cs-pfx 20170903

Easy context prefixes for messages.

The primary facility here is Pfx, a context manager which manitains a per thread stack of context prefixes. Usage is like this:

from cs.pfx import Pfx
...
def parser(filename):
  with Pfx("parse(%r)", filename):
    with open(filename) as f:
      for line, lineno in enumerate(f, 1):
        with Pfx("%d", lineno) as P:
          if line_is_invalid(line):
            raise ValueError("problem!")
          P.info("line = %r", line)

This produces log messages like:

datafile: 1: line = 'foo\n'

and exception messages like:

datafile: 17: problem!

which lets one put just the relevant complaint in exception and log messages and get useful calling context on the output. This does make for wordier logs and exceptions but used with a little discretion produces far more debugable results.