Iterpolators and Exceptions

You may want to write your own functions that take strings and interpolate {} format templates in them. The easy way is:

from say import caller_fmt

def ucfmt(s):
    return caller_fmt(s).upper()

If ucfmt() had used fmt(), it would not have worked. fmt() would look for interpolating values within the context of ucfmt() and, not finding any, probably raised an exception. But using caller_fmt() it looks into the context of the caller of ucfmt(), which is exactly where those values would reside. Voila!

And example of how this can work–and a useful tool in its own right–is FmtException. If you want to have comprehensible error messages when something goes wrong, you could use fmt():

if bad_thing_has_happened:
    raise ValueError(fmt("Parameters {x!r} or {y!r} invalid."))

But if you define your own exceptions, consider subclassing FmtException:

class InvalidParameters(FmtException, ValueError):


if bad_thing_has_happened:
    raise InvalidParameters("Parameters {x!r} or {y!r} invalid.")

You’ll save a few characters, and the code will be simpler and more comprehensible.