Raise HookMissing instead of falling back, if requested.

pep517/_in_process.py

@@ -38,6 +38,10 @@ def __init__(self, message):
         self.message = message
 
 
+class HookMissing(Exception):
+    """Raised if a hook is missing and we are not executing the fallback"""
+
+
 def contained_in(filename, directory):
     """Test if a file is located within the given directory."""
     filename = os.path.normcase(os.path.abspath(filename))
@@ -87,15 +91,19 @@ def get_requires_for_build_wheel(config_settings):
         return hook(config_settings)
 
 
-def prepare_metadata_for_build_wheel(metadata_directory, config_settings):
+def prepare_metadata_for_build_wheel(
+        metadata_directory, config_settings, _allow_fallback):
     """Invoke optional prepare_metadata_for_build_wheel
 
-    Implements a fallback by building a wheel if the hook isn't defined.
+    Implements a fallback by building a wheel if the hook isn't defined,
+    unless _allow_fallback is False in which case HookMissing is raised.
     """
     backend = _build_backend()
     try:
         hook = backend.prepare_metadata_for_build_wheel
     except AttributeError:
+        if not _allow_fallback:
+            raise HookMissing()
         return _get_wheel_metadata_from_wheel(backend, metadata_directory,
                                               config_settings)
     else:
@@ -239,6 +247,8 @@ def main():
     except GotUnsupportedOperation as e:
         json_out['unsupported'] = True
         json_out['traceback'] = e.traceback
+    except HookMissing:
+        json_out['hook_missing'] = True
 
     compat.write_json(json_out, pjoin(control_dir, 'output.json'), indent=2)
 

pep517/wrappers.py

@@ -35,6 +35,13 @@ def __init__(self, backend_name, backend_path, message):
         self.message = message
 
 
+class HookMissing(Exception):
+    """Will be raised on missing hooks."""
+    def __init__(self, hook_name):
+        super(HookMissing, self).__init__(hook_name)
+        self.hook_name = hook_name
+
+
 class UnsupportedOperation(Exception):
     """May be raised by build_sdist if the backend indicates that it can't."""
     def __init__(self, traceback):
@@ -134,18 +141,21 @@ def get_requires_for_build_wheel(self, config_settings=None):
         })
 
     def prepare_metadata_for_build_wheel(
-            self, metadata_directory, config_settings=None):
+            self, metadata_directory, config_settings=None,
+            _allow_fallback=True):
         """Prepare a *.dist-info folder with metadata for this project.
 
         Returns the name of the newly created folder.
 
         If the build backend defines a hook with this name, it will be called
         in a subprocess. If not, the backend will be asked to build a wheel,
-        and the dist-info extracted from that.
+        and the dist-info extracted from that (unless _allow_fallback is
+        False).
         """
         return self._call_hook('prepare_metadata_for_build_wheel', {
             'metadata_directory': abspath(metadata_directory),
             'config_settings': config_settings,
+            '_allow_fallback': _allow_fallback,
         })
 
     def build_wheel(
@@ -237,6 +247,8 @@ def _call_hook(self, hook_name, kwargs):
                     backend_path=self.backend_path,
                     message=data.get('backend_error', '')
                 )
+            if data.get('hook_missing'):
+                raise HookMissing(hook_name)
             return data['return_val']
 
 

tests/test_hook_fallbacks.py

@@ -1,9 +1,10 @@
 from os.path import dirname, abspath, join as pjoin
+import pytest
 import pytoml
 from testpath import modified_env, assert_isfile
 from testpath.tempdir import TemporaryDirectory
 
-from pep517.wrappers import Pep517HookCaller
+from pep517.wrappers import HookMissing, Pep517HookCaller
 
 SAMPLES_DIR = pjoin(dirname(abspath(__file__)), 'samples')
 BUILDSYS_PKGS = pjoin(SAMPLES_DIR, 'buildsys_pkgs')
@@ -37,3 +38,18 @@ def test_prepare_metadata_for_build_wheel():
             hooks.prepare_metadata_for_build_wheel(metadatadir, {})
 
         assert_isfile(pjoin(metadatadir, 'pkg2-0.5.dist-info', 'METADATA'))
+
+
+def test_prepare_metadata_for_build_wheel_no_fallback():
+    hooks = get_hooks('pkg2')
+
+    with TemporaryDirectory() as metadatadir:
+        with modified_env({'PYTHONPATH': BUILDSYS_PKGS}):
+            with pytest.raises(HookMissing) as exc_info:
+                hooks.prepare_metadata_for_build_wheel(
+                    metadatadir, {}, _allow_fallback=False
+                )
+
+            e = exc_info.value
+            assert 'prepare_metadata_for_build_wheel' == e.hook_name
+            assert 'prepare_metadata_for_build_wheel' in str(e)