Quoting https://docs.python.org/3/library/tempfile.html
"""
tempfile.tempdir
When set to a value other than None, this variable defines the default value for the dir argument to the functions defined in this module.

If tempdir is unset or None at any call to any of the above functions except gettempprefix() it is initialized following the algorithm described in gettempdir().
"""

"If tempdir is unset ..." is not true:

$ python3 -c 'import tempfile; del tempfile.tempdir; t = tempfile.mkdtemp()'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3.2/tempfile.py", line 298, in mkdtemp
    dir = gettempdir()
  File "/usr/lib/python3.2/tempfile.py", line 238, in gettempdir
    if tempdir is None:
NameError: global name 'tempdir' is not defined

No strong preference whether the docs change to match the implementation, or vice-versa.

"If tempdir is unset or None" means "If tempdir has not been assigned to, or if it has been assigned the value None". "Unset" doesn't mean "delete from the module".

I agree the documentation wording could be improved.

How about

- If tempdir is unset or None at any call to
+ If tempdir is None (the default) at any call to

This avoids headaches over the meaning of "unset", and accurately reflects the code at:
https://github.com/python/cpython/blob/3.6/Lib/tempfile.py#L289

What about:

"If tempdir is unset or None (the default value has not been modified) ..." 

?

I like philiprowlands' version:

- If tempdir is unset or None at any call to
+ If tempdir is None (the default) at any call to

Care to produce a PR?

New changeset b225cb770fb17596298f5a05c41a7c90c470c4f8 by Eric V. Smith (Miss Islington (bot)) in branch '3.6':
bpo-33894: Clarified the tempfile.tempdir documentation (GH-7829) (GH-7842)
https://github.com/python/cpython/commit/b225cb770fb17596298f5a05c41a7c90c470c4f8

Thanks for the edit. I did try a PR but was defeated by build/doc tool's recent version requirements (and didn't want to send untested changes, however minor).

The reason for getting rid of "unset" is the confusion of "state", i.e. it has never been assigned to, or "action", i.e. the user is unsetting it.

This is complete and can be closed.