Format() spec for deferred str.format()

[kurtbrose]

class _Format(object):
    def __init__(self, fmt_str):
        self.fmt_str = fmt_str
    def glomit(self, target, scope):
        return self.fmt_str.format(**target)

motivating use case: generating a SAML IDP config as needed by pysaml2:

_SAML_IDP_CONFIG = Fill({
    'debug': T['debug'],
    'xmlsec_binary': saml2.sigver.get_xmlsec_binary(['/opt/local/bin', '/usr/bin/xmlsec1']),
    'entityid': T['entity_id'],
    'description': 'EBill/Tableau IdP',
    'service': {
        'idp': {
            'name': 'Ebill IdP',
            'endpoints': {
                'single_sign_on_service': [
                    (_Format('{base_url}/sso/post/'), saml2.BINDING_HTTP_POST),
                    (_Format('{base_url}/sso/redirect/'), saml2.BINDING_HTTP_REDIRECT),
                ],
            },
            'name_id_format': [saml2.saml.NAMEID_FORMAT_EMAILADDRESS],
            'sign_response': False,
            'sign_assertion': True,
        },
    },
    'metadata': {
        'local': [T['metadata']],
    },
    # Signing
    'key_file': _Format('{cert_dir}/private.key'),
    'cert_file': _Format('{cert_dir}/public.cert'),
    # Encryption
    'encryption_keypairs': [{
        'key_file': _Format('{cert_dir}/private.key'),
        'cert_file': _Format('{cert_dir}/public.cert'),
    }],
    'valid_for': 365 * 24,
})

[kurtbrose]

maybe it could be made a BIT more general by taking a second argument of the dictionary which should be **'d into the format call (default T)

class _Format(object):
    def __init__(self, fmt_str, spec=T):
        self.fmt_str, self.spec = fmt_str, spec
    def glomit(self, target, scope):
        fmt_args = scope[glom](self.spec, target, scope)
        return self.fmt_str.format(**fmt_args)

[kurtbrose]

#131