Correct handle default backend.

See comments in `matplotlib/__init__.py` and `setup.py`.  Test
by updating your local repo to this branch and `pip install
git+file:///path/to/repo` (doing so with the current master throws
`ValueError: Key backend: '' is not a valid value for backend`.

lib/matplotlib/__init__.py

@@ -842,6 +842,12 @@ def rc_params_from_file(fname, fail_on_error=False, use_default_template=True):
     transform=lambda line: line[1:] if line.startswith("#") else line,
     fail_on_error=True)
 dict.update(rcParamsDefault, rcsetup._hardcoded_defaults)
+# Normally, the default matplotlibrc file contains *no* entry for backend (the
+# corresponding line starts with ##, not #; we fill on _auto_backend_sentinel
+# in that case.  However, packagers can set a different default backend
+# (resulting in a normal `#backend: foo` line) in which case we should *not*
+# fill in _auto_backend_sentinel.
+dict.setdefault(rcParamsDefault, "backend", rcsetup._auto_backend_sentinel)
 rcParams = RcParams()  # The global instance.
 dict.update(rcParams, dict.items(rcParamsDefault))
 dict.update(rcParams, _rc_params_in_file(matplotlib_fname()))

lib/matplotlib/mpl-data/matplotlibrc

@@ -78,7 +78,7 @@
 ##     PS PDF SVG Template
 ## You can also deploy your own backend outside of Matplotlib by referring to
 ## the module name (which must be in the PYTHONPATH) as 'module://my_backend'.
-#backend: Agg
+##backend: Agg
 
 ## The port to use for the web server in the WebAgg backend.
 #webagg.port: 8988

lib/matplotlib/rcsetup.py

@@ -1411,14 +1411,13 @@ def _convert_validator_spec(key, conv):
     "_internal.classic_mode": validate_bool
 }
 _hardcoded_defaults = {  # Defaults not inferred from matplotlibrc.template...
-    # ... because it can"t be:
-    "backend": _auto_backend_sentinel,
     # ... because they are private:
     "_internal.classic_mode": False,
     # ... because they are deprecated:
     "animation.avconv_path": "avconv",
     "animation.avconv_args": [],
     "animation.html_args": [],
+    # backend is handled separately when constructing rcParamsDefault.
 }
 _validators = {k: _convert_validator_spec(k, conv)
                for k, conv in _validators.items()}

setup.py

@@ -194,16 +194,18 @@ def build_extensions(self):
 
 
 def update_matplotlibrc(path):
-    # Update the matplotlibrc file if packagers want to change the default
-    # backend.
+    # If packagers want to change the default backend, insert a `#backend: ...`
+    # line.  Otherwise, use the default `##backend: Agg` which has no effect
+    # even after decommenting, which allows _auto_backend_sentinel to be filled
+    # in at import time.
     template_lines = path.read_text().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 "#backend:" in line]
     template_lines[backend_line_idx] = (
         "#backend: {}".format(setupext.options["backend"])
         if setupext.options["backend"]
-        else "#backend:")
+        else "##backend: Agg")
     path.write_text("".join(template_lines))