Making Python loggers output all messages to stdout in addition to log file

Question

Is there a way to make Python logging using the logging module automatically output things to stdout in addition to the log file where they are supposed to go  For example  I d like all calls to logger warning  logger critical  logger error to go to their intended places but in addition always be copied to stdout  This is to avoid duplicating messages like   mylogger critical  something failed   print  something failed

User · Answer

The simplest way to log to stdout   import logging import sys logging basicConfig stream sys stdout  level logging DEBUG

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Here s an extremely simple example   import logging l   logging getLogger  test      Add a file logger f   logging FileHandler  test log   l addHandler f     Add a stream logger s   logging StreamHandler   l addHandler s     Send a test message to both -- critical will always log l critical  test msg     The output will show  test msg  on stdout and also in the file

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I simplified my source code  whose original version is OOP and uses a configuration file   to give you an alternative solution to  EliasStrehle s one  without using the dictConfig  thus easiest to integrate with existing source code   import logging import sys   def create stream handler stream  formatter  level  message filter None       handler   logging StreamHandler stream stream      handler setLevel level      handler setFormatter formatter      if message filter          handler addFilter message filter      return handler   def configure logger logger  logging Logger  enable console  bool   True  enable file  bool   True       if not logger handlers          if enable console              message format  str     asctime 20   name 16   levelname 8   message               date format  str     Y  m  d  H  M  S              level  int   logging DEBUG             formatter   logging Formatter message format  date format                      Configures error output  from Warning levels               error output handler   create stream handler sys stderr  formatter                                                           max level  logging WARNING               logger addHandler error output handler                 Configures standard output  from configured Level  if lower than Warning                 and excluding everything from Warning and higher               if level  lt  logging WARNING                  standard output filter   lambda record  record levelno  lt  logging WARNING                 standard output handler   create stream handler sys stdout  formatter  level                                                                  standard output filter                  logger addHandler standard output handler           if enable file              message format  str     asctime 20   name 16   levelname 8   message               date format  str     Y  m  d  H  M  S              level  int   logging DEBUG             output file  str     tmp so test log               handler   logging FileHandler output file              formatter   logging Formatter message format  date format                   handler setLevel level              handler setFormatter formatter              logger addHandler handler   This is a very simple way to test it  logger  logging Logger   logging getLogger  MyLogger   logger setLevel logging DEBUG  configure logger logger  True  True  logger debug  Debug message       logger info  Info message       logger warning  Warning       logger error  Error       logger fatal  Fatal message

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Since no one has shared a neat two liner  I will share my own   logging basicConfig filename  logs log   level logging DEBUG  format    asctime s   levelname s    message s   logging getLogger   addHandler logging StreamHandler

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Here is a solution based on the powerful but poorly documented logging config dictConfig method  Instead of sending every log message to stdout  it sends messages with log level ERROR and higher to stderr and everything else to stdout  This can be useful if other parts of the system are listening to stderr or stdout  import logging import logging config import sys  class  ExcludeErrorsFilter logging Filter       def filter self  record            quot  quot  quot Only returns log messages with log level below ERROR  numeric value  40   quot  quot  quot          return record levelno  lt  40   config          version   1       filters              exclude errors                        ExcludeErrorsFilter                       formatters               Modify log message format here or replace with your custom formatter class          my formatter                  format       process d    asctime s   name s  line   lineno s      levelname s   message s                        handlers              console stderr                   Sends log messages with log level ERROR or higher to stderr              class    logging StreamHandler                level    ERROR                formatter    my formatter                stream   sys stderr                     console stdout                   Sends log messages with log level lower than ERROR to stdout              class    logging StreamHandler                level    DEBUG                formatter    my formatter                filters     exclude errors                 stream   sys stdout                     file                   Sends all log messages to a file              class    logging FileHandler                level    DEBUG                formatter    my formatter                filename    my log                encoding    utf8                        root               In general  this should be kept at  NOTSET             Otherwise it would interfere with the log levels set for each handler           level    NOTSET            handlers     console stderr    console stdout    file             logging config dictConfig config

