DirichletBC: sometimes `.function_arg` exists, sometimes it doesn't

Question

When defining a DirichletBC, one might want to access its values after the object has been created (e.g., to alter a parameter in it in case it is an Expression).

I've always used function_arg for it, i.e.,

d1 = DirichletBC(V, '0.0', 'on_boundary')
d1.function_arg.t = 42.0

but now I noticed that it just _does not exist_ in some cases. A common example would be

ex1 = Function(V)
d2 = DirichletBC(V, ex1, 'on_boundary')
print(d2.function_arg)
# AttributeError: 'DirichletBC' object has no attribute 'function_arg'

What is the reason for this?

Answer 1

It was never meant to be used. It is there to keep a reference to an Expression avoiding garbage collection in Python. The easiest thing for you is probably to keep a reference to your value either outside the DirichletBC or just attach it to it as an attribute your self.