Apply capture_logs to initialized bound loggers (#412)

Fixes #408

CHANGELOG.md

@@ -49,6 +49,8 @@ So please make sure to **always** properly configure your applications.
   [#401](https://github.com/hynek/structlog/pull/401)
 - `structlog.PrintLogger` -- that is used by default -- now uses `print()` for printing, making it a better citizen for interactive terminal applications.
   [#399](https://github.com/hynek/structlog/pull/399)
+- `structlog.testing.capture_logs` now works for already initialized bound loggers.
+  [#408](https://github.com/hynek/structlog/pull/412)
 
 
 ### Fixed

src/structlog/testing.py

@@ -59,12 +59,22 @@ def capture_logs() -> Generator[List[EventDict], None, None]:
     .. versionadded:: 20.1.0
     """
     cap = LogCapture()
-    old_processors = get_config()["processors"]
+    # Modify `_Configuration.default_processors` set via `configure` but always
+    # keep the list instance intact to not break references held by bound
+    # loggers.
+    processors = get_config()["processors"]
+    old_processors = processors.copy()
     try:
-        configure(processors=[cap])
+        # clear processors list and use LogCapture for testing
+        processors.clear()
+        processors.append(cap)
+        configure(processors=processors)
         yield cap.entries
     finally:
-        configure(processors=old_processors)
+        # remove LogCapture and restore original processors
+        processors.clear()
+        processors.extend(old_processors)
+        configure(processors=processors)
 
 
 class ReturnLogger:

tests/test_testing.py

@@ -10,6 +10,7 @@
     CapturedCall,
     CapturingLogger,
     CapturingLoggerFactory,
+    LogCapture,
     ReturnLogger,
     ReturnLoggerFactory,
 )
@@ -46,11 +47,16 @@ def test_restores_processors_on_success(self):
         exit.
         """
         orig_procs = self.get_active_procs()
+        assert len(orig_procs) > 1
 
         with testing.capture_logs():
-            assert orig_procs is not self.get_active_procs()
+            modified_procs = self.get_active_procs()
+            assert len(modified_procs) == 1
+            assert isinstance(modified_procs[0], LogCapture)
 
-        assert orig_procs is self.get_active_procs()
+        restored_procs = self.get_active_procs()
+        assert orig_procs is restored_procs
+        assert len(restored_procs) > 1
 
     def test_restores_processors_on_error(self):
         """
@@ -64,6 +70,26 @@ def test_restores_processors_on_error(self):
 
         assert orig_procs is self.get_active_procs()
 
+    def test_captures_bound_logers(self):
+        """
+        Even logs from already bound loggers are captured and their processors
+        restored on exit.
+        """
+        logger = get_logger("bound").bind(foo="bar")
+        logger.info("ensure logger is bound")
+
+        with testing.capture_logs() as logs:
+            logger.info("hello", answer=42)
+
+        assert logs == [
+            {
+                "event": "hello",
+                "answer": 42,
+                "foo": "bar",
+                "log_level": "info",
+            }
+        ]
+
 
 class TestReturnLogger:
     # @pytest.mark.parametrize("method", stdlib_log_methods)