# 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
from astropy.utils.misc import InheritDocstrings
__all__ = ['Constant', 'EMConstant']
class ConstantMeta(InheritDocstrings):
"""Metaclass for the :class:`Constant`. The primary purpose of this is to
wrap the double-underscore methods of :class:`Quantity` which is the
superclass of :class:`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):
@functools.wraps(meth)
def wrapper(self, *args, **kwargs):
name_lower = self.name.lower()
instances = self._registry[name_lower]
if not self._checked_units:
for inst in instances.values():
try:
self.unit.to(inst.unit)
except UnitsError:
self._has_incompatible_units.add(name_lower)
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 {0!r} does not have physically compatible '
'units across all systems of units and cannot be '
'combined with other values without specifying a '
'system (eg. {1}.{2})'.format(self.abbrev, 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 = 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("{} requires a reference.".format(cls))
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 {0!r} already has a definition in the '
'{1!r} system from {2!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 ('<{0} name={1!r} value={2} uncertainty={3} unit={4!r} '
'reference={5!r}>'.format(self.__class__, self.name, self.value,
self.uncertainty, str(self.unit),
self.reference))
def __str__(self):
return (' Name = {0}\n'
' Value = {1}\n'
' Uncertainty = {2}\n'
' Unit = {3}\n'
' Reference = {4}'.format(self.name, 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 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[self.name.lower()]
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.).")