Substitute in sympy wihout evaluating or simplifying the expression

问题

Consider the follwing example:

from sympy import *
x = Symbol('x')
f = sqrt(3*x + 2)

Now I want to substitute a number, say 5 for x and get a LaTeX represenation, in this case it should return

\\sqrt(3\\cdot 5 + 2)

How can I do this?

I tried latex(f.subs(x,2,evaluate=False)) but this results just in \\sqrt(17).

回答1:

Use UnevaluatedExpr(5) instead of 5:

>>> latex(f.subs(x, UnevaluatedExpr(5)))
'\\sqrt{2 + 3 \\cdot 5}'

This wrapper prevents the expression inside of it ("5") from interacting with the outside terms. Reference: Prevent expression evaluation

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The order of addends isn't the same after substitution, as 3*x + 2 and 3*5 + 2 get sorted differently by the printer. To avoid this, one can use order='none' which keeps whatever internal order the arguments have, without trying to put them in a human-friendly arrangement.

>>> latex(f, order='none')
'\\sqrt{2 + 3 x}'
>>> latex(f.subs(x, UnevaluatedExpr(5)), order='none')
'\\sqrt{2 + 3 \\cdot 5}'

来源：https://stackoverflow.com/questions/49842196/substitute-in-sympy-wihout-evaluating-or-simplifying-the-expression