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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def _get_api_url(self, secure=None, **formatters):
        '''Constructs Postmark API url

        :param secure: Use the https Postmark API.
        :param \*\*formatters: :func:`string.format` keyword arguments to
            format the url with.
        :rtype: Postmark API url
        '''
        if self.endpoint is None:
            raise NotImplementedError('endpoint must be defined on a subclass')
        if secure is None:
            secure = self.secure
        if secure:
            api_url = POSTMARK_API_URL_SECURE
        else:
            api_url = POSTMARK_API_URL
        url = urljoin(api_url, self.endpoint)
        if formatters:
            url = url.format(**formatters)
        return url