def _eval_subs(self, old, new):
        from sympy.functions.elementary.complexes import sign
        from sympy.ntheory.factor_ import multiplicity
        from sympy.simplify.powsimp import powdenest
        from sympy.simplify.radsimp import fraction

        if not old.is_Mul:
            return None

        # try keep replacement literal so -2*x doesn't replace 4*x
        if old.args[0].is_Number and old.args[0] < 0:
            if self.args[0].is_Number:
                if 0 < self.args[0]:
                    return self._subs(-old, -new)
                return None

        def base_exp(a):
            # if I and -1 are in a Mul, they get both end up with
            # a -1 base (see issue 6421); all we want here are the
            # true Pow or exp separated into base and exponent
            from sympy import exp
            if a.is_Pow or a.func is exp:
                return a.as_base_exp()
            return a, S.One

        def breakup(eq):
            """break up powers of eq when treated as a Mul:
                   b**(Rational*e) -> b**e, Rational
                commutatives come back as a dictionary {b**e: Rational}
                noncommutatives come back as a list [(b**e, Rational)]
            """

            (c, nc) = (defaultdict(int), list())
            for a in Mul.make_args(eq):
                a = powdenest(a)
                (b, e) = base_exp(a)
                if e is not S.One:
                    (co, _) = e.as_coeff_mul()
                    b = Pow(b, e/co)
                    e = co
                if a.is_commutative:
                    c[b] += e
                else:
                    nc.append([b, e])
            return (c, nc)

        def rejoin(b, co):
            """
            Put rational back with exponent; in general this is not ok, but
            since we took it from the exponent for analysis, it's ok to put
            it back.
            """

            (b, e) = base_exp(b)
            return Pow(b, e*co)

        def ndiv(a, b):
            """if b divides a in an extractive way (like 1/4 divides 1/2
            but not vice versa, and 2/5 does not divide 1/3) then return
            the integer number of times it divides, else return 0.
            """
            if not b.q % a.q or not a.q % b.q:
                return int(a/b)
            return 0

        # give Muls in the denominator a chance to be changed (see issue 5651)
        # rv will be the default return value
        rv = None
        n, d = fraction(self)
        self2 = self
        if d is not S.One:
            self2 = n._subs(old, new)/d._subs(old, new)
            if not self2.is_Mul:
                return self2._subs(old, new)
            if self2 != self:
                rv = self2

        # Now continue with regular substitution.

        # handle the leading coefficient and use it to decide if anything
        # should even be started; we always know where to find the Rational
        # so it's a quick test

        co_self = self2.args[0]
        co_old = old.args[0]
        co_xmul = None
        if co_old.is_Rational and co_self.is_Rational:
            # if coeffs are the same there will be no updating to do
            # below after breakup() step; so skip (and keep co_xmul=None)
            if co_old != co_self:
                co_xmul = co_self.extract_multiplicatively(co_old)
        elif co_old.is_Rational:
            return rv

        # break self and old into factors

        (c, nc) = breakup(self2)
        (old_c, old_nc) = breakup(old)

        # update the coefficients if we had an extraction
        # e.g. if co_self were 2*(3/35*x)**2 and co_old = 3/5
        # then co_self in c is replaced by (3/5)**2 and co_residual
        # is 2*(1/7)**2

        if co_xmul and co_xmul.is_Rational and abs(co_old) != 1:
            mult = S(multiplicity(abs(co_old), co_self))
            c.pop(co_self)
            if co_old in c:
                c[co_old] += mult
            else:
                c[co_old] = mult
            co_residual = co_self/co_old**mult
        else:
            co_residual = 1

        # do quick tests to see if we can't succeed

        ok = True
        if len(old_nc) > len(nc):
            # more non-commutative terms
            ok = False
        elif len(old_c) > len(c):
            # more commutative terms
            ok = False
        elif set(i[0] for i in old_nc).difference(set(i[0] for i in nc)):
            # unmatched non-commutative bases
            ok = False
        elif set(old_c).difference(set(c)):
            # unmatched commutative terms
            ok = False
        elif any(sign(c[b]) != sign(old_c[b]) for b in old_c):
            # differences in sign
            ok = False
        if not ok:
            return rv

        if not old_c:
            cdid = None
        else:
            rat = []
            for (b, old_e) in old_c.items():
                c_e = c[b]
                rat.append(ndiv(c_e, old_e))
                if not rat[-1]:
                    return rv
            cdid = min(rat)

        if not old_nc:
            ncdid = None
            for i in range(len(nc)):
                nc[i] = rejoin(*nc[i])
        else:
            ncdid = 0  # number of nc replacements we did
            take = len(old_nc)  # how much to look at each time
            limit = cdid or S.Infinity  # max number that we can take
            failed = []  # failed terms will need subs if other terms pass
            i = 0
            while limit and i + take <= len(nc):
                hit = False

                # the bases must be equivalent in succession, and
                # the powers must be extractively compatible on the
                # first and last factor but equal inbetween.

                rat = []
                for j in range(take):
                    if nc[i + j][0] != old_nc[j][0]:
                        break
                    elif j == 0:
                        rat.append(ndiv(nc[i + j][1], old_nc[j][1]))
                    elif j == take - 1:
                        rat.append(ndiv(nc[i + j][1], old_nc[j][1]))
                    elif nc[i + j][1] != old_nc[j][1]:
                        break
                    else:
                        rat.append(1)
                    j += 1
                else:
                    ndo = min(rat)
                    if ndo:
                        if take == 1:
                            if cdid:
                                ndo = min(cdid, ndo)
                            nc[i] = Pow(new, ndo)*rejoin(nc[i][0],
                                    nc[i][1] - ndo*old_nc[0][1])
                        else:
                            ndo = 1

                            # the left residual

                            l = rejoin(nc[i][0], nc[i][1] - ndo*
                                    old_nc[0][1])

                            # eliminate all middle terms

                            mid = new

                            # the right residual (which may be the same as the middle if take == 2)

                            ir = i + take - 1
                            r = (nc[ir][0], nc[ir][1] - ndo*
                                 old_nc[-1][1])
                            if r[1]:
                                if i + take < len(nc):
                                    nc[i:i + take] = [l*mid, r]
                                else:
                                    r = rejoin(*r)
                                    nc[i:i + take] = [l*mid*r]
                            else:

                                # there was nothing left on the right

                                nc[i:i + take] = [l*mid]

                        limit -= ndo
                        ncdid += ndo
                        hit = True
                if not hit:

                    # do the subs on this failing factor

                    failed.append(i)
                i += 1
            else:

                if not ncdid:
                    return rv

                # although we didn't fail, certain nc terms may have
                # failed so we rebuild them after attempting a partial
                # subs on them

                failed.extend(range(i, len(nc)))
                for i in failed:
                    nc[i] = rejoin(*nc[i]).subs(old, new)

        # rebuild the expression

        if cdid is None:
            do = ncdid
        elif ncdid is None:
            do = cdid
        else:
            do = min(ncdid, cdid)

        margs = []
        for b in c:
            if b in old_c:

                # calculate the new exponent

                e = c[b] - old_c[b]*do
                margs.append(rejoin(b, e))
            else:
                margs.append(rejoin(b.subs(old, new), c[b]))
        if cdid and not ncdid:

            # in case we are replacing commutative with non-commutative,
            # we want the new term to come at the front just like the
            # rest of this routine

            margs = [Pow(new, cdid)] + margs
        return co_residual*self2.func(*margs)*self2.func(*nc)
dataset/ETHPy150Open sympy/sympy/sympy/core/mul.py/Mul._eval_subs/swapped operands of `<`