How to save traceback sys exc info values in a variable

Question

I want to save the name of the error and the traceback details into a variable   Here s is my attempt   import sys try      try          print x     except Exception  ex          raise NameError except Exception  er      print  0   sys exc info   0      print  1   sys exc info   1      print  2   sys exc info   2    Output   0  lt type  exceptions NameError  gt  1  2  lt traceback object at 0xbd5fc8 gt    Desired Output   0 NameError 1 2 Traceback  most recent call last     File  exception py   line 6  in  lt module gt      raise NameError   P S  I know this can be done easily using the traceback module  but I want to know usage of sys exc info   2  object here

User · Accepted Answer

This is how I do it    gt  gt  gt  import traceback  gt  gt  gt  try        int  k       except        var   traceback format exc         gt  gt  gt  print var Traceback  most recent call last     File   lt stdin gt    line 2  in  lt module gt  ValueError  invalid literal for int   with base 10   k    You should however take a look at the traceback documentation  as you might find there more suitable methods  depending to how you want to process your variable afterwards

User · Answer

sys exc info   returns a tuple with three values  type  value  traceback      Here type gets the exception type of the Exception being handled value is the arguments that are being passed to constructor of exception class  traceback contains the stack information like where the exception occurred etc    For Example  In the following program  try       a   1 0  except Exception e       exc tuple   sys exc info     Now If we print the tuple the values will be this    exc tuple 0  value will be  ZeroDivisionError  exc tuple 1  value will be  integer division or modulo by zero   String passed as parameter to the exception class  exc tuple 2  value will be  trackback object at  some memory address     The above details can also be fetched by simply printing the exception in string format    print str e

User · Answer

Be careful when you take the exception object or the traceback object out of the exception handler  since this causes circular references and gc collect   will fail to collect  This appears to be of a particular problem in the ipython jupyter notebook environment where the traceback object doesn t get cleared at the right time and even an explicit call to gc collect   in finally section does nothing  And that s a huge problem if you have some huge objects that don t get their memory reclaimed because of that  e g  CUDA out of memory exceptions that w o this solution require a complete kernel restart to recover    In general if you want to save the traceback object  you need to clear it from references to locals    like so   import sys  traceback  gc type  val  tb   None  None  None try      myfunc   except      type  val  tb   sys exc info       traceback clear frames tb    some cleanup code gc collect     and then use the tb  if tb      raise type val  with traceback tb    In the case of jupyter notebook  you have to do that at the very least inside the exception handler   try      myfunc   except      type  val  tb   sys exc info       traceback clear frames tb      raise type val  with traceback tb  finally        cleanup code in here     gc collect     Tested with python 3 7   p s  the problem with ipython or jupyter notebook env is that it has  tb magic which saves the traceback and makes it available at any point later  And as a result any locals   in all frames participating in the traceback will not be freed until the notebook exits or another exception will overwrite the previously stored backtrace  This is very problematic  It should not store the traceback w o cleaning its frames  Fix submitted here

User · Answer

Use traceback extract stack   if you want convenient access to module and function names and line numbers   Use    join traceback format stack    if you just want a string that looks like the traceback print stack   output   Notice that even with    join   you will get a multi-line string  since the elements of format stack   contain  n   See output below   Remember to import traceback   Here s the output from traceback extract stack     Formatting added for readability    gt  gt  gt  traceback extract stack           lt string gt    1    lt module gt    None        C   Python  lib  idlelib  run py   126   main    ret   method  args    kwargs          C   Python  lib  idlelib  run py   353   runcode    exec code  self locals           lt pyshell 1 gt    1    lt module gt    None      Here s the output from    join traceback format stack      Formatting added for readability    gt  gt  gt     join traceback format stack       File   lt string gt    line 1  in  lt module gt  n    File  C   Python  lib  idlelib  run py   line 126  in main n        ret   method  args    kwargs  n    File  C   Python  lib  idlelib  run py   line 353  in runcode n        exec code  self locals  n  File   lt pyshell 2 gt    line 1  in  lt module gt  n

User · Answer

The object can be used as a parameter in Exception with traceback   function   except Exception as e      tb   sys exc info       print e with traceback tb 2

[python] How to save traceback / sys.exc_info() values in a variable?

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