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You can see which symbolic variables are used by the input V with V.variables(). Then, you can define the corresponding derivation symbols by looking to their string representation, add the letter 'd' in front of them, and make them symbols with SR.var function. So, the following should work:

def implicit_derivative(V):
    w1, w2 = V.variables()
    dw1 = SR.var('d{}'.format(w1))
    dw2 = SR.var('d{}'.format(w2))
    V_w1 = diff(V, w1)
    V_w2 = diff(V, w2)
    # Differential
    dV = V_w1 * dw1 + V_w2 * dw2 
    # Dérivée du premier ordre
    sol=solve(dV==0, dw2)
    impder=(sol[0]/dw1)
    return impder