Force SymPy to keep the order of terms

Question

I have the following code:

from sympy import *
init_printing()

x,y = symbols(\'x y\')
u = Function(\'u\')(x,y)
ux,uy,uxx,uxy,uyy = symbols(\"u_x u_y u_xx u_         


        

Answer 1

try read this http://docs.sympy.org/0.7.2/modules/utilities/misc.html, may be could help you

Note:

The key returned is useful for getting items into a canonical order that will be the same across platforms. It is not directly useful for sorting lists of expressions:

>>> a, b = x, 1/x

Since a has only 1 term, its value of sort_key is unaffected by order:

>>> a.sort_key() == a.sort_key('rev-lex')
True

If a and b are combined then the key will differ because there are terms that can be ordered:

>>> eq = a + b
>>> eq.sort_key() == eq.sort_key('rev-lex')
False
>>> eq.as_ordered_terms()
[x, 1/x]
>>> eq.as_ordered_terms('rev-lex')
[1/x, x]

But since the keys for each of these terms are independent of order‘s value, they don’t sort differently when they appear separately in a list:

>>> sorted(eq.args, key=default_sort_key)
[1/x, x]
>>> sorted(eq.args, key=lambda i: default_sort_key(i, order='rev-lex'))
[1/x, x]

The order of terms obtained when using these keys is the order that would be obtained if those terms were factors in a product.