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(
                        'Constant {!r} does not have physically compatible '
                        'units across all systems of units and cannot be '
                        'combined with other values without specifying a '
                        'system (eg. {}.{})'.format(self.abbrev, self.abbrev,

                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 = set(['__new__', '__array_finalize__', '__array_wrap__',
                       '__dir__', '__getattr__', '__init__', '__str__',
                       '__repr__', '__hash__', '__iter__', '__getitem__',
                       '__len__', '__bool__', '__quantity_subclass__',
        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. """ _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('Constant {!r} already has a definition in the ' '{!r} system from {!r} reference'.format( name, system, 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 ('<{} name={!r} value={} uncertainty={} unit={!r} ' 'reference={!r}>'.format(self.__class__,, self.value, self.uncertainty, str(self.unit), self.reference)) def __str__(self): return (' Name = {}\n' ' Value = {}\n' ' Uncertainty = {}\n' ' Unit = {}\n' ' Reference = {}'.format(, self.value, self.uncertainty, self.unit, 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.).")