Source code for gala.units

__all__ = [

import astropy.constants as const

# Third-party
import astropy.units as u
from astropy.units.physical import _physical_unit_mapping

_greek_letters = [

[docs]class UnitSystem: _required_physical_types = [ u.get_physical_type("length"), u.get_physical_type("time"), u.get_physical_type("mass"), u.get_physical_type("angle"), ] def __init__(self, units, *args): """ Represents a system of units. At minimum, this consists of a set of length, time, mass, and angle units, but may also contain preferred representations for composite units. For example, the base unit system could be ``{kpc, Myr, Msun, radian}``, but you can also specify a preferred velocity unit, such as ``km/s``. This class behaves like a dictionary with keys set by physical types. If a unit for a particular physical type is not specified on creation, a composite unit will be created with the base units. See the examples below for some demonstrations. Parameters ---------- *units The units that define the unit system. At minimum, this must contain length, time, mass, and angle units. Examples -------- If only base units are specified, any physical type specified as a key to this object will be composed out of the base units:: >>> usys = UnitSystem(u.m, u.s,, u.radian) >>> usys['energy'] # doctest: +SKIP Unit("kg m2 / s2") However, custom representations for composite units can also be specified when initializing:: >>> usys = UnitSystem(u.m, u.s,, u.radian, u.erg) >>> usys['energy'] Unit("erg") This is useful for Galactic dynamics where lengths and times are usually given in terms of ``kpc`` and ``Myr``, but velocities are given in ``km/s``:: >>> usys = UnitSystem(u.kpc, u.Myr, u.Msun, u.radian, >>> usys['velocity'] Unit("km / s") """ self._core_units = [] if isinstance(units, UnitSystem): self._registry = units._registry.copy() self._core_units = units._core_units return if len(args) > 0: units = (units,) + tuple(args) self._registry = dict() for unit in units: if not isinstance(unit, u.UnitBase): # hopefully a quantity q = unit new_unit = u.def_unit(f"{q!s}", q) unit = new_unit typ = unit.physical_type if typ in self._registry: raise ValueError(f"Multiple units passed in with type '{typ}'") self._registry[typ] = unit for phys_type in self._required_physical_types: if phys_type not in self._registry: raise ValueError( "You must specify a unit for the physical type" f"'{phys_type}'" ) self._core_units.append(self._registry[phys_type]) def __getitem__(self, key): key = u.get_physical_type(key) if key in self._registry: return self._registry[key] else: unit = None for k, v in _physical_unit_mapping.items(): if v == key: unit = u.Unit(" ".join([f"{x}**{y}" for x, y in k])) break if unit is None: raise ValueError( f"Physical type '{key}' doesn't exist in unit " "registry." ) unit = unit.decompose(self._core_units) unit._scale = 1.0 return unit def __len__(self): return len(self._core_units) def __iter__(self): for uu in self._core_units: yield uu def __str__(self): core_units = ", ".join([str(uu) for uu in self._core_units]) return f"UnitSystem ({core_units})" def __repr__(self): return f"<{self.__str__()}>" def __eq__(self, other): for k in self._registry: if not self[k] == other[k]: return False for k in other._registry: if not self[k] == other[k]: return False return True def __ne__(self, other): return not self.__eq__(other)
[docs] def to_dict(self): """ Return a dictionary representation of the unit system with keys set by the physical types and values set by the unit objects. """ return self._registry.copy()
[docs] def decompose(self, q): """ A thin wrapper around :meth:`astropy.units.Quantity.decompose` that knows how to handle Quantities with physical types with non-default representations. Parameters ---------- q : :class:`~astropy.units.Quantity` An instance of an astropy Quantity object. Returns ------- q : :class:`~astropy.units.Quantity` A new quantity, decomposed to represented in this unit system. """ try: ptype = q.unit.physical_type except AttributeError: raise TypeError( "Object must be an astropy.units.Quantity, not " f"a '{q.__class__.__name__}'." ) if ptype in self._registry: return[ptype]) else: return q.decompose(self)
[docs] def get_constant(self, name): """ Retrieve a constant with specified name in this unit system. Parameters ---------- name : str The name of the constant, e.g., G. Returns ------- const : float The value of the constant represented in this unit system. Examples -------- >>> usys = UnitSystem(u.kpc, u.Myr, u.radian, u.Msun) >>> usys.get_constant('c') # doctest: +SKIP 306.6013937855506 """ try: c = getattr(const, name) except AttributeError: raise ValueError( f"Constant name '{name}' doesn't exist in " "astropy.constants" ) return c.decompose(self._core_units).value
[docs]class DimensionlessUnitSystem(UnitSystem): _required_physical_types = [] def __init__(self): self._core_units = [] self._registry = {"dimensionless":} def __getitem__(self, key): return def __str__(self): return "UnitSystem (dimensionless)"
[docs] def to_dict(self): raise ValueError("Cannot represent dimensionless unit system as dict!")
[docs] def get_constant(self, name): raise ValueError("Cannot get constant in dimensionless units!")
# define galactic unit system galactic = UnitSystem(u.kpc, u.Myr, u.Msun, u.radian, / u.s) # solar system units solarsystem = UnitSystem(, u.M_sun, u.yr, u.radian) # dimensionless dimensionless = DimensionlessUnitSystem()