@classmethod
    def flatten(cls, seq):
        """Return commutative, noncommutative and order arguments by
        combining related terms.

        Notes
        =====
            * In an expression like ``a*b*c``, python process this through sympy
              as ``Mul(Mul(a, b), c)``. This can have undesirable consequences.

              -  Sometimes terms are not combined as one would like:
                 {c.f. https://github.com/sympy/sympy/issues/4596}

                >>> from sympy import Mul, sqrt
                >>> from sympy.abc import x, y, z
                >>> 2*(x + 1) # this is the 2-arg Mul behavior
                2*x + 2
                >>> y*(x + 1)*2
                2*y*(x + 1)
                >>> 2*(x + 1)*y # 2-arg result will be obtained first
                y*(2*x + 2)
                >>> Mul(2, x + 1, y) # all 3 args simultaneously processed
                2*y*(x + 1)
                >>> 2*((x + 1)*y) # parentheses can control this behavior
                2*y*(x + 1)

                Powers with compound bases may not find a single base to
                combine with unless all arguments are processed at once.
                Post-processing may be necessary in such cases.
                {c.f. https://github.com/sympy/sympy/issues/5728}

                >>> a = sqrt(x*sqrt(y))
                >>> a**3
                (x*sqrt(y))**(3/2)
                >>> Mul(a,a,a)
                (x*sqrt(y))**(3/2)
                >>> a*a*a
                x*sqrt(y)*sqrt(x*sqrt(y))
                >>> _.subs(a.base, z).subs(z, a.base)
                (x*sqrt(y))**(3/2)

              -  If more than two terms are being multiplied then all the
                 previous terms will be re-processed for each new argument.
                 So if each of ``a``, ``b`` and ``c`` were :class:`Mul`
                 expression, then ``a*b*c`` (or building up the product
                 with ``*=``) will process all the arguments of ``a`` and
                 ``b`` twice: once when ``a*b`` is computed and again when
                 ``c`` is multiplied.

                 Using ``Mul(a, b, c)`` will process all arguments once.

            * The results of Mul are cached according to arguments, so flatten
              will only be called once for ``Mul(a, b, c)``. If you can
              structure a calculation so the arguments are most likely to be
              repeats then this can save time in computing the answer. For
              example, say you had a Mul, M, that you wished to divide by ``d[i]``
              and multiply by ``n[i]`` and you suspect there are many repeats
              in ``n``. It would be better to compute ``M*n[i]/d[i]`` rather
              than ``M/d[i]*n[i]`` since every time n[i] is a repeat, the
              product, ``M*n[i]`` will be returned without flattening -- the
              cached value will be returned. If you divide by the ``d[i]``
              first (and those are more unique than the ``n[i]``) then that will
              create a new Mul, ``M/d[i]`` the args of which will be traversed
              again when it is multiplied by ``n[i]``.

              {c.f. https://github.com/sympy/sympy/issues/5706}

              This consideration is moot if the cache is turned off.

            NB
            --
              The validity of the above notes depends on the implementation
              details of Mul and flatten which may change at any time. Therefore,
              you should only consider them when your code is highly performance
              sensitive.

              Removal of 1 from the sequence is already handled by AssocOp.__new__.
        """

        from sympy.calculus.util import AccumBounds
        rv = None
        if len(seq) == 2:
            a, b = seq
            if b.is_Rational:
                a, b = b, a
            assert not a is S.One
            if not a.is_zero and a.is_Rational:
                r, b = b.as_coeff_Mul()
                if b.is_Add:
                    if r is not S.One:  # 2-arg hack
                        # leave the Mul as a Mul
                        rv = [cls(a*r, b, evaluate=False)], [], None
                    elif b.is_commutative:
                        if a is S.One:
                            rv = [b], [], None
                        else:
                            r, b = b.as_coeff_Add()
                            bargs = [_keep_coeff(a, bi) for bi in Add.make_args(b)]
                            _addsort(bargs)
                            ar = a*r
                            if ar:
                                bargs.insert(0, ar)
                            bargs = [Add._from_args(bargs)]
                            rv = bargs, [], None
            if rv:
                return rv

        # apply associativity, separate commutative part of seq
        c_part = []         # out: commutative factors
        nc_part = []        # out: non-commutative factors

        nc_seq = []

        coeff = S.One       # standalone term
                            # e.g. 3 * ...

