How to catch and print the full exception traceback without halting exiting the program

Question

I want to catch and log exceptions without exiting  e g   try      do stuff   except Exception  err      print Exception  err        I want to print the entire traceback here        not just the exception name and details  I want to print the exact same output that is printed when the exception is raised without the try  except intercepting the exception  and I do not want it to exit my program   How do I do this

User · Answer

How to print the full traceback without halting the program?

When you don't want to halt your program on an error, you need to handle that error with a try/except:

try:
    do_something_that_might_error()
except Exception as error:
    handle_the_error(error)

To extract the full traceback, we'll use the traceback module from the standard library:

import traceback

And to create a decently complicated stacktrace to demonstrate that we get the full stacktrace:

def raise_error():
    raise RuntimeError('something bad happened!')

def do_something_that_might_error():
    raise_error()

Printing

To print the full traceback, use the traceback.print_exc method:

try:
    do_something_that_might_error()
except Exception as error:
    traceback.print_exc()

Which prints:

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 2, in do_something_that_might_error
  File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!

Better than printing, logging:

However, a best practice is to have a logger set up for your module. It will know the name of the module and be able to change levels (among other attributes, such as handlers)

import logging
logging.basicConfig(level=logging.DEBUG)
logger = logging.getLogger(__name__)

In which case, you'll want the logger.exception function instead:

try:
    do_something_that_might_error()
except Exception as error:
    logger.exception(error)

Which logs:

ERROR:__main__:something bad happened!
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 2, in do_something_that_might_error
  File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!

Or perhaps you just want the string, in which case, you'll want the traceback.format_exc function instead:

try:
    do_something_that_might_error()
except Exception as error:
    logger.debug(traceback.format_exc())

Which logs:

DEBUG:__main__:Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 2, in do_something_that_might_error
  File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!

Conclusion

And for all three options, we see we get the same output as when we have an error:

>>> do_something_that_might_error()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in do_something_that_might_error
  File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!

Which to use

Performance concerns aren't important here as IO usually dominates. I'd prefer, since it does precisely what's being requested in a forward compatible way:

logger.exception(error)

Logging levels and outputs can be adjusted, making it easy to turn off without touching the code. And usually doing what's directly needed is the most efficient way to do it.

User · Answer

python 3 solution stacktrace helper py  from linecache import getline import sys import traceback   def get stack trace        exc type  exc value  exc tb   sys exc info       trace   traceback format stack       trace   list filter lambda x    quot   lib   quot  not in x and  quot  lib  quot  not in x and  quot stacktrace helper py quot  not in x   trace       ex type   exc type   name       ex line   exc tb tb lineno     ex file   exc tb tb frame f code co filename     ex message   str exc value      line code    quot  quot      try          line code   getline ex file  ex line  strip       except          pass      trace insert          0  f File  quot  ex file  quot   line  ex line   line code   line code    ex   ex type   ex message              return trace   def get stack trace str msg  str    quot  quot        trace   list get stack trace        trace str    quot  n quot  join list map str  trace        trace str   msg    quot  n quot    trace str     return trace str

User · Answer

You want the traceback module  It will let you print stack dumps like Python normally does  In particular  the print last function will print the last exception and a stack trace

