open() in Python does not create a file if it doesn't exist

Question

What is the best way to open a file as read/write if it exists, or if it does not, then create it and open it as read/write? From what I read, file = open('myfile.dat', 'rw') should do this, right?

It is not working for me (Python 2.6.2) and I'm wondering if it is a version problem, or not supposed to work like that or what.

The bottom line is, I just need a solution for the problem. I am curious about the other stuff, but all I need is a nice way to do the opening part.

The enclosing directory was writeable by user and group, not other (I'm on a Linux system... so permissions 775 in other words), and the exact error was:

IOError: no such file or directory.

Solution 1

You should use open with the w+ mode:

file = open('myfile.dat', 'w+')

Solution 2

The advantage of the following approach is that the file is properly closed at the block's end, even if an exception is raised on the way. It's equivalent to try-finally, but much shorter.

with open("file.dat","a+") as f:
    f.write(...)
    ...

a+ Opens a file for both appending and reading. The file pointer is at the end of the file if the file exists. The file opens in the append mode. If the file does not exist, it creates a new file for reading and writing. -Python file modes

seek() method sets the file's current position.

f.seek(pos [, (0|1|2)])
pos .. position of the r/w pointer
[] .. optionally
() .. one of ->
  0 .. absolute position
  1 .. relative position to current
  2 .. relative position from end

Only "rwab+" characters are allowed; there must be exactly one of "rwa" - see Stack Overflow question Python file modes detail.

Solution 3

'''
w  write mode
r  read mode
a  append mode

w+  create file if it doesn't exist and open it in write mode
r+  open for reading and writing. Does not create file.
a+  create file if it doesn't exist and open it in append mode
'''

example:

file_name = 'my_file.txt'
f = open(file_name, 'w+')  # open file in write mode
f.write('python rules')
f.close()

[FYI am using Python version 3.6.2]

Solution 4

Good practice is to use the following:

import os

writepath = 'some/path/to/file.txt'

mode = 'a' if os.path.exists(writepath) else 'w'
with open(writepath, mode) as f:
    f.write('Hello, world!\n')

Solution 5

Change "rw" to "w+"

Or use 'a+' for appending (not erasing existing content)

View more solutions

Comments

[SilentGhost]

w truncates existing file. docs: Modes 'r+', 'w+' and 'a+' open the file for updating (note that 'w+' truncates the file).

[trh178]

this did the trick. thank you. i feel like an idiot now for not reading the spec. i dont think 'rw' is even acceptable there. i must have been thinking of something else.

[Daniel Hepper]

Testing for existence before opening might introduce a race condition. Probably not a big deal in this case, but something to keep in mind.

[Nick Zalutskiy]

Note that a+ creates a file if it does not exist and, crucially, seeks the file to the end. So if you do a read immediately after opening this way, you'll get nothing. You need to seek back to the beginning first: f.seek(0)

[AJP]

For the verbose, example-less documentation

[Brian Peterson]

"If you need to read from a file, the file has to be exist before you open it." Thank you for saving my sanity.

[Blacklight Shining]

This is Unpythonic. Rather than checking if the file exists first, one should assume it does first, then handle the case that it doesn't.

[Daniel F]

This is not the solution. The problem is the directory. Either the script lacks the permissions to create a file in that directory, or the directory simply doesn't exist. open('myfile.dat', 'w') is then enough.

[The Red Pea]

If the path is pointing at a directory, make sure that directory exists: i.e. (file = open('dir/myfile.dat', 'w+') ), make sure the dir folder, aka directory exists; Python won't create that for you

[antibus]

even worse, this code is prone to a race condition. thus, after checking if the file exists, the process could be interrupted and another process could create this file.

[Loretta]

I try this with open(filename, 'a+') as myfile: and get IOError: [Errno 2] No such file or directory: - why it doesn't create the file?

[Qwerty]

@Loretta Have you checked the value of filename?

[Loretta]

Yes, I did. It is a unicode string. I also tried with open('{}.txt'.format(filename), 'a+') as myfile:

[Loretta]

I am not using a path. and I tried open('test.txt', 'a+') it gets following exception 'TypeError: coercing to Unicode: need string or buffer, file found' in the line if os.stat(myfile).st_size == 0:

[Sergiy Kolodyazhnyy]

You need to properly define encoding for this to work. stackoverflow.com/q/728891/3701431

[smci]

This is not the solution. @TheRedPea: the OP said that the directory indeed did exist, but had 775 permissions ('o-w'), hence they couldn't write it.

[sleblanc]

It is bad to test a file before opening it, as it can lead to race conditions (file is deleted before it is opened). Race conditions can sometimes be used to exploit vulnerabilities in a system. "a+" mode is the best way to open the file: it creates a new file, and appends to existing files. Do not forget to wrap this in a try/except.

[The Matt]

You would also need the "w+" flag so both files will be in a read and write modes.

[David Parks]

touch does update the last modified time when used.

[Granitosaurus]

@DavidParks good point, just tested it and it is indeed true on ext4 file system and python3.7.2. I don't think that's intended or desired behaviour, maybe it's a bug wtih python?

[David Parks]

Same thing when using touch at the command line in linux, so I assume it's intended behavior.

[Jean-François Fabre]

computing mode write or append has no interest. If file doesn't exist, append mode creates it.

[Eric Bellet]

Exception has occurred: FileNotFoundError... This does no work

[chenjesu]

I have the same problem as Loretta. I try with open(self.save_to_filepath, 'a+') as f: and it gives "FileNotFoundError: [Errno 2] No such file or directory". The filepath is not in Unicode, either.

[chenjesu]

It turns out I was using the wrong directory path- I had been running the script from 2 different current directory locations. So I used the workaround in stackoverflow.com/questions/918154/relative-paths-in-python and it worked fine.

[ishandutta2007]

@DanielF op has accepted this answer without further research. There should be option to unaccept an answer by public voting.

[NONONONONO]

CAREFUL about this! It will truncate the content if the file exists.

[Chenglong Ma]

Hi @NONONONONO, yes, it is what w mode does. If you want to keep the existing content, you can use a append mode. Refer to open() doc

[Kai Petzke]

Mode "w+" has the disadvantage of truncating the file, if it already exists. In many cases, this is NOT what people may want.

[Tordek]

This introduces a race condition.

[Granitosaurus]

@Tordek how? I don't thinks that's true.