evalf not giving the expect result

[miguelwon]

Hi,
Looking for some help related to evalf. I show bellow and example where evalf does not evaluate at required point:

x = {i:Symbol(f'x{i}') for i in range(2)}
w = {i:Symbol(f'w{i}') for i in range(2)}
soma = sum([x[i] * w[i] for i in range(2)])
f = tanh(soma)
print(f)

gives:

tanh(w0*x0 + w1*x1)

Then:

df = diff(tanh(soma), x[1])
print(df)

gives:

w1*(1 - tanh(w0*x0 + w1*x1)**2)

and the evaluation:

df.evalf(subs={x[i]:0 for i in range(2)})

results in:

w1*(1 - tanh(w0*x0 + w1*x1)**2)

when it should result in justw1.

[asmeurer]

This is a known issue with evalf(subs=...) #6974. It only works if you substitute every symbol. I would just use normal subs, especially if you are just going to substitute an exact value like 0.