        c_powers = []       # (base,exp)      n
                            # e.g. (x,n) for x

        num_exp = []        # (num-base, exp)           y
                            # e.g.  (3, y)  for  ... * 3  * ...

        neg1e = S.Zero      # exponent on -1 extracted from Number-based Pow and I

        pnum_rat = {}       # (num-base, Rat-exp)          1/2
                            # e.g.  (3, 1/2)  for  ... * 3     * ...

        order_symbols = None

        # --- PART 1 ---
        #
        # "collect powers and coeff":
        #
        # o coeff
        # o c_powers
        # o num_exp
        # o neg1e
        # o pnum_rat
        #
        # NOTE: this is optimized for all-objects-are-commutative case
        for o in seq:
            # O(x)
            if o.is_Order:
                o, order_symbols = o.as_expr_variables(order_symbols)

            # Mul([...])
            if o.is_Mul:
                if o.is_commutative:
                    seq.extend(o.args)    # XXX zerocopy?

                else:
                    # NCMul can have commutative parts as well
                    for q in o.args:
                        if q.is_commutative:
                            seq.append(q)
                        else:
                            nc_seq.append(q)

                    # append non-commutative marker, so we don't forget to
                    # process scheduled non-commutative objects
                    seq.append(NC_Marker)

                continue

            # 3
            elif o.is_Number:
                if o is S.NaN or coeff is S.ComplexInfinity and o is S.Zero:
                    # we know for sure the result will be nan
                    return [S.NaN], [], None
                elif coeff.is_Number:  # it could be zoo
                    coeff *= o
                    if coeff is S.NaN:
                        # we know for sure the result will be nan
                        return [S.NaN], [], None
                continue

            elif isinstance(o, AccumBounds):
                coeff = o.__mul__(coeff)
                continue

            elif o is S.ComplexInfinity:
                if not coeff:
                    # 0 * zoo = NaN
                    return [S.NaN], [], None
                if coeff is S.ComplexInfinity:
                    # zoo * zoo = zoo
                    return [S.ComplexInfinity], [], None
                coeff = S.ComplexInfinity
                continue

            elif o is S.ImaginaryUnit:
                neg1e += S.Half
                continue

            elif o.is_commutative:
                #      e
                # o = b
                b, e = o.as_base_exp()

                #  y
                # 3
                if o.is_Pow:
                    if b.is_Number:

                        # get all the factors with numeric base so they can be
                        # combined below, but don't combine negatives unless
                        # the exponent is an integer
                        if e.is_Rational:
                            if e.is_Integer:
                                coeff *= Pow(b, e)  # it is an unevaluated power
                                continue
                            elif e.is_negative:    # also a sign of an unevaluated power
                                seq.append(Pow(b, e))
                                continue
                            elif b.is_negative:
                                neg1e += e
                                b = -b
                            if b is not S.One:
                                pnum_rat.setdefault(b, []).append(e)
                            continue
                        elif b.is_positive or e.is_integer:
                            num_exp.append((b, e))
                            continue

                    elif b is S.ImaginaryUnit and e.is_Rational:
                        neg1e += e/2
                        continue

                c_powers.append((b, e))

            # NON-COMMUTATIVE
            # TODO: Make non-commutative exponents not combine automatically
            else:
                if o is not NC_Marker:
                    nc_seq.append(o)

                # process nc_seq (if any)
                while nc_seq:
                    o = nc_seq.pop(0)
                    if not nc_part:
                        nc_part.append(o)
                        continue

                    #                             b    c       b+c
                    # try to combine last terms: a  * a   ->  a
                    o1 = nc_part.pop()
                    b1, e1 = o1.as_base_exp()
                    b2, e2 = o.as_base_exp()
                    new_exp = e1 + e2
                    # Only allow powers to combine if the new exponent is
                    # not an Add. This allow things like a**2*b**3 == a**5
                    # if a.is_commutative == False, but prohibits
                    # a**x*a**y and x**a*x**b from combining (x,y commute).
                    if b1 == b2 and (not new_exp.is_Add):
                        o12 = b1 ** new_exp

                        # now o12 could be a commutative object
                        if o12.is_commutative:
                            seq.append(o12)
                            continue
                        else:
                            nc_seq.insert(0, o12)

                    else:
                        nc_part.append(o1)
                        nc_part.append(o)

        # We do want a combined exponent if it would not be an Add, such as
        #  y    2y     3y
        # x  * x   -> x
        # We determine if two exponents have the same term by using
        # as_coeff_Mul.
        #
        # Unfortunately, this isn't smart enough to consider combining into
        # exponents that might already be adds, so things like:
        #  z - y    y
        # x      * x  will be left alone.  This is because checking every possible
        # combination can slow things down.

