Sympy 公式重新排列器不适用于字母“E”，我不知道为什么

Question

So as the title suggests I'm making a formula rearranger for some physics formulas, and I'm trying to make a function for E=h*f the formula for energy with Plank's constant and frequency.

For some reason when you type the letter "E" into the input() of the E_query() function the answer comes out as just "[]". Every other letter, capitol, lowercase, capitol and lowercase, doesn't have this problem. It is just the letter E, and I can't figure out why.

Different equation example:

def Fg_query():
Fg_variables = ["G", "M", "m", "r", "Fg"]
print("Fg=(G*M*m)/r^2 is the formula you selected.")
print("What variable are you trying to solve for?")
Fg_userchoice = input()
G = 0.0000000000667259 
if Fg_userchoice in Fg_variables:
   G,M,m,r,Fg = symbols('G M m r Fg')
   r = (G*M*m)/r**2
   l = Fg
   r = solve(l-r, Fg_userchoice)
   l = Fg_userchoice
   print(r)

elif Fg_userchoice is not Fg_variables:
    print("Invalid input")
    Fg_query()

This is the E=f*h function:

def E_query():
E_variables = ["E","h","f"]

print("E=h*f is the formula you selected.")
print("What variable are you trying to solve for?")
E_userchoice = input()
h = 6.62607004e-34

if E_userchoice in E_variables:
   h,E,f = symbols('h E f')
   r = (h*f)
   l = E
   r = solve(l-r, E_userchoice)
   l = E_userchoice
   print(r)

elif E_userchoice is not E_variables:
    print("Invalid input")
    E_query()

I don't know if it is a problem with the Sympy library, or something I'm doing wrong, but if someone could tell me what's going on that would be a big help.

Answer 1

SymPy sometimes lets you get by with entering a string and will sympify it for you. In the case of E , however, the number constant E = exp(1) is created. So to be safe, don't send the user's choice as a string to solve , send it as Symbol(user_choice) .