Logging variable data with new format string
PythonPython 2.7String FormattingPython Problem Overview
I use logging facility for python 2.7.3. Documentation for this Python version say:
> the logging package pre-dates newer formatting options such as str.format() and string.Template. These newer formatting options are supported...
I like 'new' format with curly braces. So i'm trying to do something like:
log = logging.getLogger("some.logger")
log.debug("format this message {0}", 1)
And get error:
> TypeError: not all arguments converted during string formatting
What I miss here?
P.S. I don't want to use
log.debug("format this message {0}".format(1))
because in this case the message is always being formatted regardless of logger level.
Python Solutions
Solution 1 - Python
EDIT: take a look at the StyleAdapter approach in @Dunes' answer unlike this answer; it allows to use alternative formatting styles without the boilerplate while calling logger's methods (debug(), info(), error(), etc).
From the docs — Use of alternative formatting styles:
> Logging calls (logger.debug(), logger.info() etc.) only take > positional parameters for the actual logging message itself, with > keyword parameters used only for determining options for how to handle > the actual logging call (e.g. the exc_info keyword parameter to > indicate that traceback information should be logged, or the extra > keyword parameter to indicate additional contextual information to be > added to the log). So you cannot directly make logging calls using > str.format() or string.Template syntax, because internally the logging > package uses %-formatting to merge the format string and the variable > arguments. There would no changing this while preserving backward > compatibility, since all logging calls which are out there in existing > code will be using %-format strings.
And: > > There is, however, a way that you can use {}- and $- formatting to > construct your individual log messages. Recall that for a message you > can use an arbitrary object as a message format string, and that the > logging package will call str() on that object to get the actual > format string.
Copy-paste this to wherever module:
class BraceMessage(object):
def __init__(self, fmt, *args, **kwargs):
self.fmt = fmt
self.args = args
self.kwargs = kwargs
def __str__(self):
return self.fmt.format(*self.args, **self.kwargs)
Then:
from wherever import BraceMessage as __
log.debug(__('Message with {0} {name}', 2, name='placeholders'))
Note: actual formatting is delayed until it is necessary e.g., if DEBUG messages are not logged then the formatting is not performed at all.
Solution 2 - Python
Here is another option that does not have the keyword problems mentioned in Dunes' answer. It can only handle positional ({0}) arguments and not keyword ({foo}) arguments. It also does not require two calls to format (using the underscore). It does have the ick-factor of subclassing str:
class BraceString(str):
def __mod__(self, other):
return self.format(*other)
def __str__(self):
return self
class StyleAdapter(logging.LoggerAdapter):
def __init__(self, logger, extra=None):
super(StyleAdapter, self).__init__(logger, extra)
def process(self, msg, kwargs):
if kwargs.pop('style', "%") == "{": # optional
msg = BraceString(msg)
return msg, kwargs
You use it like this:
logger = StyleAdapter(logging.getLogger(__name__))
logger.info("knights:{0}", "ni", style="{")
logger.info("knights:{}", "shrubbery", style="{")
Of course, you can remove the check noted with # optional to force all messages through the adapter to use new-style formatting.
Note for anyone reading this answer years later: Starting with Python 3.2, you can use the style parameter with Formatter objects:
> Logging (as of 3.2) provides improved support for these two additional formatting styles. The
> Formatter class been enhanced to take an additional, optional keyword parameter named style. This
> defaults to '%', but other possible values are '{' and '$', which correspond to the other two
> formatting styles. Backwards compatibility is maintained by default (as you would expect), but by
> explicitly specifying a style parameter, you get the ability to specify format strings which work
> with str.format() or
> string.Template.