        # gather exponents of common bases...
        def _gather(c_powers):
            common_b = {}  # b:e
            for b, e in c_powers:
                co = e.as_coeff_Mul()
                common_b.setdefault(b, {}).setdefault(
                    co[1], []).append(co[0])
            for b, d in common_b.items():
                for di, li in d.items():
                    d[di] = Add(*li)
            new_c_powers = []
            for b, e in common_b.items():
                new_c_powers.extend([(b, c*t) for t, c in e.items()])
            return new_c_powers

        # in c_powers
        c_powers = _gather(c_powers)

        # and in num_exp
        num_exp = _gather(num_exp)

        # --- PART 2 ---
        #
        # o process collected powers  (x**0 -> 1; x**1 -> x; otherwise Pow)
        # o combine collected powers  (2**x * 3**x -> 6**x)
        #   with numeric base

        # ................................
        # now we have:
        # - coeff:
        # - c_powers:    (b, e)
        # - num_exp:     (2, e)
        # - pnum_rat:    {(1/3, [1/3, 2/3, 1/4])}

        #  0             1
        # x  -> 1       x  -> x

        # this should only need to run twice; if it fails because
        # it needs to be run more times, perhaps this should be
        # changed to a "while True" loop -- the only reason it
        # isn't such now is to allow a less-than-perfect result to
        # be obtained rather than raising an error or entering an
        # infinite loop
        for i in range(2):
            new_c_powers = []
            changed = False
            for b, e in c_powers:
                if e.is_zero:
                    continue
                if e is S.One:
                    if b.is_Number:
                        coeff *= b
                        continue
                    p = b
                if e is not S.One:
                    p = Pow(b, e)
                    # check to make sure that the base doesn't change
                    # after exponentiation; to allow for unevaluated
                    # Pow, we only do so if b is not already a Pow
                    if p.is_Pow and not b.is_Pow:
                        bi = b
                        b, e = p.as_base_exp()
                        if b != bi:
                            changed = True
                c_part.append(p)
                new_c_powers.append((b, e))
            # there might have been a change, but unless the base
            # matches some other base, there is nothing to do
            if changed and len(set(
                    b for b, e in new_c_powers)) != len(new_c_powers):
                # start over again
                c_part = []
                c_powers = _gather(new_c_powers)
            else:
                break

        #  x    x     x
        # 2  * 3  -> 6
        inv_exp_dict = {}   # exp:Mul(num-bases)     x    x
                            # e.g.  x:6  for  ... * 2  * 3  * ...
        for b, e in num_exp:
            inv_exp_dict.setdefault(e, []).append(b)
        for e, b in inv_exp_dict.items():
            inv_exp_dict[e] = cls(*b)
        c_part.extend([Pow(b, e) for e, b in inv_exp_dict.items() if e])

        # b, e -> e' = sum(e), b
        # {(1/5, [1/3]), (1/2, [1/12, 1/4]} -> {(1/3, [1/5, 1/2])}
        comb_e = {}
        for b, e in pnum_rat.items():
            comb_e.setdefault(Add(*e), []).append(b)
        del pnum_rat
        # process them, reducing exponents to values less than 1
        # and updating coeff if necessary else adding them to
        # num_rat for further processing
        num_rat = []
        for e, b in comb_e.items():
            b = cls(*b)
            if e.q == 1:
                coeff *= Pow(b, e)
                continue
            if e.p > e.q:
                e_i, ep = divmod(e.p, e.q)
                coeff *= Pow(b, e_i)
                e = Rational(ep, e.q)
            num_rat.append((b, e))
        del comb_e

