Traceback parser and merger

traceback.py

class Traceback(object):
    @classmethod
    def parse(cls, tb_str):
        tb_lines = tb_str.strip().split('\n')
        firstline = tb_lines.pop(0)

        step_lines = []
        while True:
            if tb_lines[0].startswith(' '):
                step_lines.append(tb_lines.pop(0))
                continue

            assert len(step_lines) % 2 == 0
            steps = ['%s\n%s' % x for x in zip(step_lines[::2], step_lines[1::2])]
            exc_descr = '\n'.join(tb_lines)
            break

        return cls(firstline=firstline, steps=steps, exc_descr=exc_descr)

    firstline = None
    steps = None
    exc_descr = None

    def __init__(self, firstline, steps, exc_descr):
        self.firstline = firstline
        self.steps = steps
        self.exc_descr = exc_descr

    def without_steps(self, starting=None, count=None, until=None):
        if count is not None and until is not None:
            raise TypeError("both 'count' and 'until' cannot be given")
        if count is not None:
            if starting < 0:
                assert starting + count <= 0
            until = starting + count
            if starting < 0 and until == 0:
                until = None
        steps = self.steps[:]
        steps[starting:until] = []
        return Traceback(firstline=self.firstline, steps=steps, exc_descr=self.exc_descr)

    def without_step(self, index):
        return self.without_steps(starting=index, count=1)

    def attach(self, other, splitter_title=None):
        line = "/\\" * 10
        splitter = (
            "  %s  %s  %s" % (line + line[-2], splitter_title, line + line[-2])
            if splitter_title is not None else
            line * 2 + line[-2]
        )
        return Traceback(
            firstline=self.firstline,
            steps=self.steps + [splitter] + other.steps,
            exc_descr=other.exc_descr
        )

    def __repr__(self):
        return "%s\n%s\n%s" % (self.firstline, '\n'.join(self.steps), self.exc_descr)

    __str__ = __repr__