# Force Convert a Polynomial Ring into Multivariate Polynomial

I have the following code

```
xi = var('xi')
eta = var('eta')
P = 3*eta - 1
Pr = P.polynomial(QQ)
Pr.coefficients(sparse=False)
```

which gives the output

```
[-1,3]
```

I would like to get the output in terms of both `xi`

and `eta`

that is, I want the output to be `[-1,0,3]`

. How to force that? The reason I ask this is because my P is actually a mode in list of polyomials and generally function of both `xi`

and `eta`

and for different modes, either one or the other variable might not be there in the expression at all, and I want a general way to get the coefficients in a list corresponding to the coefficients of the terms `1,xi,eta,xi^2,xi *eta, eta^2`

and so on.