        # extract gcd of bases in num_rat
        # 2**(1/3)*6**(1/4) -> 2**(1/3+1/4)*3**(1/4)
        pnew = defaultdict(list)
        i = 0  # steps through num_rat which may grow
        while i < len(num_rat):
            bi, ei = num_rat[i]
            grow = []
            for j in range(i + 1, len(num_rat)):
                bj, ej = num_rat[j]
                g = bi.gcd(bj)
                if g is not S.One:
                    # 4**r1*6**r2 -> 2**(r1+r2)  *  2**r1 *  3**r2
                    # this might have a gcd with something else
                    e = ei + ej
                    if e.q == 1:
                        coeff *= Pow(g, e)
                    else:
                        if e.p > e.q:
                            e_i, ep = divmod(e.p, e.q)  # change e in place
                            coeff *= Pow(g, e_i)
                            e = Rational(ep, e.q)
                        grow.append((g, e))
                    # update the jth item
                    num_rat[j] = (bj/g, ej)
                    # update bi that we are checking with
                    bi = bi/g
                    if bi is S.One:
                        break
            if bi is not S.One:
                obj = Pow(bi, ei)
                if obj.is_Number:
                    coeff *= obj
                else:
                    # changes like sqrt(12) -> 2*sqrt(3)
                    for obj in Mul.make_args(obj):
                        if obj.is_Number:
                            coeff *= obj
                        else:
                            assert obj.is_Pow
                            bi, ei = obj.args
                            pnew[ei].append(bi)

            num_rat.extend(grow)
            i += 1

        # combine bases of the new powers
        for e, b in pnew.items():
            pnew[e] = cls(*b)

        # handle -1 and I
        if neg1e:
            # treat I as (-1)**(1/2) and compute -1's total exponent
            p, q =  neg1e.as_numer_denom()
            # if the integer part is odd, extract -1
            n, p = divmod(p, q)
            if n % 2:
                coeff = -coeff
            # if it's a multiple of 1/2 extract I
            if q == 2:
                c_part.append(S.ImaginaryUnit)
            elif p:
                # see if there is any positive base this power of
                # -1 can join
                neg1e = Rational(p, q)
                for e, b in pnew.items():
                    if e == neg1e and b.is_positive:
                        pnew[e] = -b
                        break
                else:
                    # keep it separate; we've already evaluated it as
                    # much as possible so evaluate=False
                    c_part.append(Pow(S.NegativeOne, neg1e, evaluate=False))

        # add all the pnew powers
        c_part.extend([Pow(b, e) for e, b in pnew.items()])

        # oo, -oo
        if (coeff is S.Infinity) or (coeff is S.NegativeInfinity):
            def _handle_for_oo(c_part, coeff_sign):
                new_c_part = []
                for t in c_part:
                    if t.is_positive:
                        continue
                    if t.is_negative:
                        coeff_sign *= -1
                        continue
                    new_c_part.append(t)
                return new_c_part, coeff_sign
            c_part, coeff_sign = _handle_for_oo(c_part, 1)
            nc_part, coeff_sign = _handle_for_oo(nc_part, coeff_sign)
            coeff *= coeff_sign

        # zoo
        if coeff is S.ComplexInfinity:
            # zoo might be
            #   infinite_real + bounded_im
            #   bounded_real + infinite_im
            #   infinite_real + infinite_im
            # and non-zero real or imaginary will not change that status.
            c_part = [c for c in c_part if not (fuzzy_not(c.is_zero) and
                                                c.is_real is not None)]
            nc_part = [c for c in nc_part if not (fuzzy_not(c.is_zero) and
                                                  c.is_real is not None)]

        # 0
        elif coeff is S.Zero:
            # we know for sure the result will be 0 except the multiplicand
            # is infinity
            if any(c.is_finite == False for c in c_part):
                return [S.NaN], [], order_symbols
            return [coeff], [], order_symbols

        # check for straggling Numbers that were produced
        _new = []
        for i in c_part:
            if i.is_Number:
                coeff *= i
            else:
                _new.append(i)
        c_part = _new

        # order commutative part canonically
        _mulsort(c_part)

        # current code expects coeff to be always in slot-0
        if coeff is not S.One:
            c_part.insert(0, coeff)

        # we are done
        if (not nc_part and len(c_part) == 2 and c_part[0].is_Number and
                c_part[1].is_Add):
            # 2*(1+a) -> 2 + 2 * a
            coeff = c_part[0]
            c_part = [Add(*[coeff*f for f in c_part[1].args])]

        return c_part, nc_part, order_symbols
dataset/ETHPy150Open sympy/sympy/sympy/core/mul.py/Mul.flatten/original