fix(reporter): describe_build_func fails on functools.partial

PR lektor#1104 uses a functools.partial as a sub-artifact's
_build_func_. When verbose logging is enabled, this caused an
exception from `lektor.reporter.describe_build_func`.

CHANGES.md

@@ -8,6 +8,8 @@ These are all the changes in Lektor since the first public release.
 
 - Pin `watchfiles>=0.12`. (Our tests use the `stop_event` parameter of `watchfiles.watch`.)
 
+- Fix exception from `describe_build_func` when building thumbnails with verbose logging enabled.
+
 ## 3.4.0b6 (2023-05-05)
 
 ### Possibly Breaking Changes

lektor/reporter.py

@@ -13,12 +13,17 @@
 
 
 def describe_build_func(func):
-    self = getattr(func, "__self__", None)
-    if self is not None and any(
-        x.__name__ == "BuildProgram" for x in self.__class__.__mro__
-    ):
-        return self.__class__.__module__ + "." + self.__class__.__name__
-    return func.__module__ + "." + func.__name__
+    if hasattr(func, "func"):
+        func = func.func  # unwrap functools.partial
+    try:
+        qualname = func.__qualname__
+        class_name, _, method = qualname.rpartition(".")
+        if class_name and method == "build_artifact":
+            # Strip method name from methods of BuildProgram instances
+            qualname = class_name
+        return f"{func.__module__}.{qualname}"
+    except AttributeError:
+        return repr(func)
 
 
 class Reporter:

tests/test_reporter.py

@@ -0,0 +1,39 @@
+from functools import partial
+
+import pytest
+
+from lektor.build_programs import FileAssetBuildProgram
+from lektor.builder import Builder
+from lektor.reporter import describe_build_func
+
+
+@pytest.fixture
+def build_state(pad, tmp_path):
+    builder = Builder(pad, destination_path=tmp_path)
+    return builder.new_build_state()
+
+
+def dummy_build_func(*args):
+    pass
+
+
+def test_describe_build_func():
+    assert describe_build_func(dummy_build_func) == "test_reporter.dummy_build_func"
+
+
+def test_describe_build_func_describes_partial():
+    build_func = partial(dummy_build_func, "dummy-arg")
+    assert describe_build_func(build_func) == "test_reporter.dummy_build_func"
+
+
+def test_describe_build_func_BuildProgram(pad, build_state):
+    build_program = FileAssetBuildProgram(pad.get_asset("static/demo.css"), build_state)
+    build_func = build_program.build_artifact
+    assert (
+        describe_build_func(build_func) == "lektor.build_programs.FileAssetBuildProgram"
+    )
+
+
+def test_describe_build_func_deals_with_garbage():
+    garbage = object()
+    assert isinstance(describe_build_func(garbage), str)