D.walk() -> iterator over files and subdirs, recursively.

        The iterator yields path objects naming each child item of
        this directory and its descendants.  This requires that
        D.isdir().

        This performs a depth-first traversal of the directory tree.
        Each directory is returned just before all its children.

        The ``errors`` keyword argument controls behavior when an
        error occurs.  The default is ``strict``, which causes an
        exception.  The other allowed values are 'warn', which
        reports the error via ``warnings.warn()``, and ``ignore``.

def walk(self, pattern=None, errors='strict'):
        """ D.walk() -> iterator over files and subdirs, recursively.

        The iterator yields path objects naming each child item of
        this directory and its descendants.  This requires that
        D.isdir().

        This performs a depth-first traversal of the directory tree.
        Each directory is returned just before all its children.

        The ``errors`` keyword argument controls behavior when an
        error occurs.  The default is ``strict``, which causes an
        exception.  The other allowed values are 'warn', which
        reports the error via ``warnings.warn()``, and ``ignore``.
        """
        if errors not in ('strict', 'warn', 'ignore'):
            raise ValueError("invalid errors parameter")

        try:
            childList = self.listdir()
        except Exception:
            if errors == 'ignore':
                return
            elif errors == 'warn':
                warnings.warn(
                    "Unable to list directory '%s': %s"
                    % (self, sys.exc_info()[1]),
                    TreeWalkWarning)
                return
            else:
                raise

        for child in childList:
            if pattern is None or child.fnmatch(pattern):
                yield child
            try:
                isdir = child.isdir()
            except Exception:
                if errors == 'ignore':
                    isdir = False
                elif errors == 'warn':
                    warnings.warn(
                        "Unable to access '%s': %s"
                        % (child, sys.exc_info()[1]),
                        TreeWalkWarning)
                    isdir = False
                else:
                    raise

            if isdir:
                for item in child.walk(pattern, errors):
                    yield item