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All logging output is handled by the handlers  just add a logging StreamHandler   to the root logger   Here s an example configuring a stream handler  using stdout instead of the default stderr  and adding it to the root logger   import logging import sys  root   logging getLogger   root setLevel logging DEBUG   handler   logging StreamHandler sys stdout  handler setLevel logging DEBUG  formatter   logging Formatter    asctime s -   name s -   levelname s -   message s   handler setFormatter formatter  root addHandler handler

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The simplest way to log to file and to stderr   import logging  logging basicConfig filename  logfile txt   stderrLogger logging StreamHandler   stderrLogger setFormatter logging Formatter logging BASIC FORMAT   logging getLogger   addHandler stderrLogger

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For more detailed explanations - great documentation at that link  For example  It s easy  you only need to set up two loggers  import sys import logging  logger   logging getLogger     logger setLevel logging DEBUG  fh   logging FileHandler  my log info log   sh   logging StreamHandler sys stdout  formatter   logging Formatter     asctime s    levelname s    filename s   funcName s   lineno d    message s   datefmt   a   d  b  Y  H  M  S   fh setFormatter formatter  sh setFormatter formatter  logger addHandler fh  logger addHandler sh   def hello logger        logger info  quot Hello info quot       logger critical  quot Hello critical quot       logger warning  quot Hello warning quot       logger debug  quot Hello debug quot    if   name       quot   main   quot       print hello logger     Output - terminal   Mon  10 Aug 2020 12 44 25  INFO  TestLoger py hello logger 15  Hello info  Mon  10 Aug 2020 12 44 25  CRITICAL  TestLoger py hello logger 16  Hello critical  Mon  10 Aug 2020 12 44 25  WARNING  TestLoger py hello logger 17  Hello warning  Mon  10 Aug 2020 12 44 25  DEBUG  TestLoger py hello logger 18  Hello debug None  Output - in file    UPDATE  color terminal Package  pip install colorlog  Code  import sys import logging import colorlog  logger   logging getLogger     logger setLevel logging DEBUG  fh   logging FileHandler  my log info log   sh   logging StreamHandler sys stdout  formatter   logging Formatter     asctime s    levelname s    filename s   funcName s   lineno d    message s   datefmt   a   d  b  Y  H  M  S   fh setFormatter formatter  sh setFormatter colorlog ColoredFormatter    log color s    asctime s    levelname s    filename s   funcName s   lineno d    message s   datefmt   a   d  b  Y  H  M  S    logger addHandler fh  logger addHandler sh   def hello logger        logger info  quot Hello info quot       logger critical  quot Hello critical quot       logger warning  quot Hello warning quot       logger debug  quot Hello debug quot       logger error  quot Error message quot    if   name       quot   main   quot       hello logger    output   Recommendation  Complete logger configuration from INI file  which also includes setup for stdout and debug log   handler file  level WARNING   handler screen  level DEBUG

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It s possible using multiple handlers   import logging import auxiliary module    create logger with  spam application  log   logging getLogger  spam application   log setLevel logging DEBUG     create formatter and add it to the handlers formatter   logging Formatter    asctime s -   name s -   levelname s -   message s      create file handler which logs even debug messages fh   logging FileHandler  spam log   fh setLevel logging DEBUG  fh setFormatter formatter  log addHandler fh     create console handler with a higher log level ch   logging StreamHandler   ch setLevel logging ERROR  ch setFormatter formatter  log addHandler ch   log info  creating an instance of auxiliary module Auxiliary   a   auxiliary module Auxiliary   log info  created an instance of auxiliary module Auxiliary    log info  calling auxiliary module Auxiliary do something   a do something   log info  finished auxiliary module Auxiliary do something    log info  calling auxiliary module some function     auxiliary module some function   log info  done with auxiliary module some function        remember to close the handlers for handler in log handlers      handler close       log removeFilter handler    Please see  https   docs python org 2 howto logging-cookbook html

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You could create two handlers for file and stdout and then create one logger with handlers argument to basicConfig  It could be useful if you have the same log level and format output for both handlers   import logging import sys  file handler   logging FileHandler filename  tmp log   stdout handler   logging StreamHandler sys stdout  handlers    file handler  stdout handler   logging basicConfig      level logging DEBUG       format     asctime s     filename s   lineno d    levelname s -   message s       handlers handlers    logger   logging getLogger  LOGGER NAME

[python] Making Python loggers output all messages to stdout in addition to log file

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