How do I log a Python error with debug information?

Question

I am printing Python exception messages to a log file with logging.error:

import logging
try:
    1/0
except ZeroDivisionError as e:
    logging.error(e)  # ERROR:root:division by zero

Is it possible to print more detailed information about the exception and the code that generated it than just the exception string? Things like line numbers or stack traces would be great.

Answer 1

The best and minimal library for doing this is loguru, which shows an in-baked traceback and provides debug info:

import sys
from loguru import logger
logger.add(sys.stdout, level="TRACE")

try:
    a = 1/0
except Exception as err:
    logger.exception(str(err), backtrace=True, diagnose=True)

Outputs:

2023-08-09 10:46:53.669 | ERROR    | __main__:<module>:8 - division by zero
Traceback (most recent call last):

> File "new.py", line 6, in <module>
    a = 1/0

ZeroDivisionError: division by zero

Answer 2

Since Python 3.5, it's possible to explicitly specify the exception in the logging function:

try:
    1/0
except Exception as ex:
    logging.error("Error occurred", exc_info = ex)

The logging.exception function still works but must be called only within the except block and does not allow specifying log level.

Answer 3

My approach was to create a context manager, to log and raise Exceptions:

import logging
from contextlib import AbstractContextManager


class LogError(AbstractContextManager):

    def __init__(self, logger=None):
        self.logger = logger.name if isinstance(logger, logging.Logger) else logger

    def __exit__(self, exc_type, exc_value, traceback):
        if exc_value is not None:
            logging.getLogger(self.logger).exception(exc_value)


with LogError():
    1/0

You can either pass a logger name or a logger instance to LogError(). By default it will use the base logger (by passing None to logging.getLogger). One could also simply add a switch for raising the error or just logging it.

Answer 4

I wrap all functions around my custom designed logger:

import json
import timeit
import traceback
import sys
import unidecode

def main_writer(f,argument):
  try:
    f.write(str(argument))
  except UnicodeEncodeError:
    f.write(unidecode.unidecode(argument))


def logger(*argv,logfile="log.txt",singleLine = False):
  """
  Writes Logs to LogFile
  """
  with open(logfile, 'a+') as f:
    for arg in argv:
      if arg == "{}":
        continue
      if type(arg) == dict and len(arg)!=0:
        json_object = json.dumps(arg, indent=4, default=str)
        f.write(str(json_object))
        f.flush()
        """
        for key,val in arg.items():
          f.write(str(key) + " : "+ str(val))
          f.flush()
        """
      elif type(arg) == list and len(arg)!=0:
        for each in arg:
          main_writer(f,each)
          f.write("\n")
          f.flush()
      else:
        main_writer(f,arg)
        f.flush()
      if singleLine==False:
        f.write("\n")
    if singleLine==True:
      f.write("\n")

def tryFunc(func, func_name=None, *args, **kwargs):
  """
  Time for Successfull Runs
  Exception Traceback for Unsuccessful Runs
  """
  stack = traceback.extract_stack()
  filename, codeline, funcName, text = stack[-2]
  func_name = func.__name__ if func_name is None else func_name # sys._getframe().f_code.co_name # func.__name__
  start = timeit.default_timer()
  x = None
  try:
    x = func(*args, **kwargs)
    stop = timeit.default_timer()
    # logger("Time to Run {} : {}".format(func_name, stop - start))
  except Exception as e:
    logger("Exception Occurred for {} :".format(func_name))
    logger("Basic Error Info :",e)
    logger("Full Error TraceBack :")
    # logger(e.message, e.args)
    logger(traceback.format_exc())
  return x

def bad_func():
  return 'a'+ 7

if __name__ == '__main__':
    logger(234)
    logger([1,2,3])
    logger(['a','b','c'])
    logger({'a':7,'b':8,'c':9})
    tryFunc(bad_func)

Answer 5

If "debugging information" means the values present when exception was raised, then logging.exception(...) won't help. So you'll need a tool that logs all variable values along with the traceback lines automatically.

