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.