adding a test.

Author: mmatera

test/test_custom_boxconstruct.py

@@ -0,0 +1,46 @@
+from .helper import session, check_evaluation
+import sys
+import pytest
+
+from mathics.core.expression import (Symbol, Atom, String, Expression)
+from mathics.builtin.base import BoxConstruct, AtomBuiltin, Predefined
+
+class CustomBoxConstruct(BoxConstruct):
+    def __init__(self, evaluation):
+        super().__init__(evaluation=evaluation)
+        self._leaves = [1,2,3]
+
+    def boxes_to_text(self, leaves=None, **options):
+        if not leaves:
+            leaves = self._leaves
+        return 'CustomBoxConstruct<<' + self._leaves.__str__()   + '>>'
+
+    def boxes_to_xml(self, leaves=None, **options):
+        if not leaves:
+            leaves = self._leaves
+        return 'CustomBoxConstruct<<' + self._leaves.__str__()   + '>>'
+
+    def boxes_to_tex(self, leaves=None, **options):
+        if not leaves:
+            leaves = self._leaves
+        return 'CustomBoxConstruct<<' + int(self_leaves)   + '>>'
+
+class CustomAtom(Predefined):
+    """
+    just a test
+    """
+    context = "System`"
+    rules = {"N[System`CustomAtom]": "37", }
+    
+    def apply_to_boxes(self, evaluation):
+        'System`MakeBoxes[System`CustomAtom, StandardForm|TraditionalForm|OutputForm|InputForm]'
+        return CustomBoxConstruct(evaluation=evaluation)
+
+def test_custom_boxconstruct():
+    defs = session.evaluation.definitions
+    instance_custom_atom = CustomAtom(expression=False)
+    instance_custom_atom.contribute(defs, is_pymodule=True)
+    expr = session.evaluate("MakeBoxes[CustomAtom, InputForm]")
+    formatted = session.format_result().boxes_to_xml()
+    assert formatted == "CustomBoxConstruct<<[1, 2, 3]>>"
+