Python logging variables: interpolation or format?

Question

What is the recommended method for logging variables using Python logging, and why is it better?

Old-style interpolation:

apple = "green"
logger.debug("apple is %s", apple)

or

New-style .format()

logger.debug("apple is {}".format(apple))

I have heard that interpolation is preferred, because it only evaluates the string if it's going to be printed, but I haven’t verified if it actually matters.

Answer 1

logger.debug("apple is {}".format(apple))

logger.debug(f"apple is {apple}")

New-style formatting will always build a string. If this is expensive (e.g. logging a large pandas DataFrame), this will impact performance.

---

logger.debug("apple is %s", apple)

Old-style interpolation only builds the logging string if the message will actually end up being logged. If you app's loglevel is one of INFO/WARNING/ERROR/CRITICAL, it will only log messages of at least that loglevel. So this DEBUG-level message would not get logged and the interpolation is skipped.

In conclusion, yes, interpolation saves formatting effort (effectively calls to __str__) -- if this actually matters depends on your app.