bpo-40275: Avoid importing logging in test.support (GH-19601)

Import logging lazily in assertLogs() in unittest.
Move TestHandler from test.support to logging_helper.

1 files changed, 29 insertions, 0 deletions

diff --git a/Lib/test/support/logging_helper.py b/Lib/test/support/logging_helper.py
new file mode 100644
index 0000000..12fcca4
--- /dev/null
+++ b/Lib/test/support/logging_helper.py
@@ -0,0 +1,29 @@
+import logging.handlers
+
+class TestHandler(logging.handlers.BufferingHandler):
+    def __init__(self, matcher):
+        # BufferingHandler takes a "capacity" argument
+        # so as to know when to flush. As we're overriding
+        # shouldFlush anyway, we can set a capacity of zero.
+        # You can call flush() manually to clear out the
+        # buffer.
+        logging.handlers.BufferingHandler.__init__(self, 0)
+        self.matcher = matcher
+
+    def shouldFlush(self):
+        return False
+
+    def emit(self, record):
+        self.format(record)
+        self.buffer.append(record.__dict__)
+
+    def matches(self, **kwargs):
+        """
+        Look for a saved dict whose keys/values match the supplied arguments.
+        """
+        result = False
+        for d in self.buffer:
+            if self.matcher.matches(d, **kwargs):
+                result = True
+                break
+        return result