wip: start to try string tests

bindings/pydrake/multibody/_inertia_fixer.py

@@ -105,6 +105,14 @@ def format_inertia(self, input_text: str, inertial_facts: ElementFacts,
         raise NotImplementedError
 
 
+def is_synth_element(el: ElementFacts) -> bool:
+    """Return true if the ElementFacts is synthetic.
+
+    Synthetic elements are placeholders for later expansion into output XML.
+    """
+    return el.end.index == el.start.index
+
+
 def make_synth_element(parent: ElementFacts, name: str) -> ElementFacts:
     """Make a new synthetic child ElementFacts for a given parent ElementFacts.
     The parent must already have at least one child.
@@ -116,7 +124,7 @@ def make_synth_element(parent: ElementFacts, name: str) -> ElementFacts:
     kid0 = parent.children[0]
     synth_el = ElementFacts(kid0.prior_data, name, {},
                             kid0.depth, kid0.start)
-    synth_el.end = kid0.start
+    synth_el.end = synth_el.start
     return synth_el
 
 
@@ -152,19 +160,20 @@ def adjusted_element_end_index(input_text: str, facts: ElementFacts) -> int:
     """Returns the index of the end of all of the text (including the element
     closure), when given an ElementFacts produced by the expat parse.
     """
-    if facts.start.index == facts.end.index:
+    if is_synth_element(facts):
         # Empty, synthetic, "pseudo" element.
         return facts.end.index
+
     if input_text[facts.end.index:].startswith(f"</{facts.name}"):
         # Typical case:
         # `<inertial>...</inertial>`.
         #               ^                # Raw index points here.
         #                          ^     # Adjusted index points here.
         return facts.end.index + input_text[facts.end.index:].find('>') + 1
-    else:
-        # `<inertial/>` corner case.
-        #             ^     # Raw index points here; good to go
-        return facts.end.index
+
+    # `<inertial/>` corner case.
+    #             ^     # Raw index points here; good to go
+    return facts.end.index
 
 
 def make_format_helpers(facts: ElementFacts) -> (

bindings/pydrake/multibody/test/fix_inertia_test.py

@@ -9,10 +9,28 @@
     temp_directory,
 )
 from pydrake.multibody._inertia_fixer import (
+    fix_inertia_from_string,
     InertiaFixer,
 )
 
 
+class TestFixInertiaFromString(unittest.TestCase):
+    def test_min_urdf(self):
+        input_text = '<robot name="a"/>'
+        output = fix_inertia_from_string(input_text, "urdf")
+        self.assertEqual(output, input_text)
+
+    def test_min_sdf(self):
+        input_text = """\
+<sdf version="1.6">
+<model name="a">
+  <link name="b"/>
+</model>
+</sdf>"""
+        output = fix_inertia_from_string(input_text, "sdf")
+        self.assertEqual(output, input_text)
+
+
 class TestInertiaFixer(unittest.TestCase):
     def setUp(self):
         self._temp_dir = Path(temp_directory())