<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def is_url(default_scheme='http', **kwargs):
 """Return a converter that converts a clean string to an URL."""

    def converter(value):
        if value is None:
            return value
        if '://' not in value and default_scheme:
            value = '://'.join((default_scheme, value.strip()))
        try:
            return uris.validate(value)
        except uris.ValidationError as e:
            raise Invalid(e.message)
    return converter