Remove logic for optionally building Agg and TkAgg.

They are actually not optional (that's the `force = True` setting which
overrides the OptionalBackendPackage logic), so just make them
SetupPackages and remove the corresponding (outdated) comments in
setup.cfg.template.

Author: anntzer

setup.cfg.template

@@ -1,5 +1,4 @@
-# Rename this file to setup.cfg to modify Matplotlib's
-# build options.
+# Rename this file to setup.cfg to modify Matplotlib's build options.
 
 [egg_info]
 
@@ -18,25 +17,10 @@
 #sample_data = True
 
 [gui_support]
-# Matplotlib supports multiple GUI toolkits, including
-# GTK3, MacOSX, Qt4, Qt5, Tk, and WX. Support for many of
-# these toolkits requires AGG, the Anti-Grain Geometry library,
-# which is provided by Matplotlib and built by default.
-#
-# Some backends are written in pure Python, and others require
-# extension code to be compiled. By default, Matplotlib checks for
-# these GUI toolkits during installation and, if present, compiles the
-# required extensions to support the toolkit.
-#
-# - Tk support requires Tk development headers and Tkinter.
-# - Mac OSX backend requires the Cocoa headers included with XCode.
-#
-# The other GUI toolkits do not require any extension code, and can be
-# used as long as the libraries are installed on your system --
-# therefore they are installed unconditionally.
-#
-# You can uncomment any the following lines to change this
-# behavior. Acceptable values are:
+# Matplotlib supports multiple GUI toolkits, known as backends.
+# The Mac OSX backend requires the Cocoa headers included with XCode.
+# You can select whether to build it by uncommenting the following line.
+# Acceptable values are:
 #
 #     True: build the extension. Exits with a warning if the
 #           required dependencies are not available
@@ -45,18 +29,16 @@
 #           otherwise skip silently. This is the default
 #           behavior
 #
-#agg = auto
 #macosx = auto
-#tkagg = auto
 
 [rc_options]
 # User-configurable options
 #
 # Default backend, one of: Agg, Cairo, GTK3Agg, GTK3Cairo, MacOSX, Pdf, Ps,
 # Qt4Agg, Qt5Agg, SVG, TkAgg, WX, WXAgg.
 #
-# The Agg, Ps, Pdf and SVG backends do not require external dependencies. Do
-# not choose MacOSX, or TkAgg if you have disabled the relevant extension
-# modules.  The default is determined by fallback.
+# The Agg, Ps, Pdf and SVG backends do not require external dependencies.  Do
+# not choose MacOSX if you have disabled the relevant extension modules.  The
+# default is determined by fallback.
 #
 #backend = Agg

setupext.py

@@ -302,7 +302,6 @@ def do_custom_build(self):
 
 class OptionalPackage(SetupPackage):
     optional = True
-    force = False
     config_category = "packages"
     default_config = "auto"
 
@@ -328,26 +327,13 @@ def check(self):
 
         May be overridden by subclasses for additional checks.
         """
-        # Check configuration file
-        conf = self.get_config()
-        # Default "auto" state or install forced by user
-        if conf in [True, 'auto']:
-            # Set non-optional if user sets `True` in config
-            if conf is True:
+        conf = self.get_config()  # Check configuration file
+        if conf in [True, 'auto']:  # Default "auto", or install forced by user
+            if conf is True:  # Set non-optional if user sets `True` in config
                 self.optional = False
             return "installing"
-        # Configuration opt-out by user
-        else:
-            # Some backend extensions (e.g. Agg) need to be built for certain
-            # other GUI backends (e.g. TkAgg) even when manually disabled
-            if self.force is True:
-                return "installing forced (config override)"
-            else:
-                raise CheckFailed("skipping due to configuration")
-
-
-class OptionalBackendPackage(OptionalPackage):
-    config_category = "gui_support"
+        else:  # Configuration opt-out by user
+            raise CheckFailed("skipping due to configuration")
 
 
 class Platform(SetupPackage):
@@ -707,9 +693,8 @@ def get_extension(self):
         return ext
 
 
-class BackendAgg(OptionalBackendPackage):
+class BackendAgg(SetupPackage):
     name = "agg"
-    force = True
 
     def get_extension(self):
         sources = [
@@ -725,9 +710,8 @@ def get_extension(self):
         return ext
 
 
-class BackendTkAgg(OptionalBackendPackage):
+class BackendTkAgg(SetupPackage):
     name = "tkagg"
-    force = True
 
     def check(self):
         return "installing; run-time loading from Python Tcl/Tk"
@@ -755,7 +739,8 @@ def add_flags(self, ext):
             ext.libraries.extend(['dl'])
 
 
-class BackendMacOSX(OptionalBackendPackage):
+class BackendMacOSX(OptionalPackage):
+    config_category = 'gui_support'
     name = 'macosx'
 
     def check(self):