Source code for cumin.grammar

"""Query grammar definition."""
import importlib
import pkgutil

from collections import namedtuple

import pyparsing as pp

from cumin import backends, CuminError


INTERNAL_BACKEND_PREFIX = 'cumin.backends'
""":py:class:`str` with the prefix for built-in backends."""


Backend = namedtuple('Backend', ['keyword', 'name', 'cls'])
""":py:func:`collections.namedtuple` that define a Backend object.

Keyword Arguments:
    keyword (str): The backend keyword to be used in the grammar.
    name (str): The backend name.
    cls (BaseQuery): The backend class object.
"""


[docs]def get_registered_backends(external=()): """Get a mapping of all the registered backends with their keyword. Arguments: external (list, tuple, optional): external backend modules to register. Returns: dict: A dictionary with a ``{keyword: Backend object}`` mapping for each available backend. Raises: cumin.CuminError: If unable to register a backend. """ available_backends = {} backend_names = ['{prefix}.{backend}'.format(prefix=INTERNAL_BACKEND_PREFIX, backend=name) for _, name, ispkg in pkgutil.iter_modules(backends.__path__) if not ispkg] for name in backend_names + list(external): keyword, backend = _import_backend(name, available_backends) if keyword is not None and backend is not None: available_backends[keyword] = backend return available_backends
[docs]def grammar(backend_keys): """Define the main multi-query grammar. Cumin provides a user-friendly generic query language that allows to combine the results of subqueries for multiple backends: * Each query part can be composed with the others using boolean operators ``and``, ``or``, ``and not``, ``xor``. * Multiple query parts can be grouped together with parentheses ``(``, ``)``. * Specific backend query ``I{backend-specific query syntax}``, where ``I`` is an identifier for the specific backend. * Alias replacement, according to aliases defined in the configuration file ``A:group1``. * The identifier ``A`` is reserved for the aliases replacement and cannot be used to identify a backend. * A complex query example: ``(D{host1 or host2} and (P{R:Class = Role::MyClass} and not A:group1)) or D{host3}`` Backus-Naur form (BNF) of the grammar:: <grammar> ::= <item> | <item> <boolean> <grammar> <item> ::= <backend_query> | <alias> | "(" <grammar> ")" <backend_query> ::= <backend> "{" <query> "}" <alias> ::= A:<alias_name> <boolean> ::= "and not" | "and" | "xor" | "or" Given that the pyparsing library defines the grammar in a BNF-like style, for the details of the tokens not specified above check directly the source code. Arguments: backend_keys (list): list of the GRAMMAR_PREFIX for each registered backend. Returns: pyparsing.ParserElement: the grammar parser. """ # Boolean operators boolean = (pp.CaselessKeyword('and not').leaveWhitespace() | pp.CaselessKeyword('and') | pp.CaselessKeyword('xor') | pp.CaselessKeyword('or'))('bool') # Parentheses lpar = pp.Literal('(')('open_subgroup') rpar = pp.Literal(')')('close_subgroup') # Backend query: P{PuppetDB specific query} query_start = pp.Combine(pp.oneOf(backend_keys, caseless=True)('backend') + pp.Literal('{')) query_end = pp.Literal('}') # Allow the backend specific query to use the end_query token as well, as long as it's in a quoted string # and fail if there is a query_start token before the first query_end is reached query = pp.SkipTo(query_end, ignore=pp.quotedString, failOn=query_start)('query') backend_query = pp.Combine(query_start + query + query_end) # Alias alias = pp.Combine(pp.CaselessKeyword('A') + ':' + pp.Word(pp.alphanums + '-_.+')('alias')) # Final grammar, see the docstring for its BNF based on the tokens defined above # Group are used to have an easy dictionary access to the parsed results full_grammar = pp.Forward() item = backend_query | alias | lpar + full_grammar + rpar full_grammar << pp.Group(item) + pp.ZeroOrMore(pp.Group(boolean + item)) # pylint: disable=expression-not-assigned return full_grammar
[docs]def _import_backend(module, available_backends): """Dynamically import a backend for Cumin and validate it. Arguments: module (str): the full module name of the backend to register. Must be importable from Python ``PATH``. available_backends (dict): dictionary with a ``{keyword: Backend object}`` mapping for all registered backends. Returns: tuple: with two elements: ``(keyword, Backend object)`` of the imported backend. """ try: backend = importlib.import_module(module) except ImportError as e: if not module.startswith(INTERNAL_BACKEND_PREFIX): raise CuminError("Unable to import backend '{module}': {e}".format(module=module, e=e)) from e return (None, None) # Internal backend not available, are all the dependencies installed? name = module.split('.')[-1] message = "Unable to register backend '{name}' in module '{module}'".format(name=name, module=module) try: keyword = backend.GRAMMAR_PREFIX except AttributeError as e: raise CuminError('{message}: GRAMMAR_PREFIX module attribute not found'.format(message=message)) from e if keyword in available_backends: raise CuminError(("{message}: keyword '{key}' already registered: {backends}").format( message=message, key=keyword, backends=available_backends)) try: class_obj = backend.query_class except AttributeError as e: raise CuminError('{message}: query_class module attribute not found'.format(message=message)) from e if not issubclass(class_obj, backends.BaseQuery): raise CuminError('{message}: query_class module attribute is not a subclass of cumin.backends.BaseQuery') return (keyword, Backend(name=name, keyword=keyword, cls=class_obj))