def test_expr_units():
'''
Test getting/checking the units of an expression.
'''
tau = 5 * ms
expr = Expression('-v / tau', namespace={'tau': tau})
expr.resolve(['v'])
expr.check_units(volt / second, {'v': volt})
assert_raises(DimensionMismatchError, lambda: expr.check_units(volt / second,
{'v': second}))
assert_raises(DimensionMismatchError, lambda: expr.check_units(volt,
{'v': volt}))
assert expr.get_dimensions({'v': volt}) == get_dimensions(volt / second)
