python sympy cannot sub value when using letter I

I am using sympy expressions to rearrange and solve equations i have been given.

from sympy import *
V = symbols("V")
I = symbols("I")
R = symbols("R")
expr = "R*I-V"
expr = parse_expr(expr)
expr = expr.subs("I",20)
expr = expr.subs("R",10)
answer = solve(expr,V)
print(answer)

this should output 200 but insted outputs I*10

for some reason it does not substitute the value for I

when a differnt value is used it works fine:

V = symbols("V")
i = symbols("i")
R = symbols("R")
expr = "R*i-V"
expr = parse_expr(expr)
expr = expr.subs("i",20)
expr = expr.subs("R",10)
answer = solve(expr,V)
print(answer)

this outputs the correct value of 200
other values work such as other capitals and non capitals.

Could someone explain if there is something i have missed or if this behaviour is strange

as i mentioned before i tried using a different value and it worked as expected.

rearranging the entered equation did nothing so expr = "R*I-V" had the same output as expr = "I*R-V"

Sympy uses some letters to identify specific objects. In particular:

• "O" is an alias to the Order class.
• "Q" is an alias for the AssumptionKeys class.
• "S" is an alias to the sympify() function, or to the registry of singleton objects.
• "N" is an alias to numerical evaluation with the evalf() function.
• "E" represents the base of the natural logarithm.
• "I" represents the imaginary unit.

Hence, when parse_expr sees the "I" character, it converts it to the imaginary unit! You can provide the locad_dict dictionary to associate your symbols to a particular character of the string. For example:

expr = "R*I-V"
I = symbols("I")
expr = parse_expr(expr, local_dict={"I": I})
expr = expr.subs("I",20)
expr = expr.subs("R",10)
answer = solve(expr,V)
print(answer)
# [200]