Update ft11_magnetostatics.py

[benjaminknebusch]

Expression needs Cpp code to be provided use UserExpression instead

[smartalecH]

UserExpression throws an error:

AttributeError: 'Permeability' object has no attribute '_ufl_shape'

Any ideas on a fix?

[smartalecH]

It looks like in addition to changing Expression to UserExpression, you need to initialize the superclass. The child class definition should look like this:

class Permeability(UserExpression):
    def __init__(self, markers, **kwargs):
        super().__init__(**kwargs) # This part is new!
        self.markers = markers
    def eval_cell(self, values, x, cell):
        if self.markers[cell.index] == 0:
            values[0] = 4*pi*1e-7 # vacuum
        elif self.markers[cell.index] == 1:
            values[0] = 1e-5      # iron (should really be 6.3e-3)
        else:
            values[0] = 1.26e-6   # copper

Finally, to get the script to run with 2019.1.0, you also need to remove Interactive() and rely on pyplot (as mentioned in other PRs).