Explicitly raise an exception if backend isn't available

pep517/_in_process.py

@@ -21,11 +21,17 @@ import sys
 # This is run as a script, not a module, so it can't do a relative import
 import compat
 
+class BackendUnavailable(Exception):
+    """Raised if we cannot import the backend"""
+
 def _build_backend():
     """Find and load the build backend"""
     ep = os.environ['PEP517_BUILD_BACKEND']
     mod_path, _, obj_path = ep.partition(':')
-    obj = import_module(mod_path)
+    try:
+        obj = import_module(mod_path)
+    except ImportError:
+        raise BackendUnavailable
     if obj_path:
         for path_part in obj_path.split('.'):
             obj = getattr(obj, path_part)
@@ -173,6 +179,8 @@ def main():
     json_out = {'unsupported': False, 'return_val': None}
     try:
         json_out['return_val'] = hook(**hook_input['kwargs'])
+    except BackendUnavailable:
+        json_out['no_backend'] = True
     except GotUnsupportedOperation:
         json_out['unsupported'] = True
     

pep517/wrappers.py

@@ -18,6 +18,9 @@ def tempdir():
     finally:
         shutil.rmtree(td)
 
+class BackendUnavailable(Exception):
+    """Will be raised if the backend cannot be imported in the hook process."""
+
 class UnsupportedOperation(Exception):
     """May be raised by build_sdist if the backend indicates that it can't."""
 
@@ -130,5 +133,7 @@ class Pep517HookCaller(object):
             data = compat.read_json(pjoin(td, 'output.json'))
             if data.get('unsupported'):
                 raise UnsupportedOperation
+            if data.get('no_backend'):
+                raise BackendUnavailable
             return data['return_val']
 

tests/test_call_hooks.py

@@ -7,7 +7,8 @@ import pytest
 import pytoml
 import zipfile
 
-from pep517.wrappers import Pep517HookCaller, UnsupportedOperation
+from pep517.wrappers import Pep517HookCaller
+from pep517.wrappers import UnsupportedOperation, BackendUnavailable
 
 SAMPLES_DIR = pjoin(dirname(abspath(__file__)), 'samples')
 BUILDSYS_PKGS = pjoin(SAMPLES_DIR, 'buildsys_pkgs')
@@ -18,6 +19,12 @@ def get_hooks(pkg):
         data = pytoml.load(f)
     return Pep517HookCaller(source_dir, data['build-system']['build-backend'])
 
+def test_missing_backend_gives_exception():
+    hooks = get_hooks('pkg1')
+    with modified_env({'PYTHONPATH': ''}):
+        with pytest.raises(BackendUnavailable):
+            res = hooks.get_requires_for_build_wheel({})
+
 def test_get_requires_for_build_wheel():
     hooks = get_hooks('pkg1')
     with modified_env({'PYTHONPATH': BUILDSYS_PKGS}):