User · Answer

I don t see this mentioned in any of the other answers  If you re passing around an Exception object for whatever reason     In Python 3 5  you can get a trace from an Exception object using traceback TracebackException from exception    For example   import traceback   def stack lvl 3        raise Exception  a1    b2    c3     def stack lvl 2        try          stack lvl 3       except Exception as e            raise         return e   def stack lvl 1        e   stack lvl 2       return e  e   stack lvl 1    tb1   traceback TracebackException from exception e  print    join tb1 format       However  the above code results in   Traceback  most recent call last     File  exc py   line 10  in stack lvl 2     stack lvl 3     File  exc py   line 5  in stack lvl 3     raise Exception  a1    b2    c3   Exception    a1    b2    c3     This is just two levels of the stack  as opposed to what would have been printed on screen had the exception been raised in stack lvl 2   and not intercepted  uncomment the   raise line    As I understand it  that s because an exception records only the current level of the stack when it is raised  stack lvl 3   in this case  As it s passed back up through the stack  more levels are being added to its   traceback    But we intercepted it in stack lvl 2    meaning all it got to record was levels 3 and 2  To get the full trace as printed on stdout we d have to catch it at the highest  lowest   level   import traceback   def stack lvl 3        raise Exception  a1    b2    c3     def stack lvl 2        stack lvl 3     def stack lvl 1        stack lvl 2     try      stack lvl 1   except Exception as exc      tb   traceback TracebackException from exception exc   print  Handled at stack lvl 0   print    join tb stack format       Which results in   Handled at stack lvl 0   File  exc py   line 17  in  lt module gt      stack lvl 1     File  exc py   line 13  in stack lvl 1     stack lvl 2     File  exc py   line 9  in stack lvl 2     stack lvl 3     File  exc py   line 5  in stack lvl 3     raise Exception  a1    b2    c3     Notice that the stack print is different  the first and last lines are missing  Because it s a different format     Intercepting the exception as far away from the point where it was raised as possible makes for simpler code while also giving more information

User · Answer

In addition to Aaron Hall s answer  if you are logging  but don t want to use logging exception    since it logs at the ERROR level   you can use a lower level and pass exc info True  e g  try      do something that might error   except Exception      logging info  General exception noted    exc info True

User · Answer

If you re debugging and just want to see the current stack trace  you can simply call   traceback print stack    There s no need to manually raise an exception just to catch it again

User · Answer

A remark about this answer s comments  print traceback format exc    does a better job for me than traceback print exc    With the latter  the hello is sometimes strangely  mixed  with the traceback text  like if both want to write to stdout or stderr at the same time  producing weird output  at least when building from inside a text editor and viewing the output in the  Build results  panel       Traceback  most recent call last      File  C  Users User Desktop test py   line 7  in    hell    do stuff       File  C  Users User Desktop test py   line 4  in do stuff       1 0   ZeroDivisionError  integer division or modulo by zero   o    Finished in 0 1s      So I use   import traceback  sys  def do stuff        1 0  try      do stuff   except Exception      print traceback format exc        print  hello

User · Answer

To get the precise stack trace  as a string  that would have been raised if no try except were there to step over it  simply place this in the except block that catches the offending exception   desired trace   traceback format exc sys exc info      Here s how to use it  assuming flaky func is defined  and log calls your favorite logging system     import traceback import sys  try      flaky func   except KeyboardInterrupt      raise except Exception      desired trace   traceback format exc sys exc info        log desired trace    It s a good idea to catch and re-raise KeyboardInterrupts  so that you can still kill the program using Ctrl-C   Logging is outside the scope of the question  but a good option is logging   Documentation for the sys and traceback modules

User · Answer

traceback format exception If you only have the exception object  you can get the traceback as a string from any point of the code in Python 3 with  import traceback     join traceback format exception None  exc obj  exc obj   traceback      Full example     usr bin env python3  import traceback  def f        g    def g        raise Exception  asdf    try      g   except Exception as e      exc   e  tb str      join traceback format exception None  exc obj  exc obj   traceback     print tb str   Output  Traceback  most recent call last     File  quot   main py quot   line 12  in  lt module gt      g     File  quot   main py quot   line 9  in g     raise Exception  asdf   Exception  asdf  Documentation  https   docs python org 3 7 library traceback html traceback format exception See also  Extract traceback info from an exception object Tested in Python 3 7 3

User · Answer

You will need to put the try except inside the most innerloop where the error may occur  i e   for i in something      for j in somethingelse          for k in whatever              try                  something complex i  j  k              except Exception  e                  print e         try              something less complex i  j          except Exception  e              print e       and so on  In other words  you will need to wrap statements that may fail in try except as specific as possible  in the most inner-loop as possible

