Source code for sympy.physics.units.prefixes

# -*- coding: utf-8 -*-

"""
Module defining unit prefixe class and some constants.

Constant dict for SI and binary prefixes are defined as PREFIXES and
BIN_PREFIXES.
"""
from sympy import sympify, Expr


[docs]class Prefix(Expr): """ This class represent prefixes, with their name, symbol and factor. Prefixes are used to create derived units from a given unit. They should always be encapsulated into units. The factor is constructed from a base (default is 10) to some power, and it gives the total multiple or fraction. For example the kilometer km is constructed from the meter (factor 1) and the kilo (10 to the power 3, i.e. 1000). The base can be changed to allow e.g. binary prefixes. A prefix multiplied by something will always return the product of this other object times the factor, except if the other object: - is a prefix and they can be combined into a new prefix; - defines multiplication with prefixes (which is the case for the Unit class). """ _op_priority = 13.0 def __new__(cls, name, abbrev, exponent, base=sympify(10)): name = sympify(name) abbrev = sympify(abbrev) exponent = sympify(exponent) base = sympify(base) obj = Expr.__new__(cls, name, abbrev, exponent, base) obj._name = name obj._abbrev = abbrev obj._scale_factor = base**exponent obj._exponent = exponent obj._base = base return obj @property def name(self): return self._name @property def abbrev(self): return self._abbrev @property def scale_factor(self): return self._scale_factor @property def base(self): return self._base def __str__(self): # TODO: add proper printers and tests: if self.base == 10: return "Prefix(%s, %s, %s)" % (self.name, self.abbrev, self._exponent) else: return "Prefix(%s, %s, %s, %s)" % (self.name, self.abbrev, self._exponent, self.base) __repr__ = __str__ def __mul__(self, other): fact = self.scale_factor * other.scale_factor if fact == 1: return 1 elif isinstance(other, Prefix): # simplify prefix for p in PREFIXES: if PREFIXES[p].scale_factor == fact: return PREFIXES[p] return fact return self.scale_factor * other def __div__(self, other): fact = self.scale_factor / other.scale_factor if fact == 1: return 1 elif isinstance(other, Prefix): for p in PREFIXES: if PREFIXES[p].scale_factor == fact: return PREFIXES[p] return fact return self.scale_factor / other __truediv__ = __div__ def __rdiv__(self, other): if other == 1: for p in PREFIXES: if PREFIXES[p].scale_factor == 1 / self.scale_factor: return PREFIXES[p] return other / self.scale_factor __rtruediv__ = __rdiv__
def prefix_unit(unit, prefixes): """ Return a list of all units formed by unit and the given prefixes. You can use the predefined PREFIXES or BIN_PREFIXES, but you can also pass as argument a subdict of them if you don't want all prefixed units. >>> from sympy.physics.units.prefixes import (PREFIXES, ... prefix_unit) >>> from sympy.physics.units.systems import MKS >>> from sympy.physics.units import m >>> pref = {"m": PREFIXES["m"], "c": PREFIXES["c"], "d": PREFIXES["d"]} >>> prefix_unit(m, pref) #doctest: +SKIP [cm, dm, mm] """ from sympy.physics.units.quantities import Quantity prefixed_units = [] for prefix_abbr, prefix in prefixes.items(): prefixed_units.append(Quantity("%s%s" % (prefix.name, unit.name), unit.dimension, unit.scale_factor * prefix, "%s%s" % (prefix.abbrev, unit.abbrev))) return prefixed_units yotta = Prefix('yotta', 'Y', 24) zetta = Prefix('zetta', 'Z', 21) exa = Prefix('exa', 'E', 18) peta = Prefix('peta', 'P', 15) tera = Prefix('tera', 'T', 12) giga = Prefix('giga', 'G', 9) mega = Prefix('mega', 'M', 6) kilo = Prefix('kilo', 'k', 3) hecto = Prefix('hecto', 'h', 2) deca = Prefix('deca', 'da', 1) deci = Prefix('deci', 'd', -1) centi = Prefix('centi', 'c', -2) milli = Prefix('milli', 'm', -3) micro = Prefix('micro', 'mu', -6) nano = Prefix('nano', 'n', -9) pico = Prefix('pico', 'p', -12) femto = Prefix('femto', 'f', -15) atto = Prefix('atto', 'a', -18) zepto = Prefix('zepto', 'z', -21) yocto = Prefix('yocto', 'y', -24) # http://physics.nist.gov/cuu/Units/prefixes.html PREFIXES = { 'Y': yotta, 'Z': zetta, 'E': exa, 'P': peta, 'T': tera, 'G': giga, 'M': mega, 'k': kilo, 'h': hecto, 'da': deca, 'd': deci, 'c': centi, 'm': milli, 'mu': micro, 'n': nano, 'p': pico, 'f': femto, 'a': atto, 'z': zepto, 'y': yocto, } kibi = Prefix('kibi', 'Y', 10, 2), mebi = Prefix('mebi', 'Y', 20, 2), gibi = Prefix('gibi', 'Y', 30, 2), tebi = Prefix('tebi', 'Y', 40, 2), pebi = Prefix('pebi', 'Y', 50, 2), exbi = Prefix('exbi', 'Y', 60, 2) # http://physics.nist.gov/cuu/Units/binary.html BIN_PREFIXES = { 'Ki': kibi, 'Mi': mebi, 'Gi': gibi, 'Ti': tebi, 'Pi': pebi, 'Ei': exbi, }