# -*- coding: utf-8 -*-
"""
This module provides a class :class:`Expr` to subclass from in order to
describe expressions. The value of the class is that it allows straightforward
interoperability between python packages handling symbolics (SymPy) and units
(quantities) as well as working without either of those. The price one has to
pay to allow for this is a somewhat contrived syntax
"""
from __future__ import (absolute_import, division, print_function)
[docs]class Expr(object):
'''
Examples
--------
>>> class HeatCapacity(Expr):
... parameter_keys = ('temperature',)
... kw = ('substance',)
...
>>> import math
>>> class EinsteinSolid(HeatCapacity):
... """ arguments: einstein temperature """
... def __call__(self, variables, args=None, backend=math):
... molar_mass = self.substance.mass
... eps = self.arg(variables, args, 0) # einstein_temperature
... R = 8.3145
... T = variables['temperature']
... molar_c_v = 3*R*(eps/(2*R*T))**2 * backend.sinh(eps/(2*R*T))**-2
... return molar_c_v/molar_mass
...
>>> from chempy import Substance
>>> Al = Substance.from_formula('Al', other_properties={'DebyeT': 428})
>>> Be = Substance.from_formula('Be', other_properties={'DebyeT': 1440})
>>> einT = lambda s: 0.806*s.other_properties['DebyeT']
>>> cv = {s.name: EinsteinSolid([einT(s)], substance=s) for s in (Al, Be)}
>>> print('%.4f' % cv['Al']({'temperature': 273.15})) # J/(g*K)
0.9227
>>> import sympy
>>> print(cv['Be']({'temperature': sympy.Symbol('T')}, backend=sympy))
13483.113390338/(T**2*sinh(69.796139274761/T)**2)
'''
parameter_keys = ()
kw = ()
def __init__(self, args, arg_keys=None, **kwargs):
self.args = args
self.arg_keys = arg_keys
for k, v in kwargs.items():
if k not in self.kw:
raise ValueError("Unexpected keyword argument %s" % k)
setattr(self, k, v)
def __call__(self, variables, args=None, backend=None):
raise NotImplementedError("Subclass and implement __call__")
[docs] def arg(self, variables, args, index):
args = args or self.args
if self.arg_keys is None:
return args[index]
else:
return variables.get(self.arg_keys[index], args[index])
[docs] def all_args(self, variables, args):
return [self.arg(variables, args, i) for i in range(len(self.args))]