This file contains lessons in Python that I felt needed to be documented because I see them, and I consistently comment on them during code reviews.

Using "hello {0}".format(thing) is almost always the correct choice for string interpolation. In the logging package, however, it is preferrable to use it's built-in interpolation to avoid the cost of formatting the string if it's not needed.

Consider the following:

log.info("About to make 10,000 requests")
for x in xrange(10000):
    response = make_request()
    log.debug("Response: {0}".format(response))

    # code continues...

Even if we're only at the INFO level we still pay to format the whole response into a string on every iteration just to have it thrown away by the logger.

Instead, use the built-in interpolation:

log.info("About to make 10,000 requests")
for x in xrange(10000):
    response = make_request()
    log.debug("Response: %s", response)

    # code continues...

Since the arguments are passed as references in Python you only pay for the interpolation if you're at that log level.

When logging in an exception handler the Python logging package provides a straight forward method for logging not only your human-consumable message but also the complete stack trace for the current exception.

This is a common anti-pattern:

try:
    raise_an_exception()
except Exception as exc:
    log.error("Something bad happened: %s", str(exc))

Instead, use the .exception method on the log handler:

try:
    raise_an_exception()
except Exception:
    log.exception("Something bad happened")