How to use the coverage.parser.ArcStart function in coverage

def __new__(cls, lineno, cause=None):
        return super(ArcStart, cls).__new__(cls, lineno, cause)

# `except` and `else`.
                try_block.raise_from = set([])
        else:
            self.block_stack.pop()

        handler_exits = set()

        if node.handlers:
            last_handler_start = None
            for handler_node in node.handlers:
                handler_start = self.line_for_node(handler_node)
                if last_handler_start is not None:
                    self.add_arc(last_handler_start, handler_start)
                last_handler_start = handler_start
                from_cause = "the exception caught by line {lineno} didn't happen"
                from_start = ArcStart(handler_start, cause=from_cause)
                handler_exits |= self.add_body_arcs(handler_node.body, from_start=from_start)

        if node.orelse:
            exits = self.add_body_arcs(node.orelse, prev_starts=exits)

        exits |= handler_exits

        if node.finalbody:
            self.block_stack.pop()
            final_from = (                  # You can get to the `finally` clause from:
                exits |                         # the exits of the body or `else` clause,
                try_block.break_from |          # or a `break`,
                try_block.continue_from |       # or a `continue`,
                try_block.raise_from |          # or a `raise`,
                try_block.return_from           # or a `return`.
            )

"""
        node_name = node.__class__.__name__
        handler = getattr(self, "_handle__" + node_name, None)
        if handler is not None:
            return handler(node)
        else:
            # No handler: either it's something that's ok to default (a simple
            # statement), or it's something we overlooked. Change this 0 to 1
            # to see if it's overlooked.
            if 0:
                if node_name not in self.OK_TO_DEFAULT:
                    print("*** Unhandled: {}".format(node))

            # Default for simple statements: one exit from this node.
            return set([ArcStart(self.line_for_node(node))])

def _combine_finally_starts(self, starts, exits):
        """Helper for building the cause of `finally` branches.

        "finally" clauses might not execute their exits, and the causes could
        be due to a failure to execute any of the exits in the try block. So
        we use the causes from `starts` as the causes for `exits`.
        """
        causes = []
        for start in sorted(starts):
            if start.cause is not None:
                causes.append(start.cause.format(lineno=start.lineno))
        cause = " or ".join(causes)
        exits = set(ArcStart(xit.lineno, cause) for xit in exits)
        return exits

def _handle__Try(self, node):
        if node.handlers:
            handler_start = self.line_for_node(node.handlers[0])
        else:
            handler_start = None

        if node.finalbody:
            final_start = self.line_for_node(node.finalbody[0])
        else:
            final_start = None

        try_block = TryBlock(handler_start, final_start)
        self.block_stack.append(try_block)

        start = self.line_for_node(node)
        exits = self.add_body_arcs(node.body, from_start=ArcStart(start))

        # We're done with the `try` body, so this block no longer handles
        # exceptions. We keep the block so the `finally` clause can pick up
        # flows from the handlers and `else` clause.
        if node.finalbody:
            try_block.handler_start = None
            if node.handlers:
                # If there are `except` clauses, then raises in the try body
                # will already jump to them.  Start this set over for raises in
                # `except` and `else`.
                try_block.raise_from = set([])
        else:
            self.block_stack.pop()

        handler_exits = set()

def _handle__For(self, node):
        start = self.line_for_node(node.iter)
        self.block_stack.append(LoopBlock(start=start))
        from_start = ArcStart(start, cause="the loop on line {lineno} never started")
        exits = self.add_body_arcs(node.body, from_start=from_start)
        # Any exit from the body will go back to the top of the loop.
        for xit in exits:
            self.add_arc(xit.lineno, start, xit.cause)
        my_block = self.block_stack.pop()
        exits = my_block.break_exits
        from_start = ArcStart(start, cause="the loop on line {lineno} didn't complete")
        if node.orelse:
            else_exits = self.add_body_arcs(node.orelse, from_start=from_start)
            exits |= else_exits
        else:
            # No else clause: exit from the for line.
            exits.add(from_start)
        return exits

if env.PYBEHAVIOR.trace_decorated_def:
                self.add_arc(last, main_line)
                last = main_line
            # The definition line may have been missed, but we should have it
            # in `self.statements`.  For some constructs, `line_for_node` is
            # not what we'd think of as the first line in the statement, so map
            # it to the first one.
            if node.body:
                body_start = self.line_for_node(node.body[0])
                body_start = self.multiline.get(body_start, body_start)
                for lineno in range(last+1, body_start):
                    if lineno in self.statements:
                        self.add_arc(last, lineno)
                        last = lineno
        # The body is handled in collect_arcs.
        return set([ArcStart(last)])

def _handle__Return(self, node):
        here = self.line_for_node(node)
        return_start = ArcStart(here, cause="the return on line {lineno} wasn't executed")
        self.process_return_exits([return_start])
        # `return` statement jumps away, no exits from here.
        return set()