Is this a bug?

[Chengpu Wang]

Hi, I am a new user of SymPy.  I wonder if the following result is a bug?  Anyway, the following code is a nuance.   I am using the latest SymPy

class TestSymPy (unittest.TestCase):

    def testSin(self):

        '''

        The expansion is not ordered by x**n: x**7 before x**5

        '''

        x = sympy.Symbol('x')

        sin = sympy.sin(x)

        self.assertTupleEqual(sin.args, (x,))

        e = sin.series(x, 0, 10)

        self.assertTupleEqual(e.args, (x, -x**3/6, -x**7/5040, x**5/120, x**9/362880, sympy.O(x**10)))

[emanuel.c...@gmail.com]

Doesn't matter :

```

>>> from sympy import *
>>> x=symbols("x")
>>> sin(x).series(x,0,10).args
(x, -x**3/6, -x**7/5040, x**5/120, x**9/362880, O(x**10))

```

The arguments tuple doesn't order `x` powers (and this isn't a problem, since (complex) multiplication is commutative. But

```
>>> sin(x).series(x,0,10)
x - x**3/6 + x**5/120 - x**7/5040 + x**9/362880 + O(x**10)
```

The expression is still printed with ordered powers of `x`.

HTH,

[Chengpu Wang]

How do I detect which order of x?

[Oscar Benjamin]

Ideally series would produce something more like a Poly object rather
than just an Add with an O() term. You can use Poly to get the
coefficients in order:

In [6]: sin(x).series(x,0,10).removeO().as_poly(x).coeffs()[::-1]
Out[6]: [1, -1/6, 1/120, -1/5040, 1/362880]

In [7]: sin(x).series(x,0,10).removeO().as_poly(x).all_coeffs()[::-1]
Out[7]: [0, 1, 0, -1/6, 0, 1/120, 0, -1/5040, 0, 1/362880]

--
Oscar

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