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Lib/test/test_tcl.py fails with:
test test_tcl failed -- Traceback (most recent call last):
 File "/home/abuild/rpmbuild/BUILD/Python-2.7.7/Lib/test/test_tcl.py", line 430
, in test_user_command
 check(float('nan'), 'NaN', eq=nan_eq)
 File "/home/abuild/rpmbuild/BUILD/Python-2.7.7/Lib/test/test_tcl.py", line 397
, in check
 eq(result[0], expected2)
 File "/home/abuild/rpmbuild/BUILD/Python-2.7.7/Lib/test/test_tcl.py", line 405
, in nan_eq
 actual = float(actual)
ValueError: invalid literal for float(): NaN(7ffffffffffff)

What version of Tcl are you using and on what platform?

You will see this on any architecture where the canonical NaN has all bits set (or a subset of them). This include mips and m68k.

Interesting, what result of getdouble?
>>> import _tkinter
>>> tk = _tkinter.create()
>>> nan = float('nan')
>>> tk.getdouble(nan)
nan
What returns getdouble() (note that _tkinter is imported instead of tkinter!) for NaN value?

>>> _tkinter.create().getdouble(float('nan'))
nan

Here is a patch which omits using of float() to convert Tcl's NaN values.

Thanks, this is working now.

New changeset 4514804d0e50 by Serhiy Storchaka in branch '2.7':
Issue #21881: Be more tolerant in test_tcl to not parsable by float() NaN
http://hg.python.org/cpython/rev/4514804d0e50
New changeset 4879f6337ef6 by Serhiy Storchaka in branch '3.4':
Issue #21881: Be more tolerant in test_tcl to not parsable by float() NaN
http://hg.python.org/cpython/rev/4879f6337ef6
New changeset 72289f574664 by Serhiy Storchaka in branch 'default':
Issue #21881: Be more tolerant in test_tcl to not parsable by float() NaN
http://hg.python.org/cpython/rev/72289f574664 

New changeset 01ec8bb7187f by Serhiy Storchaka in branch '2.7':
Issue #21881: Just omit tests for platform-specific NaN representation in test_tcl.
http://hg.python.org/cpython/rev/01ec8bb7187f
New changeset 5ac811cbec87 by Serhiy Storchaka in branch '3.4':
Issue #21881: Just omit tests for platform-specific NaN representation in test_tcl.
http://hg.python.org/cpython/rev/5ac811cbec87
New changeset f138a74ee611 by Serhiy Storchaka in branch 'default':
Issue #21881: Just omit tests for platform-specific NaN representation in test_tcl.
http://hg.python.org/cpython/rev/f138a74ee611 

On some platforms NaN representation has a minus before "nan" (see http://buildbot.python.org/all/builders/x86%20OpenIndiana%202.7/builds/2384/steps/test/logs/stdio). So it is easier just omit tests for these platform-specific values.
Regardless of the fact that it is very unlikely to get NaN in Tk, in future we should fix getdouble() (it should work with Tcl_Obj) and use it everywhere in tkinter instead of float().