问题
SymPy does a wonderful work keeping track of all the operations I do to my symbolic expressions. But a the moment of printing the result for latex output I would like to enforce a certain ordering of the term. This is just for convention, and unfortunately that convention is not alphabetical on the symbol name(as reasonably sympy does)
import sympy as sp
sp.init_printing()
U,tp, z, d = sp.symbols('U t_\perp z d')
# do many operations with those symbols
# the final expression is:
z+tp**2+U+U/(z-3*tp)+d
My problem is that SymPy presents the expression ordered as
U + U/(-3*t_\perp + z) + d + t_\perp**2 + z
But this ordering is not the convention in my field. For us z has to be the leftmost expression, then tp, then U even if it is capitalized, d is the most irrelevant and is put at the right. All this variables hold a particular meaning and that is the reason we write them in such order, and the reason in the code variables are named in such way.
I don't want to rename z to a and as suggested in Prevent Sympy from rearranging the equation and then at the moment of printing transform that a into z. In Force SymPy to keep the order of terms there is a hint I can write a sorting function but I could not find documentation about it.
回答1:
If you can put the terms in the order you want then setting the order flag for the Latex printer to 'none' will print them in that order.
>>> import sympy as sp
>>> sp.init_printing()
>>> U,tp, z, d = sp.symbols('U t_\perp z d')
>>> eq=z+tp**2+U+U/(z-3*tp)+d
Here we put them in order (knowing the power of tp is 2) and rebuild as an Add with evaluate=False to keep the order unchanged
>>> p = Add(*[eq.coeff(i)*i for i in (z, U, tp**2, d)],evaluate=False)
And now we print that expression with a printer instance with order='none':
>>> from sympy.printing.latex import LatexPrinter
>>> s=LatexPrinter(dict(order='none'))
>>> s._print_Add(p)
z + U \left(1 + \frac{1}{z - 3 t_\perp}\right) + t_\perp^{2} + d
来源：https://stackoverflow.com/questions/44414547/enforce-custom-ordering-on-sympy-print