How to catch exception output from Python subprocess check output

Question

I m trying to do a Bitcoin payment from within Python  In bash I would normally do this   bitcoin sendtoaddress  lt bitcoin address gt   lt amount gt    so for example   bitcoin sendtoaddress 1HoCUcbK9RbVnuaGQwiyaJGGAG6xrTPC9y 1 4214   if it is successfull I get a transaction id as output but if I try to transfer an amount larger than my bitcoin balance  I get the following output   error    code  -4  message   Insufficient funds     In my Python program I now try to do the payment as follows   import subprocess  try      output   subprocess check output   bitcoin    sendtoaddress   address  str amount    except      print  Unexpected error    sys exc info     If there s enough balance it works fine  but if there s not enough balance sys exc info   prints out this     lt class  subprocess CalledProcessError  gt   CalledProcessError     lt traceback object at 0x7f339599ac68 gt     It doesn t include the error which I get on the command line though  So my question is  how can I get the outputted  error    code  -4  message   Insufficient funds    from within Python   All tips are welcome

User · Answer

As mentioned by  Sebastian the default solution should aim to use run    https   docs python org 3 library subprocess html subprocess run  Here a convenient implementation  feel free to change the log class with print statements or what ever other logging functionality you are using    import subprocess  def  run command command       log debug  Command      format command       result   subprocess run command  shell True  capture output True      if result stderr          raise subprocess CalledProcessError                  returncode   result returncode                  cmd   result args                  stderr   result stderr                       if result stdout          log debug  Command Result      format result stdout decode  utf-8         return result   And sample usage  code is unrelated  but I think it serves as example of how readable and easy to work with errors it is with this simple implementation    try        Unlock PIN Card      run command           sudo qmicli --device  dev cdc-wdm0 -p --uim-verify-pin PIN1              format pin         except subprocess CalledProcessError as error      if  couldn t verify PIN  in error stderr decode  utf-8            log error                   SIM card could not be unlocked                     Either the PIN is wrong or the card is not properly connected                     Resetting module                                reset 4g hat           return

User · Answer

According to the subprocess check output   docs  the exception raised on error has an output attribute that you can use to access the error details   try      subprocess check output      except subprocess CalledProcessError as e      print e output    You should then be able to analyse this string and parse the error details with the json module   if e output startswith  error           error   json loads e output 7      Skip  error        print error  code        print error  message

User · Answer

Trying to  transfer an amount larger than my bitcoin balance  is not an unexpected error  You could use Popen communicate   directly instead of check output   to avoid raising an exception unnecessarily   from subprocess import Popen  PIPE  p   Popen   bitcoin    sendtoaddress        stdout PIPE  output   p communicate   0  if p returncode    0      print  bitcoin failed  d  s     p returncode  output

User · Answer

I don t think the accepted solution handles the case where the error text is reported on stderr  From my testing the exception s output attribute did not contain the results from stderr and the docs warn against using stderr PIPE in check output     Instead  I would suggest one small improvement to J F Sebastian s solution by adding stderr support   We are  after all  trying to handle errors and stderr is where they are often reported     from subprocess import Popen  PIPE  p   Popen   bitcoin    sendtoaddress        stdout PIPE  stderr PIPE  output  error   p communicate   if p returncode    0      print  bitcoin failed  d  s  s     p returncode  output  error

User · Answer

This did the trick for me  It captures all the stdout output from the subprocess For python 3 8   from subprocess import check output  STDOUT cmd    quot Your Command goes here quot  try      cmd stdout   check output cmd  stderr STDOUT  shell True  decode   except Exception as e      print e output decode      print out the stdout messages up to the exception     print e    To print out the exception message

User · Answer

There are good answers here  but in these answers  there has not been an answer that comes up with the text from the stack-trace output  which is the default behavior of an exception   If you wish to use that formatted traceback information  you might wish to   import traceback  try      check call  args   except CalledProcessError      tb   traceback format exc       tb   tb replace passwd                print tb      exit 1    As you might be able to tell  the above is useful in case you have a password in the check call  args   that you wish to prevent from displaying

User · Answer

Based on the answer of  macetw I print the exception directly to stderr in a decorator   Python 3  from functools import wraps from sys import stderr from traceback import format exc from typing import Callable  Collection  Any  Mapping   def force error output func  Callable        wraps func      def forced error output  args  Collection Any     kwargs  Mapping str  Any            nonlocal func          try              func  args    kwargs          except Exception as exception              stderr write format exc                stderr write   n               stderr flush                raise exception      return forced error output   Python 2  from functools import wraps from sys import stderr from traceback import format exc   def force error output func        wraps func      def forced error output  args    kwargs           try              func  args    kwargs          except Exception as exception              stderr write format exc                stderr write   n               stderr flush                raise exception      return forced error output   Then in your worker just use the decorator   force error output def da worker arg1  int  arg2  str       pass

[python] How to catch exception output from Python subprocess.check_output()?

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