Customized Expression class example does not work

[Simon Funke]

I tried to run the customized Expression example in the dolfin documentation (last code example in https://fenicsproject.org/documentation/dolfin/dev/python/programmers-reference/functions/expression/Expression.html)

A slightly simplified version of this script is:

from fenics import *
mesh = UnitIntervalMesh(10)

class MyExpression1(Expression):
    def __init__(self, mesh):
        self._mesh = mesh
    def eval(self, values, x):
        pass

MyExpression1(degree=0, mesh=mesh)

However, when running this example on dolfin master, one gets:

git:master$ python test.py 
Traceback (most recent call last):
  File "test.py", line 10, in <module>
    MyExpression1(degree=0, mesh=mesh)
  File "/home/simon/src/fenics_master/dolfin_master/local.master/lib/python2.7/site-packages/dolfin/functions/expression.py", line 282, in __init__
    user_init(self, *args, **kwargs)
TypeError: __init__() got an unexpected keyword argument 'degree'
Aborted (core dumped)

[Miklós Homolya]

The __init__ method of your MyExpression1 class clearly does not expect a degree argument. Did you mean to write

    def __init__(self, mesh, **kwargs):
        super(MyExpression1, self).__init__(**kwargs)
        self._mesh = mesh

and then MyExpression1(mesh=mesh, degree=0)?

Edit: ah, you probably mean that the documentation is wrong...

[Simon Funke]

Yes, I was referring to the documentation. In other words, the documentation should be updated to:

from fenics import *
mesh = UnitIntervalMesh(10)

class MyExpression1(Expression):
    def __init__(self, **kwargs):
        self._mesh = kwargs["mesh"]
    def eval(self, values, x):
        pass

MyExpression1(degree=0, mesh=mesh)

Ill make a pull request

[Miklós Homolya]

Well, your proposed alternative might work, but is super misleading. kwargs is never used, so the parameters degree=0 and mesh=mesh are thrown away, never used, and the constructor of the super class is never called. self._mesh = mesh will still work though, because it picks up the global variable mesh = UnitIntervalMesh(10).

[Jan Blechta]

Not sure whether it changed recently, but Expression.__init__(self, *foo, **bar) did not use to work from Expression subclass. (I made it an error to avoid confusion.) There is (or at least was) a lot of metaclass magic and utilization of new and unused kwargs in Simon's example might be actually used there.

[Mikael Mortensen]

That's actually not true. But there's some magic to it, and you cannot call the base class__init__ because of this magic. See documentation of Expression:

The user can customize initialization of derived Expressions.
However, because of magic behind the scenes, a user needs to
pass optional arguments to __init__ using ``**kwargs``, and
_not_ calling the base class __init__:

If you check out m=MyExpression1(degree=0, mesh=mesh) and m.element you will see that it's MyExpression1.element of Coefficient(FunctionSpace(None, FiniteElement('Discontinuous Lagrange', None, 0)), 2). Try sending in degree=2.

[Simon Funke]

Sorry, my previous implementation is clearly wrong. It is fixed now.

[Jan Blechta]

On the other hand I agree that the current state is confusing. But there does not seem to be an obvious fix.

[Jan Blechta]

Yes, @mikael_mortensen expressed it clearly.

[Jan Blechta]

The other issue is that failing MWE from the description fails with TypeError: init() got an unexpected keyword argument 'degree' rather than expression without element or degree are illega`.

EDIT: Sorry, I can't read.

[Jan Blechta]

Fixed by pull request #305.