sympy中给算式赋值的例子

evalf是取浮点数，subs是替代。下面是两个例子。

import sympy
omega, t = sympy.symbols("omega, t")
s = sympy.pi * sympy.cos(omega*t)
print(s)
#pi*cos(omega*t)
print(s.evalf())
#3.14159265358979*cos(omega*t)
print(s.evalf().subs({t: 0}))
#3.14159265358979
print(s.subs({t: 0}))
#pi

import sympy as sym
def q():
    a,Om = sym.symbols('a, Om')
    E=sym.sqrt(Om*a**(-3)+1-Om)
    Ediff = sym.diff(E,a)
    q=-(a/E)*Ediff-1
    return q
print(q().simplify())
#(Om + 2*a**3*(Om - 1))/(2*(Om + a**3*(-Om + 1)))
print(q().subs('Om',.3).expand())
#-1 + 0.45/(0.7*a**3 + 0.3)
print(q().subs([('a',.5),('Om',.3)]))
#0.161290322580645
print(q().evalf(subs={'a':.5,'Om':.3}))
#0.161290322580645