[feat] Support parameter and variable access directly from the class body

docs/regression_test_api.rst

@@ -59,6 +59,7 @@ In essence, these builtins exert control over the test creation, and they allow
 
     class Foo(rfm.RegressionTest):
         variant = parameter(['A', 'B'])
+        # print(variant) # Error: a parameter may only be accessed from the class instance.
 
         def __init__(self):
             if self.variant == 'A':
@@ -67,8 +68,9 @@ In essence, these builtins exert control over the test creation, and they allow
                 do_other()
 
   One of the most powerful features about these built-in functions is that they store their input information at the class level. 
-  This means if one were to extend or specialize an existing regression test, the test attribute additions and modifications made through built-in functions in the parent class will be automatically inherited by the child test.
-  For instance, continuing with the example above, one could override the :func:`__init__` method in the :class:`MyTest` regression test as follows:
+  However, a parameter may only be accessed from the class instance and accessing it directly from the class body is disallowed.
+  With this approach, extending or specializing an existing parametrized regression test becomes straightforward, since the test attribute additions and modifications made through built-in functions in the parent class are automatically inherited by the child test.
+  For instance, continuing with the example above, one could override the :func:`__init__` method in the :class:`Foo` regression test as follows:
 
   .. code:: python
 
@@ -124,7 +126,7 @@ In essence, these builtins exert control over the test creation, and they allow
 
     class Foo(rfm.RegressionTest):
         my_var = variable(int, value=8)
-        not_a_var = 4
+        not_a_var = my_var - 4
 
         def __init__(self):
             print(self.my_var) # prints 8.
@@ -133,13 +135,15 @@ In essence, these builtins exert control over the test creation, and they allow
             self.my_var = 10 # tests may also assign values the standard way
 
   The argument ``value`` in the :func:`variable` built-in sets the default value for the variable.
-  As mentioned above, a variable may not be declared more than once, but its default value can be updated by simply assigning it a new value directly in the class body.
+  Note that a variable may be accesed directly from the class body as long as its value was previously assigned in the same class body.
+  As mentioned above, a variable may not be declared more than once, but its default value can be updated by simply assigning it a new value directly in the class body. However, a variable may only be acted upon once in the same class body.
 
   .. code:: python
 
     class Bar(Foo):
         my_var = 4
         # my_var = 'override' # Error again!
+        # my_var = 8 # Error: Double action on `my_var` is not allowed.
 
         def __init__(self):
             print(self.my_var) # prints 4.

reframe/core/meta.py

@@ -28,6 +28,38 @@ def __setitem__(self, key, value):
             else:
                 super().__setitem__(key, value)
 
+        def __getitem__(self, key):
+            '''Expose and control access to the local namespaces.
+
+            Variables may only be retrieved if their value has been previously
+            set. Accessing a parameter in the class body is disallowed (the
+            actual test parameter is set during the class instantiation).
+            '''
+            try:
+                return super().__getitem__(key)
+            except KeyError as err:
+                try:
+                    # Handle variable access
+                    var = self['_rfm_local_var_space'][key]
+                    if var.is_defined():
+                        return var.default_value
+                    else:
+                        raise ValueError(
+                            f'variable {key!r} is not assigned a value'
+                        )
+
+                except KeyError:
+                    # Handle parameter access
+                    if key in self['_rfm_local_param_space']:
+                        raise ValueError(
+                            'accessing a test parameter from the class '
+                            'body is disallowed'
+                        )
+                    else:
+                        # If 'key' is neither a variable nor a parameter,
+                        # raise the exception from the base __getitem__.
+                        raise err from None
+
     @classmethod
     def __prepare__(metacls, name, bases, **kwargs):
         namespace = super().__prepare__(name, bases, **kwargs)

reframe/core/pipeline.py

@@ -138,9 +138,22 @@ class RegressionMixin(metaclass=RegressionTestMeta):
 
     .. versionadded:: 3.4.2
     '''
+    def __getattribute__(self, name):
+        try:
+            return super().__getattribute__(name)
+        except AttributeError:
+            # Intercept the AttributeError if the name corresponds to a
+            # required variable.
+            if (name in self._rfm_var_space.vars and
+                not self._rfm_var_space.vars[name].is_defined()):
+                raise AttributeError(
+                    f'required variable {name!r} has not been set'
+                ) from None
+            else:
+                super().__getattr__(name)
 
 
-class RegressionTest(jsonext.JSONSerializable, metaclass=RegressionTestMeta):
+class RegressionTest(RegressionMixin, jsonext.JSONSerializable):
     '''Base class for regression tests.
 
     All regression tests must eventually inherit from this class.
@@ -764,20 +777,6 @@ def __new__(cls, *args, _rfm_use_params=False, **kwargs):
     def __init__(self):
         pass
 
-    def __getattribute__(self, name):
-        try:
-            return super().__getattribute__(name)
-        except AttributeError:
-            # Intercept the AttributeError if the name corresponds to a
-            # required variable.
-            if (name in self._rfm_var_space.vars and
-                not self._rfm_var_space.vars[name].is_defined()):
-                raise AttributeError(
-                    f'required variable {name!r} has not been set'
-                ) from None
-            else:
-                super().__getattr__(name)
-
     def _append_parameters_to_name(self):
         if self._rfm_param_space.params:
             return '_' + '_'.join([util.toalphanum(str(self.__dict__[key]))

unittests/test_parameters.py

@@ -228,3 +228,10 @@ class Bar(Base):
     assert Foo(_rfm_use_params=True).p0.val == -20
     assert Bar(_rfm_use_params=True).p0.val == 1
     assert Bar(_rfm_use_params=True).p0.val == 2
+
+
+def test_param_access():
+    with pytest.raises(ValueError):
+        class Foo(rfm.RegressionTest):
+            p = parameter([1, 2, 3])
+            x = f'accessing {p!r} in the class body is disallowed.'

unittests/test_variables.py

@@ -180,3 +180,15 @@ class Bar(Base):
 
     assert Foo().my_var == [1, 2, 3]
     assert Bar().my_var == [1, 2]
+
+
+def test_variable_access():
+    class Foo(rfm.RegressionMixin):
+        my_var = variable(str, value='bananas')
+        x = f'accessing {my_var!r} works because it has a default value.'
+
+    assert 'bananas' in getattr(Foo, 'x')
+    with pytest.raises(ValueError):
+        class Foo(rfm.RegressionMixin):
+            my_var = variable(int)
+            x = f'accessing {my_var!r} fails because its value is not set.'