<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def find_module(modpath):
 """Determines whether a module exists with the given modpath."""

    module_path = modpath.replace('.', '/') + '.py'
    init_path = modpath.replace('.', '/') + '/__init__.py'
    for root_path in sys.path:
        path = os.path.join(root_path, module_path)
        if os.path.isfile(path):
            return path
        path = os.path.join(root_path, init_path)
        if os.path.isfile(path):
            return path