Propagate changes to backend loading to setup/setupext.

1) Restore the ability to set the default backend from setup.cfg.
2) Remove the backend fallback code.

Note that not including `mpl-data/*.glade` in the package data is fine:
that file had been removed in 962388f anyways.

Author: anntzer

.flake8

@@ -23,7 +23,7 @@ exclude =
 
 per-file-ignores =
     setup.py: E402
-    setupext.py: E302, E501
+    setupext.py: E501
 
     tools/compare_backend_driver_results.py: E501
     tools/subset.py: E221, E231, E251, E261, E302, E501, E701

setup.cfg.template

@@ -63,15 +63,9 @@
 #           behavior
 #
 #agg = auto
-#cairo = auto
-#gtk3agg = auto
-#gtk3cairo = auto
 #macosx = auto
-#pyside = auto
-#qt4agg = auto
 #tkagg = auto
 #windowing = auto
-#wxagg = auto
 
 [rc_options]
 # User-configurable options
@@ -81,10 +75,9 @@
 #
 # 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.  Agg will be used by default.
+# modules.  The default is determined by fallback.
 #
 #backend = Agg
-#
 
 [package_data]
 # Package additional files found in the lib/matplotlib directories.

setup.py

@@ -70,18 +70,9 @@
     setupext.Tests(),
     setupext.Toolkits_Tests(),
     'Optional backend extensions',
-    # These backends are listed in order of preference, the first
-    # being the most preferred.  The first one that looks like it will
-    # work will be selected as the default backend.
-    setupext.BackendMacOSX(),
-    setupext.BackendQt5(),
-    setupext.BackendQt4(),
-    setupext.BackendGtk3Agg(),
-    setupext.BackendGtk3Cairo(),
-    setupext.BackendTkAgg(),
-    setupext.BackendWxAgg(),
     setupext.BackendAgg(),
-    setupext.BackendCairo(),
+    setupext.BackendTkAgg(),
+    setupext.BackendMacOSX(),
     setupext.Windowing(),
     'Optional package data',
     setupext.Dlls(),
@@ -133,7 +124,6 @@ def run(self):
     package_dir = {'': 'lib'}
     install_requires = []
     setup_requires = []
-    default_backend = None
 
     # If the user just queries for information, don't bother figuring out which
     # packages to build or install.
@@ -169,10 +159,6 @@ def run(self):
                         required_failed.append(package)
                 else:
                     good_packages.append(package)
-                    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.
@@ -203,14 +189,16 @@ def run(self):
             setup_requires.extend(package.get_setup_requires())
 
         # Write the default matplotlibrc file
-        if default_backend is None:
-            default_backend = 'svg'
-        if setupext.options['backend']:
-            default_backend = setupext.options['backend']
         with open('matplotlibrc.template') as fd:
-            template = fd.read()
+            template_lines = fd.read().splitlines(True)
+        backend_line_idx, = [  # Also asserts that there is a single such line.
+            idx for idx, line in enumerate(template_lines)
+            if line.startswith('#backend ')]
+        if setupext.options['backend']:
+            template_lines[backend_line_idx] = (
+                'backend: {}'.format(setupext.options['backend']))
         with open('lib/matplotlib/mpl-data/matplotlibrc', 'w') as fd:
-            fd.write(template)
+            fd.write(''.join(template_lines))
 
         # Finalize the extension modules so they can get the Numpy include
         # dirs

setupext.py

@@ -384,13 +384,6 @@ def check(self):
         """
         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.
@@ -1368,10 +1361,6 @@ 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."""
-        return importlib.util.find_spec("tkinter") is not None
-
     def get_extension(self):
         sources = [
             'src/_tkagg.cpp'
@@ -1391,57 +1380,6 @@ def add_flags(self, ext):
             ext.libraries.extend(['dl'])
 
 
-class BackendGtk3Agg(OptionalBackendPackage):
-    name = "gtk3agg"
-
-    def check_requirements(self):
-        if not any(map(importlib.util.find_spec, ["cairocffi", "cairo"])):
-            raise CheckFailed("Requires cairocffi or pycairo to be installed.")
-
-        try:
-            import gi
-        except ImportError:
-            raise CheckFailed("Requires pygobject to be installed.")
-
-        try:
-            gi.require_version("Gtk", "3.0")
-        except ValueError:
-            raise CheckFailed(
-                "Requires gtk3 development files to be installed.")
-        except AttributeError:
-            raise CheckFailed("pygobject version too old.")
-
-        try:
-            from gi.repository import Gtk, Gdk, GObject
-        except (ImportError, RuntimeError):
-            raise CheckFailed("Requires pygobject to be installed.")
-
-        return "version {}.{}.{}".format(
-            Gtk.get_major_version(),
-            Gtk.get_minor_version(),
-            Gtk.get_micro_version())
-
-    def get_package_data(self):
-        return {'matplotlib': ['mpl-data/*.glade']}
-
-
-class BackendGtk3Cairo(BackendGtk3Agg):
-    name = "gtk3cairo"
-
-
-class BackendWxAgg(OptionalBackendPackage):
-    name = "wxagg"
-
-    def check_requirements(self):
-        try:
-            import wx
-            backend_version = wx.VERSION_STRING
-        except ImportError:
-            raise CheckFailed("requires wxPython")
-
-        return "version %s" % backend_version
-
-
 class BackendMacOSX(OptionalBackendPackage):
     name = 'macosx'
 
