I want to define my own field. For example, pressure.

```python

fields = {
    'temperature' : ('real', 1, 'Omega', 1),
    'pressure' : ('real', 1, 'Omega', 1),
}

variables = {
    't' : ('unknown field', 'temperature', 0),
    's' : ('test field',    'temperature', 't'),
    'p' : ( 'parameter field', 'pressure', {'setter' : 'get_pressure'} ),
}


def get_pressure(ts, coors, region=None):
    z = coors[:,2]
    return z

functions = {
        'get_pressure': (get_pressure,),
        }

```

The idea is to create very primitive field that depends on z coordinates.

Now I want to export the grad(p) to the output mesh. And so I add this in postprocess function:

```python

p_grad = pb.evaluate('ev_grad.i.Omega(p)', mode='el_avg', verbose=False)
out['p'] = Struct(name='output_data', mode='cell', data=p_grad, dofs=None)

```

But I get the following error:

ValueError: argument p not found!

What I'm doing wrong?

And how can I export the pressure field itself (not its gradient)?

The answer from Robert Cimrman:

Hi,

In a post-processing function, the only variables that are defined are those
that are used in the equations. If p is not used, you need to create it, as
shown below (also how to save p and its cell averages.

def post_process(out, pb, state, extend=False):
     from sfepy.base.base import Struct

     pvar = pb.create_variables(['p'])['p']
     pvar.time_update(None, pb.functions)

     out.update(pvar.create_output())

     ap = pb.evaluate('ev_volume_integrate.i.Omega(p)', mode='el_avg', p=pvar,
                      verbose=False)
     out['ap'] = Struct(name='output_data', mode='cell', data=ap, dofs=None)

     p_grad = pb.evaluate('ev_grad.i.Omega(p)', mode='el_avg', p=pvar,
                          verbose=False)
     out['gp'] = Struct(name='output_data', mode='cell', data=p_grad, dofs=None)

     return out

r.

PS: This might be interesting to others, so, if you do not mind, you can
forward it to the list.