Bug fix: Restore StandardErrorHandler functionality

The StandardErrorHandler class is responsible for dynamically resolving
(looking up the value of) sys.stderr for each logged message instead of
once when coloredlogs.install() is called.

This was unintentionally broken by changes in 4f8c9aa which were
released as part of coloredlogs version 14.1.

This was brought to my attention by @hugovk here:
xolox/python-humanfriendly#51

coloredlogs/__init__.py

@@ -1,7 +1,7 @@
 # Colored terminal output for Python's logging module.
 #
 # Author: Peter Odding <peter@peterodding.com>
-# Last Change: December 10, 2020
+# Last Change: June 11, 2021
 # URL: https://coloredlogs.readthedocs.io
 
 """
@@ -394,7 +394,7 @@ def install(level=None, **kw):
     """
     logger = kw.get('logger') or logging.getLogger()
     reconfigure = kw.get('reconfigure', True)
-    stream = kw.get('stream', sys.stderr)
+    stream = kw.get('stream') or sys.stderr
     style = check_style(kw.get('style') or DEFAULT_FORMAT_STYLE)
     # Get the log level from an argument, environment variable or default and
     # convert the names of log levels to numbers to enable numeric comparison.
@@ -453,7 +453,10 @@ def install(level=None, **kw):
         # Create a stream handler and make sure to preserve any filters
         # the current handler may have (if an existing handler is found).
         filters = handler.filters if handler else None
-        handler = logging.StreamHandler(stream) if stream else StandardErrorHandler()
+        if stream is sys.stderr:
+            handler = StandardErrorHandler()
+        else:
+            handler = logging.StreamHandler(stream)
         handler.setLevel(level)
         if filters:
             handler.filters = filters

coloredlogs/tests.py

@@ -1,7 +1,7 @@
 # Automated tests for the `coloredlogs' package.
 #
 # Author: Peter Odding <peter@peterodding.com>
-# Last Change: December 10, 2020
+# Last Change: June 11, 2021
 # URL: https://coloredlogs.readthedocs.io
 
 """Automated tests for the `coloredlogs` package."""
@@ -396,6 +396,22 @@ def test_plain_text_output_format(self):
             assert severity in last_line
             assert PLAIN_TEXT_PATTERN.match(last_line)
 
+    def test_dynamic_stderr_lookup(self):
+        """Make sure coloredlogs.install() uses StandardErrorHandler when possible."""
+        coloredlogs.install()
+        # Redirect sys.stderr to a temporary buffer.
+        initial_stream = StringIO()
+        initial_text = "Which stream will receive this text?"
+        with PatchedAttribute(sys, 'stderr', initial_stream):
+            logging.info(initial_text)
+        assert initial_text in initial_stream.getvalue()
+        # Redirect sys.stderr again, to a different destination.
+        subsequent_stream = StringIO()
+        subsequent_text = "And which stream will receive this other text?"
+        with PatchedAttribute(sys, 'stderr', subsequent_stream):
+            logging.info(subsequent_text)
+        assert subsequent_text in subsequent_stream.getvalue()
+
     def test_force_enable(self):
         """Make sure ANSI escape sequences can be forced (bypassing auto-detection)."""
         interpreter = subprocess.Popen([