The docs provide the example
logging.Formatter('{asctime} {name} {levelname:8s} {message}', style='{')
Note that in this case you still can't call the logger with the new format. I.e., the following still won't work:
logger.info("knights:{say}", say="ni") # Doesn't work!
logger.info("knights:{0}", "ni") # Doesn't work either
Solution 3 - Python
This was my solution to the problem when I found logging only uses printf style formatting. It allows logging calls to remain the same -- no special syntax such as log.info(__("val is {}", "x")). The change required to code is to wrap the logger in a StyleAdapter.
from inspect import getargspec
class BraceMessage(object):
def __init__(self, fmt, args, kwargs):
self.fmt = fmt
self.args = args
self.kwargs = kwargs
def __str__(self):
return str(self.fmt).format(*self.args, **self.kwargs)
class StyleAdapter(logging.LoggerAdapter):
def __init__(self, logger):
self.logger = logger
def log(self, level, msg, *args, **kwargs):
if self.isEnabledFor(level):
msg, log_kwargs = self.process(msg, kwargs)
self.logger._log(level, BraceMessage(msg, args, kwargs), (),
**log_kwargs)
def process(self, msg, kwargs):
return msg, {key: kwargs[key]
for key in getargspec(self.logger._log).args[1:] if key in kwargs}
Usage is:
log = StyleAdapter(logging.getLogger(__name__))
log.info("a log message using {type} substitution", type="brace")
It's worth noting that this implementation has problems if key words used for brace substitution include level, msg, args, exc_info, extra or stack_info. These are argument names used by the log method of Logger. If you need to one of these names then modify process to exclude these names or just remove log_kwargs from the _log call. On a further note, this implementation also silently ignores misspelled keywords meant for the Logger (eg. ectra).
Solution 4 - Python
The easier solution would be to use the excellent logbook module
import logbook
import sys
logbook.StreamHandler(sys.stdout).push_application()
logbook.debug('Format this message {k}', k=1)
Or the more complete:
>>> import logbook
>>> import sys
>>> logbook.StreamHandler(sys.stdout).push_application()
>>> log = logbook.Logger('MyLog')
>>> log.debug('Format this message {k}', k=1)
[2017-05-06 21:46:52.578329] DEBUG: MyLog: Format this message 1
Solution 5 - Python
UPDATE: There is now a package on PyPI called bracelogger that implements the solution detailed below.
To enable brace-style formatting for log messages, we can monkey-patch a bit of the logger code.
The following patches the logging module to create a get_logger function that will return a logger that uses the new-style formatting for every log record that it handles.
import functools
import logging
import types
def _get_message(record):
"""Replacement for logging.LogRecord.getMessage
that uses the new-style string formatting for
its messages"""
msg = str(record.msg)
args = record.args
if args:
if not isinstance(args, tuple):
args = (args,)
msg = msg.format(*args)
return msg
def _handle_wrap(fcn):
"""Wrap the handle function to replace the passed in
record's getMessage function before calling handle"""
@functools.wraps(fcn)
def handle(record):
record.getMessage = types.MethodType(_get_message, record)
return fcn(record)
return handle
def get_logger(name=None):
"""Get a logger instance that uses new-style string formatting"""
log = logging.getLogger(name)
if not hasattr(log, "_newstyle"):
log.handle = _handle_wrap(log.handle)
log._newstyle = True
return log
Usage:
>>> log = get_logger()
>>> log.warning("{!r}", log)
<logging.RootLogger object at 0x4985a4d3987b>
Notes:
- Fully compatible with normal logging methods (just replace
logging.getLoggerwithget_logger) - Will only affect specific loggers created by the
get_loggerfunction (doesn't break 3rd party packages). - The formatting of the message is delayed until it is output (or not at all if the log message is filtered).
- Args are stored on
logging.LogRecordobjects as usual (useful in some cases with custom log handlers). - Works in all versions of Python from 2.7 to 3.10.
Solution 6 - Python
Try logging.setLogRecordFactory in Python 3.2+:
import collections
import logging
class _LogRecord(logging.LogRecord):
def getMessage(self):
msg = str(self.msg)
if self.args:
if isinstance(self.args, collections.Mapping):
msg = msg.format(**self.args)
else:
msg = msg.format(*self.args)
return msg
logging.setLogRecordFactory(_LogRecord)
Solution 7 - Python
I created a custom Formatter, called ColorFormatter that handles the problem like this:
class ColorFormatter(logging.Formatter):
def format(self, record):
# previous stuff, copy from logging.py…
try: # Allow {} style
message = record.getMessage() # printf
except TypeError:
message = record.msg.format(*record.args)
# later stuff…
This keeps it compatible with various libraries. The drawback is that it is probably not performant due to potentially attempting format of the string twice.
