Sympy solver bug in a for loop?

Question

So I'm playing with Sympy in an effort to build a generic solver/generator of physics problems. One component is that I'm going for a function that will take kwargs and, according to what it got, rearrange the equation and substitute values in it. Thanks to SO, I managed to find the things I need for that.

However..... I've tried putting sympy.solve in a for loop to generate all those expressions and I've ran into.... something.

import sympy
R, U, I, eq = sympy.symbols('R U I eq')
eq = R - U/I
for x in 'RUI':
    print(x)
    print(sympy.solve(eq, x))

The output?

R
[U/I]
U
[I*R]
I
[]

However, whenever I do sympy.solve(eq, I) it works and returns [U/R].

Now, I'm guessing the issue is with sympy using I for imaginary unit and with variable hiding in blocks, but even when I transfer the symbol declaration inside the for loop (and equation as well), I still get the same problem.

I'm not sure I'll need this badly in the end, but this is interesting to say the least.

Answer 1

It's more like an undocumented feature than a bug. The loop for x in 'RUI' is equivalent to for x in ['R', 'U', 'I'], meaning that x runs over one-character strings, not sympy symbols. Insert print(type(x)) in the loop to see this. And note that sympy.solve(eq, 'I') returns [].

The loop for x in [R, U, I] solves correctly for each variable. This is the right way to write this loop.

The surprising thing is that you get anything at all when passing a string as the second argument of solve. Sympy documentation does not list strings among acceptable arguments. Apparently, it tries to coerce the string to a sympy object and does not always guess your meaning correctly: works with sympy.solve(eq, 'R') but not with sympy.solve(eq, 'I')