Although this works, it is not sympy's standard way. Usually, functions are written as expressions. Note how f3 is used as an expression that contains x, instead of the less flexible f3(x) of the previous example.

from sympy import plot, Symbol, Piecewise, And

x = Symbol('x', real=True)
n = Symbol('n', real=True)

fn = Piecewise((x - n, (n <= x) & (x <= n   1)),
               (2 - x   n, (n   1 <= x) & (x <= n   2)),
               (0, True))

f3 = fn.subs(n, 3)  # Piecewise((x - 3, (x >= 3) & (x <= 4)), (5 - x, (x >= 4) & (x <= 5)), (0, True))
f8 = fn.subs(n, 8)  # Piecewise((x - 8, (x >= 8) & (x <= 9)), (10 - x, (x >= 9) & (x <= 10)), (0, True))

plot(f3, f8, (x, -1, 11))