Solution 8 - Python
Similar solution to pR0Ps' , wrapping getMessage in LogRecord by wrapping makeRecord (instead of handle in their answer) in instances of Logger that should be new-formatting-enabled:
def getLogger(name):
log = logging.getLogger(name)
def Logger_makeRecordWrapper(name, level, fn, lno, msg, args, exc_info, func=None, extra=None, sinfo=None):
self = log
record = logging.Logger.makeRecord(self, name, level, fn, lno, msg, args, exc_info, func, sinfo)
def LogRecord_getMessageNewStyleFormatting():
self = record
msg = str(self.msg)
if self.args:
msg = msg.format(*self.args)
return msg
record.getMessage = LogRecord_getMessageNewStyleFormatting
return record
log.makeRecord = Logger_makeRecordWrapper
return log
I tested this with Python 3.5.3.
Solution 9 - Python
Combined string.Formatter to add pprint.pformat type conversion and from logging: setLogRecordFactory, setLoggerClass. There's one neat trick- i create extra nested tuple for argument args for Logger._log method and then unpack it in LogRecord init to omit overriding in Logger.makeRecord. Using log.f wraps every attribute (log methods on purpose) with use_format so you don't have to write it explicitly. This solution is backward compatible.
from collections import namedtuple
from collections.abc import Mapping
from functools import partial
from pprint import pformat
from string import Formatter
import logging
Logger = logging.getLoggerClass()
LogRecord = logging.getLogRecordFactory()
class CustomFormatter(Formatter):
def format_field(self, value, format_spec):
if format_spec.endswith('p'):
value = pformat(value)
format_spec = format_spec[:-1]
return super().format_field(value, format_spec)
custom_formatter = CustomFormatter()
class LogWithFormat:
def __init__(self, obj):
self.obj = obj
def __getattr__(self, name):
return partial(getattr(self.obj, name), use_format=True)
ArgsSmuggler = namedtuple('ArgsSmuggler', ('args', 'smuggled'))
class CustomLogger(Logger):
def __init__(self, *ar, **kw):
super().__init__(*ar, **kw)
self.f = LogWithFormat(self)
def _log(self, level, msg, args, *ar, use_format=False, **kw):
super()._log(level, msg, ArgsSmuggler(args, use_format), *ar, **kw)
class CustomLogRecord(LogRecord):
def __init__(self, *ar, **kw):
args = ar[5]
# RootLogger use CustomLogRecord but not CustomLogger
# then just unpack only ArgsSmuggler instance
args, use_format = args if isinstance(args, ArgsSmuggler) else (args, False)
super().__init__(*ar[:5], args, *ar[6:], **kw)
self.use_format = use_format
def getMessage(self):
return self.getMessageWithFormat() if self.use_format else super().getMessage()
def getMessageWithFormat(self):
msg = str(self.msg)
args = self.args
if args:
fmt = custom_formatter.format
msg = fmt(msg, **args) if isinstance(args, Mapping) else fmt(msg, *args)
return msg
logging.setLogRecordFactory(CustomLogRecord)
logging.setLoggerClass(CustomLogger)
log = logging.getLogger(__name__)
log.info('%s %s', dict(a=1, b=2), 5)
log.f.info('{:p} {:d}', dict(a=1, b=2), 5)
Solution 10 - Python
Here's something real simple that works:
debug_logger: logging.Logger = logging.getLogger("app.debug")
def mydebuglog(msg: str, *args, **kwargs):
if debug_logger.isEnabledFor(logging.DEBUG):
debug_logger.debug(msg.format(*args, **kwargs))
Then:
mydebuglog("hello {} {val}", "Python", val="World")