid using write() or writelines() and joining the strings with a newline yourself, you can pass all of your lines to print(), and the newline delimiter and your file handle as keyword arguments. This snippet assumes your strings do not have trailing newlines.

print(line1, line2, sep="\n", file=f)

You don't need to put a special newline character is needed at the end, because print() does that for you.

If you have an arbitrary number of lines in a list, you can use list expansion to pass them all to print().

lines = ["The Quick Brown Fox", "Lorem Ipsum"]
print(*lines, sep="\n", file=f)

It is OK to use "\n" as the separator on Windows, because print() will also automatically convert it to a Windows CRLF newline ("\r\n").


H
Higgs

If you want to insert items in a list with a format per line, a way to start could be:

with open('somefile.txt', 'a') as the_file:
    for item in items:
        the_file.write(f"{item}\n")

S
Sashini Hettiarachchi

To write text in a file in the flask can be used:

filehandle = open("text.txt", "w")
filebuffer = ["hi","welcome","yes yes welcome"]
filehandle.writelines(filebuffer)
filehandle.close()

It is always more advisable to write to a file with a with open('file_to_write', 'w') as f: statement. It's much easier to make sure that file will not be left open if someone accidentally wrote something in between that would result in not having an explicit close() call
E
Emin Bugra Saral

You can also try filewriter

pip install filewriter

from filewriter import Writer

Writer(filename='my_file', ext='txt') << ["row 1 hi there", "row 2"]

Writes into my_file.txt

Takes an iterable or an object with __str__ support.


M
MxNx

When I need to write new lines a lot, I define a lambda that uses a print function:

out = open(file_name, 'w')
fwl = lambda *x, **y: print(*x, **y, file=out) # FileWriteLine
fwl('Hi')

This approach has the benefit that it can utilize all the features that are available with the print function.

Update: As is mentioned by Georgy in the comment section, it is possible to improve this idea further with the partial function:

from functools import partial
fwl = partial(print, file=out)

IMHO, this is a more functional and less cryptic approach.


Or another (probably cleaner) way to write this: from functools import partial; fwl = partial(print, file=out).
@Georgy Your approach is so good that it can be given as a new answer.
The idea is the same as yours, just implementation is a bit different. If you want, you can add it in an edit to your answer. I'm fine with it.
A
Abhishek Kumar

A very basic way to write lines in python to a text file. This will overwrite a file with the same name if exist.

%%writefile test.txt
 i am writing this to test.txt file \n
 writing this line also

this would only work in Jupyter or ipython since the 'normal' python interpreter does not recognize magic. Please adjust your answer to specify that
H
Hyperx837

since others have answered how to do it, I'll answer how it happens line by line.

with FileOpenerCM('file.txt') as fp: # is equal to "with open('file.txt') as fp:"
      fp.write('dummy text')

this is a so-called context manager, anything that comes with a with block is a context manager. so let's see how this happens under the hood.

class FileOpenerCM:
     def __init__(self, file, mode='w'):
         self.file = open(file, mode)
      
     def __enter__(self):
          return self.file
      
     def __exit__(self, exc_type, exc_value, exc_traceback):
         self.file.close()

the first method __init__ is (as you all know) the initialization method of an object. whenever an object is created obj.__init__ is definitely called. and that's the place where you put your all the init kinda code.

the second method __enter__ is a bit interesting. some of you might not have seen it because it is a specific method for context managers. what it returns is the value to be assigned to the variable after the as keyword. in our case, fp.

the last method is the method to run after an error is captured or if the code exits the with block. exc_type, exc_value, exc_traceback variables are the variables that hold the values of the errors that occurred inside with block. for example,

exc_type: TypeError
exc_value: unsupported operand type(s) for +: 'int' and 'str
exc_traceback: <traceback object at 0x6af8ee10bc4d>

from the first two variables, you can get info enough info about the error. honestly, I don't know the use of the third variable, but for me, the first two are enough. if you want to do more research on context managers surely you can do it and note that writing classes are not the only way to write context managers. with contextlib you can write context managers through functions(actually generators) as well. it's totally up to you to have a look at it. you can surely try generator functions with contextlib but as I see classes are much cleaner.