User · Answer

First  don t use prints for logging  there is astable  proven and well-thought out stdlib module to do that  logging  You definitely should use it instead   Second  don t be tempted to do a mess with unrelated tools when there is native and simple approach  Here it is   log   logging getLogger   name     try      call code that fails   except MyError      log exception  Any extra info you want to see in your logs     That s it  You are done now   Explanation for anyone who is interested in how things work under the hood  What log exception is actually doing is just a call to log error  that is  log event with level ERROR  and print traceback then   Why is it better   Well  here is some considerations    it is just right  it is straightforward  it is simple    Why should nobody use traceback or call logger with exc info True or get their hands dirty with sys exc info   Well  just because  They all exist for different purposes  For example  traceback print exc s output is a little bit different from tracebacks produced by the interpreter itself  If you use it  you will confuse anyone who reads your logs  they will be banging their heads against them   Passing exc info True to log calls is just inappropriate  But  it is useful when catching recoverable errors and you want to log them  using  e g INFO level  with tracebacks as well  because log exception produces logs of only one level - ERROR   And you definitely should avoid messing with sys exc info as much as you can  It s just not a public interface  it s an internal one - you can use it if you definitely know what you are doing  It is not intended for just printing exceptions

User · Answer

You could do  try      do stuff   except Exception  err      print Exception  err      raise err

User · Answer

traceback format exc   or sys exc info   will yield more info if that s what you want   import traceback import sys  try      do stuff   except Exception      print traceback format exc          or     print sys exc info   2

User · Answer

Some other answer have already pointed out the traceback module   Please notice that with print exc  in some corner cases  you will not obtain what you would expect  In Python 2 x   import traceback  try      raise TypeError  Oups    except Exception  err      try          raise TypeError  Again           except          pass      traceback print exc        will display the traceback of the last exception   Traceback  most recent call last     File  e py   line 7  in  lt module gt      raise TypeError  Again       TypeError  Again       If you really need to access the original traceback one solution is to cache the exception infos as returned from exc info in a local variable and display it using print exception   import traceback import sys  try      raise TypeError  Oups    except Exception  err      try          exc info   sys exc info              do you usefull stuff here            potentially raising an exception          try              raise TypeError  Again               except              pass           end of useful stuff       finally            Display the  original  exception         traceback print exception  exc info          del exc info   Producing   Traceback  most recent call last     File  t py   line 6  in  lt module gt      raise TypeError  Oups    TypeError  Oups    Few pitfalls with this though    From the doc of sys info      Assigning the traceback return value to a local variable in a function that is handling an exception will cause a circular reference  This will prevent anything referenced by a local variable in the same function or by the traceback from being garbage collected        If you do need the traceback  make sure to delete it after use  best done with a try     finally statement   but  from the same doc      Beginning with Python 2 2  such cycles are automatically reclaimed when garbage collection is enabled and they become unreachable  but it remains more efficient to avoid creating cycles       On the other hand  by allowing you to access the traceback associated with an exception  Python 3 produce a less surprising result   import traceback  try      raise TypeError  Oups    except Exception as err      try          raise TypeError  Again           except          pass      traceback print tb err   traceback          will display     File  e3 py   line 4  in  lt module gt      raise TypeError  Oups

User · Answer

This is my solution to write the error in a log file and also on console  import logging  sys import traceback logging basicConfig filename  error log   level logging DEBUG   def handle exception exc type  exc value  exc traceback       import sys     if issubclass exc type  KeyboardInterrupt           sys   excepthook   exc type  exc value  exc traceback          return     exc info  exc type  exc value  exc traceback      logging critical  quot  nDate  quot    str datetime datetime now     exc info  exc type  exc value  exc traceback       print  quot An error occured  check error log to see the error details quot       traceback print exception  exc info    sys excepthook   handle exception

User · Answer

If you have an Error object already  and you want to print the whole thing  you need to make this slightly awkward call   import traceback traceback print exception type err   err  err   traceback      That s right  print exception takes three positional arguments  The type of the exception  the actual exception object  and the exception s own internal traceback property   In python 3 5 or later  the type err  is optional    but it s a positional argument  so you still have to explicitly pass None in its place   traceback print exception None  err  err   traceback      I have no idea why all of this isn t just traceback print exception err   Why you would ever want to print out an error  along with a traceback other than the one that belongs to that error  is beyond me

[python] How to catch and print the full exception traceback without halting/exiting the program?

How to print the full traceback without halting the program?

Printing

Better than printing, logging:

Conclusion

Which to use

Examples related to python

Examples related to exception

Examples related to try-catch

Examples related to traceback