'Dictionary with requirements as keys, and tool_ids as values.'

@property
def tool_ids_by_requirements(self):

    tool_ids_by_requirements = {}
    if (not self._app.toolbox.tools_by_id):
        return {}
    for (tid, tool) in self._app.toolbox.tools_by_id.items():
        if (tool.tool_requirements not in tool_ids_by_requirements):
            tool_ids_by_requirements[tool.tool_requirements] = [tid]
        else:
            tool_ids_by_requirements[tool.tool_requirements].append(tid)
    return tool_ids_by_requirements
