Improve error when accessing cpp_info shorthand methods. (#16847)

conans/model/build_info.py

@@ -194,22 +194,28 @@ def frameworkdirs(self, value):
     @property
     def bindir(self):
         bindirs = self.bindirs
-        assert bindirs
-        assert len(bindirs) == 1
+        if not bindirs or len(bindirs) != 1:
+            raise ConanException(f"The bindir property is undefined because bindirs "
+                                 f"{'is empty' if not bindirs else 'has more than one element'}."
+                                 f" Consider using the bindirs property.")
         return bindirs[0]
 
     @property
     def libdir(self):
         libdirs = self.libdirs
-        assert libdirs
-        assert len(libdirs) == 1
+        if not libdirs or len(libdirs) != 1:
+            raise ConanException(f"The libdir property is undefined because libdirs "
+                                 f"{'is empty' if not libdirs else 'has more than one element'}."
+                                 f" Consider using the libdirs property.")
         return libdirs[0]
 
     @property
     def includedir(self):
         includedirs = self.includedirs
-        assert includedirs
-        assert len(includedirs) == 1
+        if not includedirs or len(includedirs) != 1:
+            raise ConanException(f"The includedir property is undefined because includedirs "
+                                 f"{'is empty' if not includedirs else 'has more than one element'}."
+                                 f" Consider using the includedirs property.")
         return includedirs[0]
 
     @property

test/integration/conanfile/conanfile_errors_test.py

@@ -192,3 +192,27 @@ def requirements(self):
     c.save({"conanfile.py": conanfile})
     c.run("create .", assert_error=True)
     assert "ERROR: hello/0.1: Dependencies options were defined incorrectly." in c.out
+
+
+@pytest.mark.parametrize("property_name", ["libdir", "bindir", "includedir"])
+@pytest.mark.parametrize("property_content", [[], ["mydir1", "mydir2"]])
+def test_shorthand_bad_interface(property_name, property_content):
+    c = TestClient(light=True)
+    conanfile = textwrap.dedent(f"""
+        from conan import ConanFile
+
+        class HelloConan(ConanFile):
+            name = "hello"
+            version = "0.1"
+
+            def package_info(self):
+                self.cpp_info.{property_name}s = {property_content}
+                self.output.info(self.cpp_info.{property_name})
+        """)
+    c.save({"conanfile.py": conanfile})
+    c.run("create .", assert_error=True)
+    if property_content:
+        assert f"The {property_name} property is undefined because {property_name}s has more than one element." in c.out
+    else:
+        assert f"The {property_name} property is undefined because {property_name}s is empty." in c.out
+