from sympy import symbols,solve

[n_,Re_,p_,i_,J2_,Pe_] = symbols(['n_','Re_','p_','i_','J2_','Pe_'])

del_ss = -((3*n_*(Re_**2)*J2_/(4*(p_**2)))*(4-5*(sin(i_)**2)))-((3*n_*(Re_**2)*J2_/(2*(p_**2)))*cos(i_))-((2*pi)/Pe_)

pprint(solve(del_ss,i_))

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:Python27libsite-packagesspyderlibwidgetsexternalshellsitecustomize.py", line 479, in runfile
    execfile(filename, namespace)
  File "C:UsersNathanPythonsympy_test_1.py", line 22, in <module>
    pprint(solve(del_ss,i_))
  File "C:Python27libsite-packagessympysolverssolvers.py", line 484, in solve
    solution = _solve(f, *symbols, **flags)
  File "C:Python27libsite-packagessympysolverssolvers.py", line 700, in _solve
    soln = tsolve(f_num, symbol)
  File "C:Python27libsite-packagessympysolverssolvers.py", line 1143, in tsolve
    "(tsolve: at least one Function expected at this point")
NotImplementedError: Unable to solve the equation(tsolve: at least one Function expected at this point

from sympy import symbols,solve,sin,cos,pi

[n_,Re_,p_,ci_,J2_,Pe_] = symbols(['n_','Re_','p_','ci_','J2_','Pe_'])

del_ss = -((3*n_*(Re_**2)*J2_/(4*(p_**2)))*(4-5*(1-ci_**2)))-((3*n_*(Re_**2)*J2_/(2*(p_**2)))*ci_)-((2*pi)/Pe_)

pprint(solve(del_ss,ci_))