Please tell me how to break down the expression by using sympy with positive and negative.

SymPy normalizes the order of the sections, so I don't know if it suits my purpose, but how about this?

 from sympathy import cos
from sympy.abc import x,y

y = 1 + 2 * x - 3 * cos(x)
print(y)
print(y.args)

Results

2*x-3*cos(x)+1
(1,-3*cos(x), 2*x)

Alternatively, you can use y.as_terms() to obtain a section.