astropy:docs

Source code for astropy.constants.constant

# Licensed under a 3-clause BSD style license - see LICENSE.rst
from __future__ import (absolute_import, division, print_function,
                        unicode_literals)
import six

import functools
import types
import warnings

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

__all__ = ['Constant', 'EMConstant']


class ConstantMeta(type):
    """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 = Constant._registry[name_lower]
                if not self._checked_units:
                    for inst in six.itervalues(instances):
                        try:
                            self.unit.to(inst.unit)
                        except UnitsError:
                            Constant._has_incompatible_units.add(name_lower)
                    self._checked_units = True

                if (not self.system and
                        name_lower in Constant._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__', '__nonzero__'])
        for attr, value in list(six.iteritems(vars(Quantity))):
            if (isinstance(value, types.FunctionType) and
                    attr.startswith('__') and attr.endswith('__') and
                    attr not in exclude):
                d[attr] = wrap(value)

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


@six.add_metaclass(ConstantMeta)
[docs]class Constant(Quantity): """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, system=None): name_lower = name.lower() instances = Constant._registry.setdefault(name_lower, {}) if system in instances: warnings.warn('Constant {0!r} is already has a definition in the ' '{1!r} system'.format(name, system), AstropyUserWarning) inst = super(Constant, cls).__new__(cls, value) for c in six.itervalues(instances): 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 return inst def __init__(self, abbrev, name, value, unit, uncertainty, reference, system=None): self._abbrev = abbrev self._name = name self._value = value self._unit = unit self._uncertainty = uncertainty self._reference = reference self._system = system self._checked_units = False def __repr__(self): return ('<Constant name={0!r} value={1} error={2} units={3!r} ' 'reference={4!r}>'.format(self.name, self.value, self.uncertainty, str(self.unit), self.reference)) def __str__(self): return (' Name = {0}\n' ' Value = {1}\n' ' Error = {2}\n' ' Units = {3}\n' ' Reference = {4}'.format(self.name, self.value, self.uncertainty, self.unit, self.reference)) @property
[docs] 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
[docs] def name(self): """The full name of the constant.""" return self._name
@lazyproperty
[docs] def unit(self): """The unit(s) in which this constant is defined.""" return Unit(self._unit)
@property
[docs] def uncertainty(self): """The known uncertainty in this constant's value.""" return self._uncertainty
@property
[docs] def reference(self): """The source used for the value of this constant.""" return self._reference
@property
[docs] 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
@property
[docs] 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. """ instances = Constant._registry[self.name.lower()] return instances.get('si') or super(Constant, self).si
@property
[docs] 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. """ instances = Constant._registry[self.name.lower()] return instances.get('cgs') or super(Constant, self).cgs
[docs]class EMConstant(Constant): """An electromagnetic constant.""" @property
[docs] 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.).")

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