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Easy context prefixes for messages.

Project description

Dynamic message prefixes providing execution context.

Latest release 20220523: Pfx.umask: promote self.mark directly to ustr.

The primary facility here is Pfx, a context manager which maintains a per thread stack of context prefixes. There are also decorators for functions. This stack is used to prefix logging messages and exception text with context.

Usage is like this:

from cs.logutils import setup_logging, info
from cs.pfx import Pfx
def parser(filename):
  with Pfx(filename):
    with open(filename) as f:
      for lineno, line in enumerate(f, 1):
        with Pfx(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.
  • print: if true, print the mark on entry to the with suite. This may be a bool, implying print() if True, a callable which works like print(), or a file-like object which implies using print(...,file=print).

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, ...):

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_call(func, *a, **kw)

Call func(*a,**kw) within an enclosing Pfx context manager reciting the function name and arguments.


>>> import os
>>> pfx_call(os.rename, "oldname", "newname")

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.

If use_str is true (default False) use str(self) instead of classname.

If with_args is true (default False) include the specified arguments in the Pfx context. If with_args is True, this includes all the arguments. Otherwise with_args should be a sequence of argument references: an int specifies one of the positional arguments and a string specifies one of the keyword arguments.


class O:
    # just use ""
    def foo(self, .....):
    # use the value of self instead of the class name
    def foo2(self, .....):
    # include all the arguments
    def foo3(self, a, b, c, *, x=1, y):
    # include the "b", "c" and "x" arguments
    def foo3(self, a, b, c, *, x=1, y):

Class PfxCallInfo(Pfx)

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

Function pfxprint(*a, print_func=None, **kw)

Call print() with the current prefix.

The optional keyword parameter print_func provides an alternative function to the builtin print().

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 20220523: Pfx.umask: promote self.mark directly to ustr.

Release 20220429:

  • New Pfx.scope() context manager and Pfx.push(msg,*a) nonindenting Pfx push.
  • pfxprint: new optional print_func keyword parameter.

Release 20220227:

  • Pfx.prefixify: change OSError.args action: prefixify the first string.
  • XP: use DEFAULT_SEPARATOR on both paths.

Release 20211031: Pfx.prefixify_exception: skip attributes which are None.

Release 20210913: Pfx: do not fiddle with LookupError.args[0], it is the key.

Release 20210906:

  • New pfxprint which calls print() with the current prefix.
  • Pfx.prefixify_exception: catch unexpected OSError.args value and report.
  • @pfx: use pfx_call if there is no presupplied message argument.

Release 20210801: Bugfix for @pfx.

Release 20210731:

  • Pfx.exit: special handling for some exception types.
  • New pfx_call(func,*a,**kw) function to concisely wrap single function calls.

Release 20210717: @pfx_method: new optional decorator argument "with_args" to include some or all arguments in the Pfx context.

Release 20201227:

  • Pfx: new print=False option to issue a print() or other call on entry to the with-suite eg with Pfx(....,print=verbose).
  • Pfx: print= now also accepts a file-like object.

Release 20201105: @pfx: bugfix for generator functions.

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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