[pytest-dev] fixture params must be immutable?

[Ofer Nave]

Been using pytest for a few days and enjoying it greatly.  However, the 
last bit of code I wrote just blew up with "INTERNALERROR> TypeError: 
unhashable type: 'dict'".

It seems that if you parameterize a fixture function, it must be with a 
list of immutable values.  This works:

@pytest.fixture(params=[1, 2, 3])
def foo(request):
     return request.param
def bar(foo):
     assert foo

This doesn't:

@pytest.fixture(params=[{a:1}, {a:2}, {a:3}])
def foo(request):
     return request.param
def bar(foo):
     assert foo

...because dict is immutable.

I didn't see this requirement mentioned in the docs, and I don't 
understand why it would be the case.  Looking for enlightenment.

-ofer