Out of the box you'll get log like

2020-03-30 18:24:31 main ERROR   File "./temp.py", line 13, in get_ratio
2020-03-30 18:24:31 main ERROR     return height / width
2020-03-30 18:24:31 main ERROR       height = 300
2020-03-30 18:24:31 main ERROR       width = 0
2020-03-30 18:24:31 main ERROR builtins.ZeroDivisionError: division by zero

Have a look at some pypi tools, I'd name:

• tbvaccine
• traceback-with-variables
• better-exceptions

Some of them give you pretty crash messages:

But you might find some more on pypi

Answer 6

logger.exception will output a stack trace alongside the error message.

For example:

import logging
try:
    1/0
except ZeroDivisionError:
    logging.exception("message")

Output:

ERROR:root:message
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: integer division or modulo by zero

@Paulo Cheque notes, "be aware that in Python 3 you must call the logging.exception method just inside the except part. If you call this method in an arbitrary place you may get a bizarre exception. The docs alert about that."

Answer 7

If you look at the this code example (which works for Python 2 and 3) you'll see the function definition below which can extract

• method
• line number
• code context
• file path

for an entire stack trace, whether or not there has been an exception:

def sentry_friendly_trace(get_last_exception=True):
    try:
        current_call = list(map(frame_trans, traceback.extract_stack()))
        alert_frame = current_call[-4]
        before_call = current_call[:-4]

        err_type, err, tb = sys.exc_info() if get_last_exception else (None, None, None)
        after_call = [alert_frame] if err_type is None else extract_all_sentry_frames_from_exception(tb)

        return before_call + after_call, err, alert_frame
    except:
        return None, None, None

Of course, this function depends on the entire gist linked above, and in particular extract_all_sentry_frames_from_exception() and frame_trans() but the exception info extraction totals less than around 60 lines.

Hope that helps!

Answer 8

In your logging module(if custom module) just enable stack_info.

api_logger.exceptionLog("*Input your Custom error message*",stack_info=True)

Answer 9

Using exc_info options may be better, to allow you to choose the error level (if you use exception, it will always be at the error level):

try:
    # do something here
except Exception as e:
    logging.critical(e, exc_info=True)  # log exception info at CRITICAL log level

Answer 10

You can log the stack trace without an exception.

https://docs.python.org/3/library/logging.html#logging.Logger.debug

The second optional keyword argument is stack_info, which defaults to False. If true, stack information is added to the logging message, including the actual logging call. Note that this is not the same stack information as that displayed through specifying exc_info: The former is stack frames from the bottom of the stack up to the logging call in the current thread, whereas the latter is information about stack frames which have been unwound, following an exception, while searching for exception handlers.

Example:

>>> import logging
>>> logging.basicConfig(level=logging.DEBUG)
>>> logging.getLogger().info('This prints the stack', stack_info=True)
INFO:root:This prints the stack
Stack (most recent call last):
  File "<stdin>", line 1, in <module>
>>>

Answer 11

A little bit of decorator treatment (very loosely inspired by the Maybe monad and lifting). You can safely remove Python 3.6 type annotations and use an older message formatting style.

fallible.py

from functools import wraps
from typing import Callable, TypeVar, Optional
import logging


A = TypeVar('A')


def fallible(*exceptions, logger=None) \
        -> Callable[[Callable[..., A]], Callable[..., Optional[A]]]:
    """
    :param exceptions: a list of exceptions to catch
    :param logger: pass a custom logger; None means the default logger, 
                   False disables logging altogether.
    """
    def fwrap(f: Callable[..., A]) -> Callable[..., Optional[A]]:

        @wraps(f)
        def wrapped(*args, **kwargs):
            try:
                return f(*args, **kwargs)
            except exceptions:
                message = f'called {f} with *args={args} and **kwargs={kwargs}'
                if logger:
                    logger.exception(message)
                if logger is None:
                    logging.exception(message)
                return None

        return wrapped

    return fwrap

Demo:

