Source code for astropy.constants.constant

# Licensed under a 3-clause BSD style license - see LICENSE.rst

import functools
import types
import warnings

import numpy as np

from astropy.units.core import Unit, UnitsError
from astropy.units.quantity import Quantity
from astropy.utils import lazyproperty
from astropy.utils.exceptions import AstropyUserWarning

__all__ = ["Constant", "EMConstant"]

class ConstantMeta(type):
    """Metaclass for `~astropy.constants.Constant`. The primary purpose of this
    is to wrap the double-underscore methods of `~astropy.units.Quantity`
    which is the superclass of `~astropy.constants.Constant`.

    In particular this wraps the operator overloads such as `__add__` to
    prevent their use with constants such as ``e`` from being used in
    expressions without specifying a system.  The wrapper checks to see if the
    constant is listed (by name) in ``Constant._has_incompatible_units``, a set
    of those constants that are defined in different systems of units are
    physically incompatible.  It also performs this check on each `Constant` if
    it hasn't already been performed (the check is deferred until the
    `Constant` is actually used in an expression to speed up import times,
    among other reasons).

    def __new__(mcls, name, bases, d):
        def wrap(meth):
            def wrapper(self, *args, **kwargs):
                name_lower =
                instances = self._registry[name_lower]
                if not self._checked_units:
                    for inst in instances.values():
                        except UnitsError:
                    self._checked_units = True

                if not self.system and name_lower in self._has_incompatible_units:
                    systems = sorted(x for x in instances if x)
                    raise TypeError(
                        f"Constant {self.abbrev!r} does not have physically compatible "
                        "units across all systems of units and cannot be "
                        "combined with other values without specifying a "
                        f"system (eg. {self.abbrev}.{systems[0]})"

                return meth(self, *args, **kwargs)

            return wrapper

        # The wrapper applies to so many of the __ methods that it's easier to
        # just exclude the ones it doesn't apply to
        exclude = {
        for attr, value in vars(Quantity).items():
            if (
                isinstance(value, types.FunctionType)
                and attr.startswith("__")
                and attr.endswith("__")
                and attr not in exclude
                d[attr] = wrap(value)

        return super().__new__(mcls, name, bases, d)

[docs] class Constant(Quantity, metaclass=ConstantMeta): """A physical or astronomical constant. These objects are quantities that are meant to represent physical constants. Parameters ---------- abbrev : str A typical ASCII text abbreviation of the constant, generally the same as the Python variable used for this constant. name : str Full constant name. value : numbers.Real Constant value. Note that this should be a bare number, not a |Quantity|. unit : str String representation of the constant units. uncertainty : numbers.Real Absolute uncertainty in constant value. Note that this should be a bare number, not a |Quantity|. reference : str, optional Reference where the value is taken from. system : str System of units in which the constant is defined. This can be `None` when the constant's units can be directly converted between systems. """ _registry = {} _has_incompatible_units = set() def __new__( cls, abbrev, name, value, unit, uncertainty, reference=None, system=None ): if reference is None: reference = getattr(cls, "default_reference", None) if reference is None: raise TypeError(f"{cls} requires a reference.") name_lower = name.lower() instances = cls._registry.setdefault(name_lower, {}) # By-pass Quantity initialization, since units may not yet be # initialized here, and we store the unit in string form. inst = np.array(value).view(cls) if system in instances: warnings.warn( f"Constant {name!r} already has a definition in " f"the {system!r} system from {reference!r} reference", AstropyUserWarning, ) for c in instances.values(): if system is not None and not hasattr(c.__class__, system): setattr(c, system, inst) if c.system is not None and not hasattr(inst.__class__, c.system): setattr(inst, c.system, c) instances[system] = inst inst._abbrev = abbrev inst._name = name inst._value = value inst._unit_string = unit inst._uncertainty = uncertainty inst._reference = reference inst._system = system inst._checked_units = False return inst def __repr__(self): return ( f"<{self.__class__} " f"name={!r} " f"value={self.value} " f"uncertainty={self.uncertainty} " f"unit={str(self.unit)!r} " f"reference={self.reference!r}>" ) def __str__(self): return ( f" Name = {}\n" f" Value = {self.value}\n" f" Uncertainty = {self.uncertainty}\n" f" Unit = {self.unit}\n" f" Reference = {self.reference}" ) def __quantity_subclass__(self, unit): return super().__quantity_subclass__(unit)[0], False
[docs] def copy(self): """ Return a copy of this `Constant` instance. Since they are by definition immutable, this merely returns another reference to ``self``. """ return self
__deepcopy__ = __copy__ = copy @property def abbrev(self): """A typical ASCII text abbreviation of the constant, also generally the same as the Python variable used for this constant. """ return self._abbrev @property def name(self): """The full name of the constant.""" return self._name @lazyproperty def _unit(self): """The unit(s) in which this constant is defined.""" return Unit(self._unit_string) @property def uncertainty(self): """The known absolute uncertainty in this constant's value.""" return self._uncertainty @property def reference(self): """The source used for the value of this constant.""" return self._reference @property def system(self): """The system of units in which this constant is defined (typically `None` so long as the constant's units can be directly converted between systems). """ return self._system def _instance_or_super(self, key): instances = self._registry[] inst = instances.get(key) if inst is not None: return inst else: return getattr(super(), key) @property def si(self): """If the Constant is defined in the SI system return that instance of the constant, else convert to a Quantity in the appropriate SI units. """ return self._instance_or_super("si") @property def cgs(self): """If the Constant is defined in the CGS system return that instance of the constant, else convert to a Quantity in the appropriate CGS units. """ return self._instance_or_super("cgs") def __array_finalize__(self, obj): for attr in ( "_abbrev", "_name", "_value", "_unit_string", "_uncertainty", "_reference", "_system", ): setattr(self, attr, getattr(obj, attr, None)) self._checked_units = getattr(obj, "_checked_units", False)
[docs] class EMConstant(Constant): """An electromagnetic constant.""" @property def cgs(self): """Overridden for EMConstant to raise a `TypeError` emphasizing that there are multiple EM extensions to CGS. """ raise TypeError( "Cannot convert EM constants to cgs because there " "are different systems for E.M constants within the " "c.g.s system (ESU, Gaussian, etc.). Instead, " "directly use the constant with the appropriate " "suffix (e.g. e.esu, e.gauss, etc.)." )