What does python3 open "x" mode do?

Question

What does the new open file mode “x” do in python 3?

here is the doc of python 3:

‘r’: open for reading (default)

‘w’: open for writing, truncating the file first

‘x’: open for exclusive creation, failing if the file already exists

‘a’: open for writing, appending to the end of the file if it exists

‘b’: binary mode

‘t’: text mode (default)

‘+’: open a disk file for updating (reading and writing)

‘U’: universal newlines mode (deprecated)

What does “exclusive creation” mean?

I test the “x” mode and find some:

It can not be used with “r/w/a”
“x” is only writeable. “x+” can write and read
The file must not exist before open
The file will be created after open

So, “x” is similar to “w”. But for “x”, if the file exists, raise FileExistsError. For “w”, it will simply create a new file / truncate the existed file.

Am I right? Is this the only difference?

Asked By: Tyler Temp

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Answer 1

As @Martjin has already said, you have already answered your own question. I would only be amplifying on the explanation in the manual so as to get a better understanding of the text

‘x’: open for exclusive creation, failing if the file already exists

When you specify exclusive creation, it clearly means, you would use this mode for exclusively creating the file. The need for this is required when you won’t accidentally truncate/append an existing file with either of the modes w or a.

In absence of this, developers should be cautious to check for the existence of the file before leaping to open the file for updation.

With this mode, your code would be simply be written as

try:
    with open("fname", "x") as fout:
        #Work with your open file
except FileExistsError:
    # Your error handling goes here

Previously though your code might had been written as

import os.path
if os.path.isfile(fname):
    # Your error handling goes here
else:
    with open("fname", "w") as fout:
        # Work with your open file

Answered By: Abhijit

Answer 2

Yes, that’s basically it. It calls the underlying operating system code with the two flags O_CREAT and O_EXCL, which attempts to open the file exclusively, creating a new one if it doesn’t currently exist.

It’s handy if you may find two instances of your program running concurrently, the use of x mode will ensure only one will successfully create a file, with the other one failing.

A classic example is daemons that write their process ID into a pid file (so that can be easily signalled later on). By using x, you can guarantee that only one daemon can be running at a time, something that’s harder to do without the x mode, and prone to race conditions.

Answered By: paxdiablo

Answer 3

To put simple, opening a file with 'x' mode means:

Atomically do: (check if exists and create file)

Answered By: Omer Dagan

What does python3 open "x" mode do?

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