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153 lines
4.6 KiB
153 lines
4.6 KiB
"""
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Physical quantities.
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"""
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from sympy.core.expr import AtomicExpr
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from sympy.core.symbol import Symbol
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from sympy.core.sympify import sympify
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from sympy.physics.units.dimensions import _QuantityMapper
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from sympy.physics.units.prefixes import Prefix
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class Quantity(AtomicExpr):
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"""
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Physical quantity: can be a unit of measure, a constant or a generic quantity.
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"""
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is_commutative = True
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is_real = True
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is_number = False
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is_nonzero = True
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is_physical_constant = False
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_diff_wrt = True
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def __new__(cls, name, abbrev=None,
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latex_repr=None, pretty_unicode_repr=None,
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pretty_ascii_repr=None, mathml_presentation_repr=None,
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is_prefixed=False,
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**assumptions):
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if not isinstance(name, Symbol):
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name = Symbol(name)
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if abbrev is None:
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abbrev = name
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elif isinstance(abbrev, str):
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abbrev = Symbol(abbrev)
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# HACK: These are here purely for type checking. They actually get assigned below.
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cls._is_prefixed = is_prefixed
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obj = AtomicExpr.__new__(cls, name, abbrev)
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obj._name = name
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obj._abbrev = abbrev
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obj._latex_repr = latex_repr
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obj._unicode_repr = pretty_unicode_repr
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obj._ascii_repr = pretty_ascii_repr
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obj._mathml_repr = mathml_presentation_repr
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obj._is_prefixed = is_prefixed
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return obj
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def set_global_dimension(self, dimension):
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_QuantityMapper._quantity_dimension_global[self] = dimension
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def set_global_relative_scale_factor(self, scale_factor, reference_quantity):
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"""
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Setting a scale factor that is valid across all unit system.
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"""
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from sympy.physics.units import UnitSystem
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scale_factor = sympify(scale_factor)
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if isinstance(scale_factor, Prefix):
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self._is_prefixed = True
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# replace all prefixes by their ratio to canonical units:
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scale_factor = scale_factor.replace(
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lambda x: isinstance(x, Prefix),
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lambda x: x.scale_factor
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)
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scale_factor = sympify(scale_factor)
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UnitSystem._quantity_scale_factors_global[self] = (scale_factor, reference_quantity)
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UnitSystem._quantity_dimensional_equivalence_map_global[self] = reference_quantity
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@property
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def name(self):
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return self._name
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@property
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def dimension(self):
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from sympy.physics.units import UnitSystem
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unit_system = UnitSystem.get_default_unit_system()
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return unit_system.get_quantity_dimension(self)
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@property
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def abbrev(self):
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"""
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Symbol representing the unit name.
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Prepend the abbreviation with the prefix symbol if it is defines.
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"""
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return self._abbrev
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@property
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def scale_factor(self):
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"""
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Overall magnitude of the quantity as compared to the canonical units.
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"""
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from sympy.physics.units import UnitSystem
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unit_system = UnitSystem.get_default_unit_system()
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return unit_system.get_quantity_scale_factor(self)
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def _eval_is_positive(self):
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return True
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def _eval_is_constant(self):
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return True
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def _eval_Abs(self):
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return self
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def _eval_subs(self, old, new):
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if isinstance(new, Quantity) and self != old:
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return self
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def _latex(self, printer):
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if self._latex_repr:
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return self._latex_repr
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else:
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return r'\text{{{}}}'.format(self.args[1] \
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if len(self.args) >= 2 else self.args[0])
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def convert_to(self, other, unit_system="SI"):
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"""
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Convert the quantity to another quantity of same dimensions.
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Examples
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========
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>>> from sympy.physics.units import speed_of_light, meter, second
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>>> speed_of_light
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speed_of_light
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>>> speed_of_light.convert_to(meter/second)
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299792458*meter/second
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>>> from sympy.physics.units import liter
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>>> liter.convert_to(meter**3)
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meter**3/1000
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"""
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from .util import convert_to
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return convert_to(self, other, unit_system)
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@property
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def free_symbols(self):
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"""Return free symbols from quantity."""
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return set()
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@property
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def is_prefixed(self):
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"""Whether or not the quantity is prefixed. Eg. `kilogram` is prefixed, but `gram` is not."""
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return self._is_prefixed
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class PhysicalConstant(Quantity):
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"""Represents a physical constant, eg. `speed_of_light` or `avogadro_constant`."""
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is_physical_constant = True
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