Add examples for opener argument of open (#13424).
Patch by Guillaume Pratte.
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index 0fb48f4..5df7b67 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -935,6 +935,36 @@
:mod:`os.open` as *opener* results in functionality similar to passing
``None``).
+ The following example is an alternative implementation for opening files
+ for exclusive writing. If we did not have support for the ``'x'`` mode,
+ we could implement it with this opener::
+
+ >>> import os
+ >>> def open_exclusive(path, mode):
+ ... return os.open(path, mode | os.O_CREAT | os.O_EXCL)
+ ...
+ >>> filename = 'spam.txt'
+ >>> fp = open(filename, 'w', opener=open_exclusive)
+ >>> fp2 = open(filename, 'w', opener=open_exclusive)
+ Traceback (most recent call last):
+ ...
+ FileExistsError: [Errno 17] File exists: 'spam.txt'
+
+ This other example uses the :ref:`dir_fd` parameter of the
+ :func:`os.open` function to open a file relative to a given directory::
+
+ >>> import os
+ >>> def open_relative(dirname):
+ ... dir_fd = os.open(dirname, os.O_RDONLY)
+ ... def opener(path, flags):
+ ... return os.open(path, flags, dir_fd=dir_fd)
+ ... return opener
+ ...
+ >>> opener = open_relative('somedir')
+ >>> with open('spamspam.txt', 'w', opener=opener) as f:
+ ... print('This will be written to somedir/spamspam.txt', file=f)
+ ...
+
.. versionchanged:: 3.3
The *opener* parameter was added.
The ``'x'`` mode was added.