contents/using-lttng/instrumenting/python-application.md

   1 ---
   2 id: python-application
   3 since: 2.7
   4 ---
   5
   6 Python 2 and Python 3 applications using the standard
   7 <a href="https://docs.python.org/3/howto/logging.html" class="ext"><code>logging</code> module</a>
   8 can be traced by LTTng using the LTTng-UST Python agent.
   9
  10 Import the `lttngust` package in your Python application. For example:
  11
  12 ~~~ python
  13 import lttngust
  14 import logging
  15 import time
  16
  17
  18 def example():
  19     logging.basicConfig()
  20     logger = logging.getLogger('my-logger')
  21
  22     while True:
  23         logger.debug('debug message')
  24         logger.info('info message')
  25         logger.warn('warn message')
  26         logger.error('error message')
  27         logger.critical('critical message')
  28         time.sleep(1)
  29
  30
  31 if __name__ == '__main__':
  32     example()
  33 ~~~
  34
  35 Importing `lttngust` adds a logging handler which emits LTTng-UST
  36 events. You do not need to get a special logger for tracing to work.
  37
  38 Note that `logging.basicConfig()`, which adds to the root logger a basic
  39 logging handler which prints to the standard error stream, is not
  40 strictly required for LTTng-UST tracing to work, but in versions of
  41 Python preceding 3.2, a warning message could be seen indicating that no
  42 handler exists for the logger `my-logger`.
  43
  44 Use the `--python` option of the `lttng enable-event`,
  45 `lttng disable-event`, and `lttng list` commands to target
  46 Python applications. For example, here's how to enable the events
  47 produced by the Python logger above:
  48
  49 <pre class="term">
  50 lttng enable-event --python my-logger
  51 </pre>
  52
  53 Standard Python log levels are supported using the `PYTHON_` prefix.
  54 For example, here's how to enable the warning (and more important)
  55 events produced by the Python logger above:
  56
  57 <pre class="term">
  58 lttng enable-event --python my-logger --loglevel PYTHON_WARNING
  59 </pre>
  60
  61 See [Enabling and disabling events](#doc-enabling-disabling-events) for
  62 more options.
  63
  64 When loading, the LTTng-UST Python agent tries to register to the
  65 [session daemon](#doc-lttng-sessiond). Note that the session daemon
  66 needs to be started _before_ the Python application is started. If a
  67 session daemon is found, the agent tries to register to it during
  68 5&nbsp;seconds, after which the application continues without LTTng
  69 tracing support. This timeout value is overriden by the
  70 `LTTNG_UST_PYTHON_REGISTER_TIMEOUT` environment variable (milliseconds).
  71
  72 If the session daemon stops while a registered Python application is
  73 registered, the application retries to connect and register to a session
  74 daemon every 3&nbsp;seconds. This timeout value is overridden by the
  75 `LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY` environment variable.