fix pypa#4170: log ignores broken-pipes in STDERR when pip | head

news/4170.bugfix

@@ -0,0 +1,3 @@
+Avoid printing a Traceback when a broken pipe is encountered in stderr
+(like when piping through `head` on Unix).
+

src/pip/_internal/utils/logging.py

@@ -1,6 +1,7 @@
 from __future__ import absolute_import
 
 import contextlib
+import errno
 import logging
 import logging.handlers
 import os
@@ -115,6 +116,15 @@ def format(self, record):
 
         return msg
 
+    def flush(self):
+        """Ignore `BrokenPipeErrors` on `flush()`."""
+        try:
+            super(ColorizedStreamHandler, self).flush()
+        except (IOError, OSError) as ex:  # Python-2 has no `BrokenPipeError`
+            if ex.errno != errno.EPIPE:
+                # Ignore BrokenPipeError eg. when piped in `head` UNIX command.
+                raise
+
 
 class BetterRotatingFileHandler(logging.handlers.RotatingFileHandler):