@symphorien, thanks for your PR! By analyzing the history of the files in this pull request, we identified @FRidh, @domenkozar and @kragniz to be potential reviewers.

I don't think this will be a mass rebuild (tk isn't used much) but even so please rebase against staging.

Indeed, you are right. I checked the source code and it always tries to find the library using an environment variable.

We shouldn't use a wrapper if we can do so without. I think we can patch the code to point to the Tix library.

The following (Python 3.5) doesn't seem to be sufficient yet

  postPatch = optionalString x11Support ''
    substituteInPlace "Lib/tkinter/tix.py" --replace "os.environ.get('TIX_LIBRARY')" "'${tix}/lib'"
  '';

because

>>> import tkinter.tix
>>> root = tkinter.tix.Tk()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/nix/store/p6m8pxyw6njqfk82bdgj08w8pdrhq17h-python3-3.5.2/lib/python3.5/tkinter/tix.py", line 221, in __init__
    self.tk.eval('package require Tix')
_tkinter.TclError: Can't find a usable Init.tcl in the following directories: 
    /nix/store/aqfcswnix9ayq1jwrh931gjimnwxc7mq-tcl-8.6.6/lib/tcl8.6/Tix8.4.3 /nix/store/aqfcswnix9ayq1jwrh931gjimnwxc7mq-tcl-8.6.6/lib/Tix8.4.3 /nix/store/p6m8pxyw6njqfk82bdgj08w8pdrhq17h-python3-3.5.2/lib/Tix8.4.3 /nix/store/8djjhdby3grc0zq90bl5qg7ckzvgrmfd-tk-8.6.6/lib/tk8.6/Tix8.4.3 /nix/store/8djjhdby3grc0zq90bl5qg7ckzvgrmfd-tk-8.6.6/lib/tk8.6/ttk/Tix8.4.3 /nix/store/p6m8pxyw6njqfk82bdgj08w8pdrhq17h-python3-3.5.2/bin/Tix8.4.3 /nix/store/95y3xa78f7ksdy9hwks01axhw738km85-tix-8.4.3/Tix8.4.3 /nix/store/lib/Tix8.4.3 /nix/store/p6m8pxyw6njqfk82bdgj08w8pdrhq17h-python3-3.5.2/library

Note that when using this method we should copy tix.py in pythonPackages.tkinter.

The Python docs write

If this fails, you have a Tk installation problem which must be resolved before proceeding. Use the environment variable TIX_LIBRARY to point to the installed Tix library directory, and make sure you have the dynamic object library (tix8183.dll or libtix8183.so) in the same directory that contains your Tk dynamic object library (tk8183.dll or libtk8183.so). The directory with the dynamic object library should also have a file called pkgIndex.tcl (case sensitive), which contains the line:

The patch you proposed should have worked, but there was a mistake in the way I packaged tix for it to work. I force pushed something which is close to what you proposed.

There is one more challenge though. The default Python python won't have the reference to Tix, which is what we want. pythonFull will have the reference, that is fine, but pythonFull is deprecated in favor of using python.withPackages(ps: [ps.tkinter]). And the latter won't work yet

$ nix-shell -p 'python.withPackages(ps: [ps.tkinter])' --run "python -c 'import Tix; Tix.Tk()'" -I nixpkgs=.
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/nix/store/i1m970i9mbrk0aqr5z5rf0rfx8jihmy4-python-2.7.13-env/lib/python2.7/lib-tk/Tix.py", line 222, in __init__
    self.tk.eval('package require Tix')
_tkinter.TclError: can't find package Tix

because it uses the interpreter from python and the _tkinter module from pythonFull. (Note that I just saw it also links to the Python interpreter from pythonFull which is a bit of a problem.)

I forgot pypy, that should be fixed.
Regarding pythonFull, do you mean that the x11support argument is to be eventually removed ?

Regarding pythonFull, do you mean that the x11support value is to be eventually removed ?

The parameter will stay; just the attibute pkgs.pythonFull is deprecated. The parameter is used to actually be able to build pythonPackages.tkinter.