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Easy context prefixes for messages.

Project description

Dynamic message prefixes providing execution context.

Latest release 20201025:

  • Refactor @pfx using @cs.deco.contextdecorator.
  • New unpfx() function, a crude hack to strip an applpied cs.pfx prefix from a string.
  • XP: just shim cs.x.X as callers expect, toss dubious extra functionality.
  • exception(): plumb keyword arguments.

The primary facility here is Pfx, a context manager which maintains a per thread stack of context prefixes. There are also decorators for functions.

Usage is like this:

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

Class Pfx

A context manager to maintain a per-thread stack of message prefixes.

Method Pfx.__init__(self, mark, *args, **kwargs)

Initialise a new Pfx instance.


  • mark: message prefix string
  • args: if not empty, apply to the prefix string with %
  • absolute: optional keyword argument, default False. If true, this message forms the base of the message prefixes; earlier prefixes will be suppressed.
  • loggers: which loggers should receive log messages.

Note: the mark and args are only combined if the Pfx instance gets used, for example for logging or to annotate an exception. Otherwise, they are not combined. Therefore the values interpolated are as they are when the Pfx is used, not necessarily as they were when the Pfx was created. If the args are subject to change and you require the original values, apply them to mark immediately, for example:

with Pfx('message %s ...' % (arg1, arg2, ...)):

This is a bit more expensive as it incurs the formatting cost whenever you enter the with clause. The common usage is:

with Pfx('message %s ...', arg1, arg2, ...):

Method Pfx.critical(*a, **kw)

Emit a critical log message.

Method Pfx.debug(*a, **kw)

Emit a debug log message.

Method Pfx.error(*a, **kw)

Emit an error log message.

Method Pfx.exception(*a, **kw)

Log an exception message to this Pfx's loggers.

Method*a, **kw)

Emit an info log message.

Method Pfx.log(*a, **kw)

Log a message at an arbitrary log level to this Pfx's loggers.

Property Pfx.loggers

Return the loggers to use for this Pfx instance.

Method Pfx.logto(self, new_loggers)

Define the Loggers anew.

Method Pfx.partial(self, func, *a, **kw)

Return a function that will run the supplied function func within a surrounding Pfx context with the current mark string.

This is intended for deferred call facilities like WorkerThreadPool, Later, and futures.

Method Pfx.prefixify(text)

Return text with the current prefix prepended. Return text unchanged if it is not a string.

Method Pfx.prefixify_exception(e)

Modify the supplied exception e with the current prefix. Return True if modified, False if unable to modify.

Property Pfx.umark

Return the unicode message mark for use with this Pfx.

This is used by Pfx._state.prefix to compute the full prefix.

Method Pfx.warning(*a, **kw)

Emit a warning log message.

Function pfx(*da, **dkw)

General purpose @pfx for generators, methods etc.


  • func: the function or generator function to decorate
  • message: optional prefix to use instead of the function name
  • message_args: optional arguments to embed in the preifx using %

Example usage:

def f(....):

Function pfx_iter(tag, iterable)

Wrapper for iterables to prefix exceptions with tag.

Function pfx_method(*da, **dkw)

Decorator to provide a Pfx context for an instance method prefixing classname.methodname (or str(self).methodname if use_str is true).

Example usage:

class O:
    def foo(self, .....):

Class PfxCallInfo(Pfx)

Subclass of Pfx to insert current function an caller into messages.

Function PfxThread(target=None, **kw)

Factory function returning a Thread which presents the current prefix as context.

Function prefix()

Return the current Pfx prefix.

Function PrePfx(tag, *args)

Push a temporary value for Pfx._state._ur_prefix to enloundenify messages.

Function unpfx(s, sep=None)

Strip the leading prefix from the string s using the prefix delimiter sep (default from DEFAULT_SEPARATOR: ': ').

This is a simple hack to support reporting error messages which have had a preifx applied, and fails accordingly if the base message itself contains the separator.

Function XP(msg, *args, **kwargs)

Variation on cs.x.X which prefixes the message with the current Pfx prefix.

Function XX(prepfx, msg, *args, **kwargs)

Trite wrapper for XP() to transiently insert a leading prefix string.


XX("NOTE!", "some message")

Release Log

Release 20201025:

  • Refactor @pfx using @cs.deco.contextdecorator.
  • New unpfx() function, a crude hack to strip an applpied cs.pfx prefix from a string.
  • XP: just shim cs.x.X as callers expect, toss dubious extra functionality.
  • exception(): plumb keyword arguments.

Release 20200517:

  • @pfx: handle normal functions and also generators, improve behaviour with the wrapped docstring.
  • @pfx_method: @pfx for methods.
  • @pfxtag obsoleted by new @pfx.

Release 20191004: @pfx_method: new optional use_str parameter to use str(self) instead of type(self).name; now requires @cs.deco.decorator

Release 20190905:

  • Pfx.exit: simplify prefixify_exc() logic, prefixify all suitable attributes.
  • New @pfx_method decorator for instance methods.

Release 20190607: Pfx.exit improved exception attribute handling.

Release 20190403: Debugging aid: Pfx.umark: emit stack traceback on format conversion error.

Release 20190327:

  • @pfx: set name on the wrapper function.
  • Bugfix some references to the internal prefixify function.

Release 20190324: Pfx.exit: apply the prefix to all the standard attributes where present, improves some message behaviour for some exception types.

Release 20181231: Bugfix for an infinite regress.

Release 20181109:

  • Update @contextmanager formalism to use try/finally for the cleanup phase.
  • New decorator @gen to manage Pfx state across generator iterations; pretty clunky.
  • Better fallback handling.
  • Some docstring updates.

Release 20170910: Slight linting.

Release 20170903.1: corrections to the module docstring

Release 20170903: Initial release for PyPI.

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