Co-Written by Hari Dara
Last week I’ve had to wrangle with Python’s documentation because I needed one of the apps I’m writing to centrally remove sensitive information from the logs. After several attempts at understanding the documentation, I’ve come to appreciate the built-in logging framework. I also have to admit that it’s more powerful than Ruby’s built in logger.
There are definitely more than one approach to solve this problem, but I will discuss a couple of approaches below:
To create a logger, use the class method (aka module-level function)
logging.gettLogger(). For example:
1 2 3 import logging logger = logging.getLogger("myapp")
This creates a logging object named “myapp”. Calling this same line from anywhere in your application will return the same logging object. You can also create hierarchies by separating names using dot notation. For example:
1 2 3 import logging logger = logging.getLogger("myapp.my_object")
Creates a logger that is a descendant of the “myapp” logger we created above.
This is handy and allows you to just say
logging.getLogger(__name__) in your
modules and they will be organized automatically.
After you’ve created your logger, creating log entries is similar to other logging frameworks:
1 2 3 4 5 logger.debug("OHAI, I CAN HAZ DEBUG MSG?") logger.info("I'M IN UR LOGGER. INFOING LOGS") logger.warning("YOU HAZ RIGHT TO REMAIN SILENT") logger.error("ERROR MSGZ") logger.critical("OH NOEZ!")
More where that came from.
There are different ways to centrally configure your application’s logs, but I prefer to use YAML. For example:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 version: 1 disable_existing_loggers: False formatters: simple: format: '%(asctime)s %(name)-30s %(levelname)-7s %(message)s' handlers: file: class: logging.handlers.RotatingFileHandler formatter: simple filename: /var/log/myapp/myapp.log root: level: DEBUG handlers: - file
Let’s break this down:
versiontells the logging framework what configuration file schema version we’re using.
disable_existing_loggerstells the framework what to do with loggers that were created before we configured it
formattersdefines the formatters that we later use in the config file. The variables in the
formatkey are actually attributes of the LogRecord class. The
7ssuffixes tell the formatter the minimum number of chars to allocate to the string. Handy for lining up your entries.
handlersdeclares and configures the handlers that we want to use with our loggers. In this example, we have a handler named
filewhich uses the built in RotatingFileHandler class.
simpleformatter with our
filehandler in line 11?
DEBUGand up and also specify that it use our
myappand use the same method that we used with
rootto configure it.
rootlogger we have
handlers(plural)? That means we can add as many handlers as we want here.
Before we can use this YAML file though, we have to convert it into a dictionary
so that Python’s logging framework can understand it. Simple enough. Let assume we
configure_logging.py file with the following contents:
1 2 3 4 5 6 7 8 import logging import logging.config import yaml with open("path/to/yaml", 'r') as the_file: config_dict = yaml.load(the_file) logging.config.dictConfig(config_dict)
After that, calling
logging.getLogger() from anywhere in your app will give you
a logger configured as the above.
Here’s a detailed flowchart of how the logging framework handles log messages. The handler flow on the right side gives us our first hint on how to centrally filter out sensitive information from our logs.
Declaring a filter requires that we create a new section in our config file called
filters like so:
1 2 3 4 5 6 7 filters: myfilter: (): filters.RedactingFilter patterns: - "filterthisstring" - "thistoo" - "metoo!"
I declared only one filter there and the class name is
RedactingFilter as indicated
() key. All other keys will be supplied to the
RedactingFilter constructor. In
the above example, an array of strings called
patterns will be passed. Let’s
see how the
RedactingFilter class might be defined. Let’s assume we have a
file with the following contents:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 import logging class RedactingFilter(logging.Filter): def __init__(self, patterns): super(RedactingFilter, self).__init__() self._patterns = patterns def filter(self, record): msg = self.redact(record.msg) if isinstance(record.args, dict): for k in record.args.keys(): record.args[k] = self.redact(record.args[k]) else: record.args = tuple(self.redact(arg) for arg in record.args) return True def redact(self, msg): msg = isinstance(msg, basestring) and msg or str(msg) for pattern in self._patterns: msg = msg.replace(pattern, "<<TOP SECRET!>>") return msg
RedactingFilterclass inherits from
filteris called automatically by the framework. This corresponds with the last diamond in this flowchart. If this method returns True, the log is written. Otherwise it is rejected. In our case, it always approves all log entries.
filtermethod expects a
recordargument. This is always a
LogRecordinstance which we can manipulate according to our needs. In our case, we take the log’s
msgand replace all occurances of the patterns we declared in the config file with “«TOP SECRET!»”. You’ll notice that it’s just a small step from here to regular expressions. I’ll let you experiment with that on your own. :-)
Finally, let’s attach this filter to our handler:
1 2 3 4 5 6 7 handlers: file: class: logging.handlers.RotatingFileHandler formatter: simple filename: /var/log/myapp/myapp.log filters: - myfilter
filters is an array. Therefore we can add as many filters as we need here.
In this approach, we will use a custom formatter objects that simply wrap the real formatter objects such that, messages can be intercepted right after they have been formatted but before the
handler gets a chance to write them. This has the advantage that it is very simple, but unlike the previous approach, it can only be configured programmatically (which is actually good in a way, as it will always get applied irrespective of what the user configures).
First we will create a formatter class which is just a wrapper that simply proxies all the original formatter attributes, except for the
format() method. Let’s assume we have a formatters.py file with the following contents:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 class RedactingFormatter(object): def __init__(self, orig_formatter, patterns): self.orig_formatter = orig_formatter self._patterns = patterns def format(self, record): msg = self.orig_formatter.format(record) for pattern in self._patterns: msg = msg.replace(pattern, "***") return msg def __getattr__(self, attr): return getattr(self.orig_formatter, attr)
In the code right after
logging is configured (see), we need to add the below code to wrap all formatters (a word of caution: the below code assumes that all the handlers are on the root logger, but this may not always be the case):
1 2 for h in logging.root.handlers: h.setFormatter(FilterFormatter(h.formatter))
There’s more to the logging framework than what I’ve discussed here. So go ahead and read some more from the official docs.