问题
Perhaps im overlooking the obvious but how do you prevent sympy from rearranging equations?
Im using Sympy in the iPython notebook so i can easily copy-paste the Latex code to Lyx, but i want the equations in the same order as i defined them.
For example, the formula for grey-body radiation as a function of its temperature:
Sympy automatically places the Temperature component to the front, which gives a very unusual representation of the formula. Is there anyway to prevent this?
回答1:
Currently, there is no way in SymPy to print things exactly as they are entered, because that information isn't even saved anywhere.
I believe in a multiplication, symbols are ordered alphabetically, with capital letters coming before lowercase letters (basically, the order from ord). The best trick I can come up with is to use the symbol_names option to latex, which lets you change the name used for a Symbol in its LaTeX representation. The ordering will still be based on the original symbol's name, so you can trick it:
>>> from sympy.abc import epsilon, omega, t
>>> latex(epsilon*sigma*t**4, symbol_names={t:"T"})
\epsilon \sigma T^{4}
If you want this to pretty print nicely in the notebook, you'll have to write a custom version of the printing extension that passes your symbol_names dict to latex. See https://github.com/sympy/sympy/blob/master/sympy/interactive/printing.py (ideally one could just pass the latex options to init_printing, I'll open an issue in the SymPy bug tracker for that).
回答2:
If you want the arguments of a product to appear in a given order you can create a Mul from those args and then tell the printer that you don't want them reordered as shown here for an Add. So for your example you could do
>>> var('epsilon, sigma, T')
(epsilon, sigma, T)
>>> this = Mul(epsilon, sigma, T**4, evaluate=False)
>>> StrPrinter({'order':'none'})._print_Mul(this)
epsilon*sigma*T**4
来源：https://stackoverflow.com/questions/14624511/prevent-sympy-from-rearranging-the-equation