@@ -1487,174 +1425,6 @@ def get_extension(self):
         return ext
 
 
-class BackendQtBase(OptionalBackendPackage):
-
-    def convert_qt_version(self, version):
-        version = '%x' % version
-        temp = []
-        while len(version) > 0:
-            version, chunk = version[:-2], version[-2:]
-            temp.insert(0, str(int(chunk, 16)))
-        return '.'.join(temp)
-
-    def check_requirements(self):
-        """
-        If PyQt4/PyQt5 is already imported, importing PyQt5/PyQt4 will fail
-        so we need to test in a subprocess (as for Gtk3).
-        """
-        try:
-            p = multiprocessing.Pool()
-
-        except:
-            # Can't do multiprocessing, fall back to normal approach
-            # (this will fail if importing both PyQt4 and PyQt5).
-            try:
-                # Try in-process
-                msg = self.callback(self)
-            except RuntimeError:
-                raise CheckFailed(
-                    "Could not import: are PyQt4 & PyQt5 both installed?")
-
-        else:
-            # Multiprocessing OK
-            try:
-                res = p.map_async(self.callback, [self])
-                msg = res.get(timeout=10)[0]
-            except multiprocessing.TimeoutError:
-                p.terminate()
-                # No result returned. Probably hanging, terminate the process.
-                raise CheckFailed("Check timed out")
-            except:
-                # Some other error.
-                p.close()
-                raise
-            else:
-                # Clean exit
-                p.close()
-            finally:
-                # Tidy up multiprocessing
-                p.join()
-
-        return msg
-
-
-def backend_pyside_internal_check(self):
-    try:
-        from PySide import __version__
-        from PySide import QtCore
-    except ImportError:
-        raise CheckFailed("PySide not found")
-    else:
-        return ("Qt: %s, PySide: %s" %
-                (QtCore.__version__, __version__))
-
-
-def backend_pyqt4_internal_check(self):
-    try:
-        from PyQt4 import QtCore
-    except ImportError:
-        raise CheckFailed("PyQt4 not found")
-
-    try:
-        qt_version = QtCore.QT_VERSION
-        pyqt_version_str = QtCore.PYQT_VERSION_STR
-    except AttributeError:
-        raise CheckFailed('PyQt4 not correctly imported')
-    else:
-        return ("Qt: %s, PyQt: %s" % (self.convert_qt_version(qt_version), pyqt_version_str))
-
-
-def backend_qt4_internal_check(self):
-    successes = []
-    failures = []
-    try:
-        successes.append(backend_pyside_internal_check(self))
-    except CheckFailed as e:
-        failures.append(str(e))
-
-    try:
-        successes.append(backend_pyqt4_internal_check(self))
-    except CheckFailed as e:
-        failures.append(str(e))
-
-    if len(successes) == 0:
-        raise CheckFailed('; '.join(failures))
-    return '; '.join(successes + failures)
-
-
-class BackendQt4(BackendQtBase):
-    name = "qt4agg"
-
-    def __init__(self, *args, **kwargs):
-        BackendQtBase.__init__(self, *args, **kwargs)
-        self.callback = backend_qt4_internal_check
-
-def backend_pyside2_internal_check(self):
-    try:
-        from PySide2 import __version__
-        from PySide2 import QtCore
-    except ImportError:
-        raise CheckFailed("PySide2 not found")
-    else:
-        return ("Qt: %s, PySide2: %s" %
-                (QtCore.__version__, __version__))
-
-def backend_pyqt5_internal_check(self):
-    try:
-        from PyQt5 import QtCore
-    except ImportError:
-        raise CheckFailed("PyQt5 not found")
-
-    try:
-        qt_version = QtCore.QT_VERSION
-        pyqt_version_str = QtCore.PYQT_VERSION_STR
-    except AttributeError:
-        raise CheckFailed('PyQt5 not correctly imported')
-    else:
-        return ("Qt: %s, PyQt: %s" % (self.convert_qt_version(qt_version), pyqt_version_str))
-
-def backend_qt5_internal_check(self):
-    successes = []
-    failures = []
-    try:
-        successes.append(backend_pyside2_internal_check(self))
-    except CheckFailed as e:
-        failures.append(str(e))
-
-    try:
-        successes.append(backend_pyqt5_internal_check(self))
-    except CheckFailed as e:
-        failures.append(str(e))
-
-    if len(successes) == 0:
-        raise CheckFailed('; '.join(failures))
-    return '; '.join(successes + failures)
-
-class BackendQt5(BackendQtBase):
-    name = "qt5agg"
-
-    def __init__(self, *args, **kwargs):
-        BackendQtBase.__init__(self, *args, **kwargs)
-        self.callback = backend_qt5_internal_check
-
-
-class BackendCairo(OptionalBackendPackage):
-    name = "cairo"
-
-    def check_requirements(self):
-        try:
-            import cairocffi
-        except ImportError:
-            try:
-                import cairo
-            except ImportError:
-                raise CheckFailed("cairocffi or pycairo not found")
-            else:
-                return "pycairo version %s" % cairo.version
-        else:
-            return "cairocffi version %s" % cairocffi.version
-
-
 class OptionalPackageData(OptionalPackage):
     config_category = "package_data"