MAINT: TkAgg default backend depends on tkinter

setup.py

@@ -193,9 +193,10 @@ def run(self):
                     required_failed.append(package)
             else:
                 good_packages.append(package)
-                if isinstance(package, setupext.OptionalBackendPackage):
-                    if default_backend is None:
-                        default_backend = package.name
+                if (isinstance(package, setupext.OptionalBackendPackage) and
+                        package.runtime_check() and
+                        default_backend is None):
+                    default_backend = package.name
     print_raw('')
 
     # Abort if any of the required packages can not be built.

setupext.py

@@ -1,5 +1,7 @@
 from __future__ import print_function, absolute_import
 
+from importlib import import_module
+
 from distutils import sysconfig
 from distutils import version
 from distutils.core import Extension
@@ -424,13 +426,20 @@ class SetupPackage(object):
 
     def check(self):
         """
-        Checks whether the dependencies are met.  Should raise a
-        `CheckFailed` exception if the dependency could not be met,
-        otherwise return a string indicating a version number or some
-        other message indicating what was found.
+        Checks whether the build dependencies are met.  Should raise a
+        `CheckFailed` exception if the dependency could not be met, otherwise
+        return a string indicating a version number or some other message
+        indicating what was found.
         """
         pass
 
+    def runtime_check(self):
+        """
+        True if the runtime dependencies of the backend are met.  Assumes that
+        the build-time dependencies are met.
+        """
+        return True
+
     def get_packages(self):
         """
         Get a list of package names to add to the configuration.
@@ -1643,6 +1652,16 @@ class BackendTkAgg(OptionalBackendPackage):
     def check(self):
         return "installing; run-time loading from Python Tcl / Tk"
 
+    def runtime_check(self):
+        """ Checks whether TkAgg runtime dependencies are met
+        """
+        pkg_name = 'tkinter' if PY3min else 'Tkinter'
+        try:
+            import_module(pkg_name)
+        except ImportError:
+            return False
+        return True
+
     def get_extension(self):
         sources = [
             'src/py_converters.cpp',