Source code for plainbox.impl.symbol

# This file is part of Checkbox.
# Copyright 2013 Canonical Ltd.
# Written by:
#   Zygmunt Krynicki <>
# Checkbox is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License version 3,
# as published by the Free Software Foundation.

# Checkbox is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# GNU General Public License for more details.
# You should have received a copy of the GNU General Public License
# along with Checkbox.  If not, see <>.

:mod:`plainbox.impl.symbol` -- Symbol Type

Symbols are special values that evaluate back to themselves. They are global,
unlike enumeration values, and are not bound to any container that defined
them. Symbols can be easily converted to strings and back and are a useful way
to store constants for use inside applications or libraries.

Applications can use Symbol class directly or use the SymbolDef helper to
quickly construct symbols without syntax overhead.

__all__ = ['Symbol', 'SymbolDef']

import functools
import inspect

[docs]@functools.total_ordering class Symbol: """ Symbol type. Instances of this class behave as self-interning strings. All instances are tracked and at most one instance with a given symbol name can be constructed. The name is immutable. """ __symbols = {} def __new__(cls, name): """ Create a new symbol instance. If the name was already used in another symbol then that object is returned directly. If the name was not used before then construct a new Symbol instance and return it. """ try: return cls.__symbols[name] except KeyError: symbol = object.__new__(cls) cls.__symbols[name] = symbol return symbol def __init__(self, name): """ Initialize a symbol with the given name """ self._name = name @property def name(self): """ name of the symbol """ return self._name def __str__(self): """ Convert the symbol object to its name """ return self._name def __repr__(self): """ Convert the symbol object to its representation in python """ return "{}({!r})".format(self.__class__.__name__, self._name) def __eq__(self, other): """ Compare two symbols or a string and a symbol for equality """ if isinstance(other, Symbol): return self is other elif isinstance(other, str): return self._name == other else: return NotImplemented def __lt__(self, other): """ Compare two symbols or a string and a symbol for inequality """ if isinstance(other, Symbol): return self._name < other._name elif isinstance(other, str): return self._name < other else: return NotImplemented def __hash__(self): """ Has the name of the symbol """ return hash(self._name)
class SymbolDefNs: """ Internal implementation detail of the symbol module. A special namespace used by :class:`SymbolDefMeta` to keep track of names that were being accessed. Each accessed name is converted to a :class:`Symbol` and added to the namespace. """ PASSTHRU = frozenset(('__name__', '__qualname__', '__doc__', '__module__')) def __init__(self, allow_outer=None): = {} self.allow_outer = allow_outer def __setitem__(self, name, value): if name in self.PASSTHRU:[name] = value elif isinstance(value, Symbol):[name] = value elif isinstance(value, str):[name] = Symbol(value) else: raise ValueError("Only Symbol() instances can be assigned here") def __getitem__(self, name): if name in self.PASSTHRU: return[name] elif self.allow_outer is not None and name in self.allow_outer: raise KeyError(name) elif name in return[name] elif name == 'Symbol': return Symbol else: symbol = Symbol(name)[name] = symbol return symbol class SymbolDefMeta(type): """ Metaclass for :class:`SymbolDef` which helps to construct multiple symbol objects easily. Uses :class:`SymbolDefNs` to keep track of all the symbol definitions inside the class and convert them to a list of candidate symbols to define. """ @classmethod def __prepare__(mcls, name, bases, allow_outer=None, **kwargs): return SymbolDefNs(allow_outer) def __new__(mcls, name, bases, ns, allow_outer=None): classdict = classdict['get_all_symbols'] = classmethod(mcls.get_all_symbols) return type.__new__(mcls, name, bases, classdict) def __init__(mcls, name, bases, ns, allow_outer=None): super().__init__(name, bases, ns) # This is inserted via a simple trick because it's very hard to do any # normal method definition inside SymbolDef blocks. @staticmethod def get_all_symbols(cls): """ Get all symbols defined by this symbol definition block """ # NOTE: This feels a bit like Enum and the extra property that it # carries which holds all values. I don't know if we should have that # as symbols are not 'bound' to any 'container' like enumeration values # are. return [value for name, kind, defcls, value in inspect.classify_class_attrs(cls) if name != '__locals__' and kind == 'data' and isinstance(value, Symbol)]
[docs]class SymbolDef(metaclass=SymbolDefMeta): """ Helper class that allows to easily define symbols. All sub-classes of SymbolDef are evaluated specially. Each word used inside the class definition becomes a Symbol() instance. In addition explicit assignment can create new symbols. This can be used to create symbols with value different from their identifiers. """
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