Comment #45 on issue 1816 by ronan.l...@gmail.com: Adding partial
derivatives and taking derivatives with respect to functions
http://code.google.com/p/sympy/issues/detail?id=1816
What users should enter is rather something like:
x, t = symbols('x, t')
xfunc = Function('x')
diff(2*x + 4, x).subs(x, xfunc(t))
2
As for inconsistencies, many of them have been pointed out already:
diff(cos(x)**2, cos(x))
2 * cos(x)
diff(1 - sin(x)**2, cos(x))
0
diff(diff(f(x), x), f(x)).subs(f, exp)
0
diff(diff(exp(x), x), exp(x))
1
Also, there's the issue of which arguments .diff() accepts. In Brian's
implementation, it seems that we have:
x = t**2; diff(2*x + 4, x)
<raises exception>
x = exp(t); diff(2*x + 4, x)
2
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