logging-tree 1.10

Propagation

A quick reminder about how logging works: by default, a node will not only submit a message to its own handlers (if any), but will also “propagate” each message up to its parent. For example, a Stream handler attached to the root logger will not only receive messages sent directly to the root, but also messages that propagate up from a child like a.b.

>>> logging.getLogger().warning('message sent to the root')
message sent to the root
>>> logging.getLogger('a.b').warning('message sent to a.b')
message sent to a.b

But messages are not subjected to filtering as they propagate. So a debug-level message, which our root node will discard because the root’s level is set to WARNING, will be accepted by the a.b node and will be allowed to propagate up to the root handler.

>>> logging.getLogger().debug('this message is ignored')
>>> logging.getLogger('a.b').debug('but this message prints!')
but this message prints!

If both the root node and a.b have a handler attached, then a message accepted by a.b will be printed twice, once by its own node, and then a second time when the message propagates up to the root.

>>> logging.getLogger('a.b').addHandler(h)
>>> logging.getLogger('a.b').warning('this message prints twice')
this message prints twice
this message prints twice

But you can stop a node from propagating messages to its parent by setting its propagate attribute to False.

>>> logging.getLogger('a.b').propagate = False
>>> logging.getLogger('a.b').warning('does not propagate')
does not propagate

The logging tree will indicate that propagate is turned off by no longer drawing the arrow <-- that points from the node to its parent:

>>> printout()
<--""
   Level WARNING
   Handler Stream <sys.stdout>
   |
   o<--"a"
   |   Level NOTSET so inherits level WARNING
   |   |
   |   o   "a.b"
   |       Level DEBUG
   |       Propagate OFF
   |       Handler Stream <sys.stdout>
   |
   o<--[x]
       |
       o<--"x.c"
           Level NOTSET so inherits level WARNING

You can turn propagate back on again by setting the attribute True.

API

Even though most users will simply call the top-level printout() routine, this package also offers a few lower-level calls. Here’s the complete list:

logging_tree.printout(node=None)

Prints the current logging tree, or the tree based at the given node, to the standard output.

logging_tree.format.build_description(node=None)

Builds and returns the multi-line description of the current logger tree, or the tree based at the given node, as a single string with newlines inside and a newline at the end.

logging_tree.format.describe(node)

A generator that yields a series of lines that describe the tree based at the given node. Note that the lines are returned without newline terminators attached.

logging_tree.tree()

Fetch the current tree of loggers from the logging module. Returns a node, that is simply a tuple with three fields:

[0] the logger name ("" for the root logger).

[1] the logging.Logger object itself.

[2] a list of zero or more child nodes.

You can find this package’s issue tracker on GitHub. You can run this package’s test suite with:

$ python -m unittest discover logging_tree

On older versions of Python you will instead have to install unittest2 and use its unit2 command line tool to run the tests.

Changelog

Version 1.10 - 2024 May 3

Declare compatibility with Python 3.12, and expand the documentation to describe the basics of log message propagation.

Version 1.9 - 2021 April 10

Declare compatibility with Python 3.9. Improve how the logging module’s built-in Formatter class is displayed under old Python versions where the logging module uses old-style classes.

Version 1.8.1 - 2020 January 26

Adjust one test to make it pass under Python 3.8, and update the distribution classifiers to declare compatibility with Python versions through 3.8.

Version 1.8 - 2018 August 5

Improve the output to better explain what happens if a “parent” attribute has been set to None.

Version 1.7 - 2016 January 23

Detect whether each logger has the correct “parent” attribute and, if not, print where its log messages are being sent instead.

Version 1.6 - 2015 January 8

Fixed a crash that would occur if a custom logging Formatter was missing its format string attributes.

Version 1.5 - 2014 December 24

Handlers now display their logging level if one has been set, and their custom logging formatter if one has been installed.

Version 1.4 - 2014 January 8

Thanks to a contribution from Dave Brondsema, disabled loggers are now actually marked as “Disabled” to make it less of a surprise that they fail to log anything.

Version 1.3 - 2013 October 29

Be explicit and display the logger level NOTSET along with the effective level inherited from the logger’s ancestors; and display the list of .filters of a custom logging handler even though it might contain custom code that ignores them.

Version 1.2 - 2013 January 19

Compatible with Python 3.3 thanks to @ralphbean.

Version 1.1 - 2012 February 17

Now compatible with 2.3 <= Python <= 3.2.

Version 1.0 - 2012 February 13

Can display the handler inside a MemoryHandler; entire public interface documented; 100% test coverage.

Version 0.6 - 2012 February 10

Added a display format for every logging.handlers class.

Version 0.5 - 2012 February 8

Initial release.