utilities.py

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"""

"""
Various helpful utility functions/classes used by PyCml.
"""

import logging
import sys
from xml.dom import Node # For nodeType values

import amara

from enum import Enum

__all__ = ['OnlyWarningsFilter', 'OnlyDebugFilter', 'OnlyTheseSourcesFilter', 'NotifyHandler',
           'DEBUG', 'LOG',
           'Colourable', 'DFS', 'Sentinel', 'unitary_iterator',
           'amara_parse', 'element_path', 'element_path_cmp', 'element_xpath', 'brief_xml',
           'call_if', 'max_i', 'prid', 'add_dicts',
           'open_output_stream', 'close_output_stream']

################################################################################
#                                                                              #
#                              Logging                                         #
#                                                                              #
################################################################################

class OnlyWarningsFilter(logging.Filter):
    """A filter that only passes warning messages."""
    def filter(self, rec):
        return (logging.WARNING <= rec.levelno < logging.ERROR)


class OnlyDebugFilter(logging.Filter):
    """A filter that only passes debug messages."""
    def filter(self, rec):
        return (logging.DEBUG <= rec.levelno < logging.INFO)


class OnlyTheseSourcesFilter(logging.Filter):
    """A filter that only emits messages from the given sources."""
    def __init__(self, sources):
        logging.Filter.__init__(self)
        self.__sources = sources
    def filter(self, rec):
        return rec.name in self.__sources


class NotifyHandler(logging.Handler):
    """
    A logging handler that just notes if any messages are logged.
    """
    def __init__(self, level=logging.NOTSET):
        logging.Handler.__init__(self, level=level)
        self.reset()

    def emit(self, record):
        self.messages = True

    def reset(self):
        """Reset the handler, as if no messages have occurred."""
        self.messages = False


def DEBUG(facility, *args):
    """Log a debug message to facility.

    Arguments are treated as for the print statement.
    """
    logger = logging.getLogger(facility)
    if logger.isEnabledFor(logging.DEBUG):
        logger.debug(' '.join(map(str, args)))


def LOG(facility, level, *args):
    """Log a message to facility with the given level.

    Arguments are treated as for the print statement.
    """
    logger = logging.getLogger(facility)
    if logger.isEnabledFor(level):
        logger.log(level, ' '.join(map(str, args)))

################################################################################
#                                                                              #
#                        Miscellaneous classes                                 #
#                                                                              #
################################################################################

# Useful constants for depth-first search
DFS = Enum('White', 'Gray', 'Black')

class Colourable(object):
    """
    A mixin class for objects that have a colour attribute, and so support
    a depth-first search.
    """
    def __init__(self, *args, **kwargs):
        super(Colourable, self).__init__(*args, **kwargs)
        self.clear_colour()
    
    def set_colour(self, colour):
        self._cml_colour = colour
    
    def get_colour(self):
        return self._cml_colour
    
    def clear_colour(self):
        self._cml_colour = DFS.White


class Sentinel(object):
    """A simple true/false store that can be used as a default parameter value in recursive calls.
    
    This provides a slightly nicer looking alternative to code such as:
    
    def f(done=[False]):
       if not done[0]:
           # do some stuff
           if xyz: done[0] = True
           f(done)

    The list can be replaced with a Sentinel instance.
    """
    def __init__(self, tf=False):
        """Create a new, unset sentinel."""
        self._tf = tf
    
    def __nonzero__(self):
        """Test whether the sentinel has been set."""
        return self._tf
    
    def set(self, tf=True):
        """Set the sentinel."""
        self._tf = tf


class unitary_iterator(object):
    """An iterator over a single item."""
    def __init__(self, start):
        self.curr = start
        return

    def __iter__(self):
        return self

    def next(self):
        if not self.curr:
            raise StopIteration()
        result = self.curr
        self.curr = None
        return result


################################################################################
#                                                                              #
#                        XML-related functions                                 #
#                                                                              #
################################################################################

def amara_parse(source, uri=None, rules=None, binderobj=None,
                prefixes=None):
    """Convenience function for parsing XML.
    
    Works just as amara.parse, except that if source is '-' then
    it reads from standard input.
    """
    if source == '-':
        return amara.parse(sys.stdin, uri=uri, rules=rules,
                           binderobj=binderobj, prefixes=prefixes)
    else:
        return amara.parse(source, uri=uri, rules=rules,
                           binderobj=binderobj, prefixes=prefixes)


def element_path(elt):
    """Find the path from the root element to this element."""
    if hasattr(elt, 'xml_parent'):
        idx = 0
        for child in elt.xml_parent.xml_children:
            if getattr(child, 'nodeType', None) == Node.ELEMENT_NODE:
                idx += 1
                if child is elt:
                    break
        return element_path(elt.xml_parent) + [idx]
    else:
        return []
    

def element_path_cmp(e1, e2):
    """Compare 2 elements by comparing their paths from the root element."""
    return cmp(element_path(e1), element_path(e2))


def element_xpath(elt):
    """Return an xpath expression that will select this element."""
    indices = element_path(elt)
    xpath = u'/*[' + u']/*['.join(map(str, indices)) + u']'
    return xpath


def brief_xml(elt):
    """Print a more concise version of elt.xml() that omits all attributes."""
    s = ''
    if getattr(elt, 'nodeType', None) == Node.ELEMENT_NODE:
        children = getattr(elt, 'xml_children', [])
        if children:
            s += '<' + elt.localName + '>'
            for child in children:
                s += brief_xml(child)
            s += '</' + elt.localName + '>'
        else:
            s += '<' + elt.localName + '/>'
    else:
        s += str(elt)
    return s


################################################################################
#                                                                              #
#                       Miscellaneous functions                                #
#                                                                              #
################################################################################

def call_if(tf, callable, *args, **kwargs):
    """Call the given callable with the given arguments iff tf is True."""
    if tf:
        return callable(*args, **kwargs)


def max_i(it):
    """Find the maximum entry of an iterable, and return it with its index.

    Returns (i, m) where i is the index of m, the maximum entry of `it`.
    """
    idx, m = None, None
    for i, val in enumerate(it):
        if m is None or val > m:
            m, idx = val, i
    return idx, m


def prid(obj, show_cls=False):
    """Get the id of an object as a hex string, optionally with its class/type."""
    if obj is None:
        s = 'None'
    else:
        s = hex(id(obj))
        if show_cls:
            s += str(type(obj))
    return s


def add_dicts(r, *ds):
    """Add multiple dictionaries together.

    Updates the first input dictionary by adding values from
    subsequent inputs.  Produces a dictionary with keys taken from the
    input dictionaries, and values being the sum of the corresponding
    values in all inputs.
    Assumes values are numeric.
    """
    for d in ds:
        for k, v in d.iteritems():
            r[k] = r.get(k, 0) + v


def open_output_stream(fname):
    """Open fname for output.
    
    fname should be a local filename, or '-' for standard output.
    Additionally, the names 'stdout' and 'stderr' have the usual
    special meanings.
    """
    if fname == '-' or fname == 'stdout':
        stream = sys.stdout
    elif fname == 'stderr':
        stream = sys.stderr
    else:
        stream = open(fname, 'w')
    return stream


def close_output_stream(stream):
    """
    Close the given output stream, unless it's one of the standard streams
    (i.e. sys.stdout or sys.stderr).
    Note that closing a stream multiple times is safe.
    """
    if not stream is sys.stdout and not stream is sys.stderr:
        stream.close()