In [1] from fallible import fallible

In [2]: @fallible(ArithmeticError)
    ...: def div(a, b):
    ...:     return a / b
    ...: 
    ...: 

In [3]: div(1, 2)
Out[3]: 0.5

In [4]: res = div(1, 0)
ERROR:root:called <function div at 0x10d3c6ae8> with *args=(1, 0) and **kwargs={}
Traceback (most recent call last):
  File "/Users/user/fallible.py", line 17, in wrapped
    return f(*args, **kwargs)
  File "<ipython-input-17-e056bd886b5c>", line 3, in div
    return a / b

In [5]: repr(res)
'None'

You can also modify this solution to return something a bit more meaningful than None from the except part (or even make the solution generic, by specifying this return value in fallible's arguments).

Answer 12

This answer builds up from the above excellent ones.

In most applications, you won't be calling logging.exception(e) directly. Most likely you have defined a custom logger specific for your application or module like this:

# Set the name of the app or module
my_logger = logging.getLogger('NEM Sequencer')
# Set the log level
my_logger.setLevel(logging.INFO)

# Let's say we want to be fancy and log to a graylog2 log server
graylog_handler = graypy.GELFHandler('some_server_ip', 12201)
graylog_handler.setLevel(logging.INFO)
my_logger.addHandler(graylog_handler)

In this case, just use the logger to call the exception(e) like this:

try:
    1/0
except ZeroDivisionError, e:
    my_logger.exception(e)

Answer 13

One nice thing about logging.exception that SiggyF's answer doesn't show is that you can pass in an arbitrary message, and logging will still show the full traceback with all the exception details:

import logging
try:
    1/0
except ZeroDivisionError:
    logging.exception("Deliberate divide by zero traceback")

With the default (in recent versions) logging behaviour of just printing errors to sys.stderr, it looks like this:

>>> import logging
>>> try:
...     1/0
... except ZeroDivisionError:
...     logging.exception("Deliberate divide by zero traceback")
... 
ERROR:root:Deliberate divide by zero traceback
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: integer division or modulo by zero

Answer 14

If you use plain logs - all your log records should correspond this rule: one record = one line. Following this rule you can use grep and other tools to process your log files.

But traceback information is multi-line. So my answer is an extended version of solution proposed by zangw above in this thread. The problem is that traceback lines could have \n inside, so we need to do an extra work to get rid of this line endings:

import logging


logger = logging.getLogger('your_logger_here')

def log_app_error(e: BaseException, level=logging.ERROR) -> None:
    e_traceback = traceback.format_exception(e.__class__, e, e.__traceback__)
    traceback_lines = []
    for line in [line.rstrip('\n') for line in e_traceback]:
        traceback_lines.extend(line.splitlines())
    logger.log(level, traceback_lines.__str__())

After that (when you'll be analyzing your logs) you could copy / paste required traceback lines from your log file and do this:

ex_traceback = ['line 1', 'line 2', ...]
for line in ex_traceback:
    print(line)

Profit!

Answer 15

Quoting

What if your application does logging some other way – not using the logging module?

Now, traceback could be used here.

import traceback

def log_traceback(ex, ex_traceback=None):
    if ex_traceback is None:
        ex_traceback = ex.__traceback__
    tb_lines = [ line.rstrip('\n') for line in
                 traceback.format_exception(ex.__class__, ex, ex_traceback)]
    exception_logger.log(tb_lines)

• Use it in Python 2:

  try:
      # your function call is here
  except Exception as ex:
      _, _, ex_traceback = sys.exc_info()
      log_traceback(ex, ex_traceback)
  

• Use it in Python 3:

  try:
      x = get_number()
  except Exception as ex:
      log_traceback(ex)
  

Answer 16

A clean way to do it is using format_exc() and then parse the output to get the relevant part:

from traceback import format_exc

try:
    1/0
except Exception:
    print 'the relevant part is: '+format_exc().split('\n')[-2]

Regards

Answer 17

If you can cope with the extra dependency then use twisted.log, you don't have to explicitly log errors and also it returns the entire traceback and time to the file or stream.