Logging Messages

This part describes how user code can log messages with twiggy.

To get started quickly, use quick_setup().:

>>> import twiggy
>>> twiggy.quick_setup()

See also

Full details on Configuring Output.

The Magic log

The main interface is the the magic log.

>>> from twiggy import log
>>> log
<twiggy.logger.Logger object at 0x...>

It works out of the box, using typical levels. Arbitrary levels are not supported. Note that when logging, you never need to refer to any level object; just use the methods on the log.

>>> log.debug('You may not care')
DEBUG|You may not care
>>> log.error('OMFG! Pants on fire!')
ERROR|OMFG! Pants on fire!

The log can handle messages in several styles of format strings, defaulting to new-style.

>>> log.info('I wear {0} on my {where}', 'pants', where='legs')
INFO|I wear pants on my legs

You can name your loggers.

>>> mylog = log.name('alfredo')
>>> mylog.debug('hello')

Better output

Twiggy’s default output strives to be user-friendly and to avoid pet peeves.

Newlines are suppressed by default; that can be turned off per-message.

>>> log.info('user\ninput\nannoys\nus')
>>> log.options(suppress_newlines=False).info('we\ndeal')

Exceptions are prefixed by TRACE. By default, tracing will use the current exception, but you can also pass an exc_info tuple.

>>> try:
...     1/0
... except:
...     log.trace('error').warning('oh noes')
WARNING|oh noes
TRACE Traceback (most recent call last):
TRACE   File "<doctest better-output[...]>", line 2, in <module>
TRACE     1/0
TRACE ZeroDivisionError: division by zero

Structured Logging

I like this method chaining style a lot.

>>> log.name('benito').info('hi there')
INFO:benito|hi there

It makes structured logging easy. In the past, fielded data was stuffed in the text of your message:

>>> log.info('Going for a walk. path: {0} roads: {1}', "less traveled", 42)
INFO|Going for a walk. path: less traveled roads: 42

Instead, you can use fields() to add arbitrary key-value pairs. Output is easily parseable.

>>> log.fields(path="less traveled", roads=42).info('Going for a walk')
INFO:path=less traveled:roads=42|Going for a walk

The struct() is a short cut for only logging fields. This is great for runtime statistics gathering.

>>> log.struct(paths=42, dolphins='thankful')

Partial Binding

Each call to fields() or options() creates a new, independent log instance that inherits all of the data of the parent. This incremental binding can be useful for webapps.

>>> ## an application-level log
... webapp_log = log.name("myblog")
>>> ## a log for the individual request
... current_request_log = webapp_log.fields(request_id='12345')
>>> current_request_log.fields(rows=100, user='frank').info('frobnicating database')
INFO:myblog:request_id=12345:rows=100:user=frank|frobnicating database
>>> current_request_log.fields(bytes=5678).info('sending page over tubes')
INFO:myblog:bytes=5678:request_id=12345|sending page over tubes
>>> ## a log for a different request
... another_log = webapp_log.fields(request_id='67890')
>>> another_log.debug('Client connected')
DEBUG:myblog:request_id=67890|Client connected

Chained style is awesome. It allows you to create complex yet parsable log messages in a concise way.

>>> log.name('donjuan').fields(pants='sexy').info("hello, {who} want to {what}?", who='ladies', what='dance')
INFO:donjuan:pants=sexy|hello, ladies want to dance?

Sample Output

Routed to a file, the above produces the following:

2010-03-28T14:23:34Z:DEBUG:You may not care
2010-03-28T14:23:34Z:ERROR:OMFG! Pants on fire!
2010-03-28T14:23:34Z:INFO:I like bikes
2010-03-28T14:23:34Z:INFO:I wear pants on my legs
2010-03-28T14:23:34Z:WARNING:oh noes
TRACE Traceback (most recent call last):
TRACE   File "<doctest better-output[...]>", line 35, in <module>
TRACE ZeroDivisionError: integer division or modulo by zero
2010-03-28T14:23:34Z:INFO:benito:hi there
2010-03-28T14:23:34Z:INFO:Going for a walk. path: less traveled roads: 42
2010-03-28T14:23:34Z:INFO:path=less traveled:roads=42:Going for a walk
2010-03-28T14:23:34Z:INFO:myblog:request_id=12345:rows=100:user=frank:frobnicating database
2010-03-28T14:23:34Z:INFO:myblog:bytes=5678:request_id=12345:sending page over tubes
2010-03-28T14:23:34Z:INFO:myblog:request_id=67890:Client connected
2010-03-28T14:23:34Z:INFO:donjuan:pants=sexy:hello, ladies want to dance?
2010-03-28T14:23:34Z:INFO:myblog:request_id=12345:rows=100:user=frank:frobnicating database
2010-03-28T14:23:34Z:INFO:myblog:bytes=5678:request_id=12345:sending page over tubes
2010-03-28T14:23:34Z:DEBUG:myblog:request_id=67890:Client connected