Jack_B <barraclough.j...@gmail.com> added the comment:
This also tripped me up recently. More broadly, I assumed that handlers
downstream of the QueueHandler/QueueListener would get complete log records.
Having looked at it, I can see they need to be pickled, and that means
stripping some information. But like Mikael, I found the current behaviour a
bit confusing.
The solution I am using is to override logging.Formatter.format and
QueueHandler.Prepare like so:
class _OptionalExcFormatter(logging.Formatter):
def format(self, record, with_exc=True, with_stack=True):
"""
Format the specified record as text.
Same as superclass except it only adds the exc_info and stack_info if
the corresponding arguments are True.
"""
record.message = record.getMessage()
if self.usesTime():
record.asctime = self.formatTime(record, self.datefmt)
s = self.formatMessage(record)
if record.exc_info and with_exc:
# Cache the traceback text to avoid converting it multiple times
# (it's constant anyway)
if not record.exc_text:
record.exc_text = self.formatException(record.exc_info)
if record.exc_text and with_exc:
if s[-1:] != "\n":
s = s + "\n"
s = s + record.exc_text
if record.stack_info and with_stack:
if s[-1:] != "\n":
s = s + "\n"
s = s + self.formatStack(record.stack_info)
return s
class _QueueHandlerExc(QueueHandler):
def prepare(self, record):
# bpo-35726: make copy of record to avoid affecting other handlers in
the chain.
record = copy.copy(record)
# Get a formatter. It must support the with_exc and with_stack args to
f.format
if self.formatter is None:
f = _OptionalExcFormatter()
else:
f = self.formatter
# Merge args into message and strip them as they may not be pickleable
msg = f.format(record, with_exc=False, with_stack=False)
record.message = msg
record.msg = msg
record.args = None
# Convert exc_info into exc_text and strip it as it may not be
pickleable
if record.exc_info is not None:
record.exc_text = f.formatException(record.exc_info)
record.exc_info = None
return record
Pros:
- The record feels "less mangled"
- It does not require another formatter to have already populated exc_text,
and allows the use of a specific formatter to do so.
- Having the message and exc_text separate seems logical, and allows
downstream handlers/formatters to treat them differently.
- logging.Formatter can get the changes above in a back-compatible way
Cons:
- exc_text now contains the stack trace as well as the usual exc_info, which
is a little odd, but it still seems to be a better place for it than the
message.
- If QueueHandler gets the changes above, it will break code where
QueueHandler is used with a custom Formatter which overrides Formatter.format.
This is probably not OK.
I think the changes to logging.Formatter might also be useful elsewhere, as it
would enable you to do:
class _NoStackFormatter(logging.Formatter):
def format(self, record):
return super().format(record, with_exc=True, with_stack=False)
to have a formatter which omits stack traces.
----------
nosy: +Jack_B
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue44924>
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