Function

class Function1(Scene):
    def construct(self):

        func = Function("\text{sin}", "\theta")
        self.add(func)

        func.function.set_color(GREEN)
        func.input.set_color(LIGHT_BROWN)
        func.parentheses.set_color(WHITE)

Function Equation Operation

class Function2(Scene):
    def construct(self):

        a = MathTex("a", color=RED)
        b = MathTex("b", color=BLUE)

        tex = MathTex(a, "=", b)
        self.add(tex)

        tex.LHS = Function("\\sin", tex.LHS)
        tex.RHS = Function("\\sin", tex.RHS)

        self.play(TransformInStages.progress(tex))

        tex.LHS = tex.LHS.input
        tex.RHS = tex.RHS.input

        self.play(TransformInStages.progress(tex))

Log of Exponent

class LogOfExponent(Scene):
    def construct(self):

        a = MathTex("a", color=RED)
        k = MathTex("k", color=BLUE)

        log = Function("\log", Term(a, k))
        tex = MathTex(log)

        self.add(tex)

        log.input = a
        tex.terms = [ k, log ]

        self.play(TransformInStages.progress(tex))