Use difflib for nicer display of changes by adjust and policies

tmt/base.py

@@ -62,6 +62,7 @@
 import tmt.utils
 import tmt.utils.git
 import tmt.utils.jira
+import tmt.utils.templates
 from tmt._compat.typing import Self
 from tmt.checks import Check
 from tmt.container import (
@@ -390,6 +391,14 @@ def resolve_dynamic_ref(self, git_repository: Path, plan: 'Plan') -> None:
 )
 
 
+#: A template showing changes made by an ``adjust`` rule.
+NODE_ADJUST_DIFF_TEMPLATE = """
+{{ RULE | trim }}
+
+{{ DIFF | join('\n') }}
+"""
+
+
 def create_adjust_callback(logger: tmt.log.Logger) -> fmf.base.AdjustCallback:
     """
     Create a custom callback for fmf's adjust.
@@ -399,10 +408,15 @@ def create_adjust_callback(logger: tmt.log.Logger) -> fmf.base.AdjustCallback:
     a callback closure with the given logger.
     """
 
-    def callback(node: fmf.Tree, rule: _RawAdjustRule, applied: Optional[bool]) -> None:
+    def callback(
+        node: fmf.Tree,
+        rule: _RawAdjustRule,
+        applied: Optional[bool],
+        before: Optional[fmf.Tree] = None,
+    ) -> None:
         if applied is None:
             logger.verbose(
-                f"Adjust rule skipped on '{node.name}'",
+                f"fmf node '{node.name}' skipped by adjust rule, condition undecided",
                 tmt.utils.format_value(rule, key_color='cyan'),
                 color='blue',
                 level=3,
@@ -411,17 +425,24 @@ def callback(node: fmf.Tree, rule: _RawAdjustRule, applied: Optional[bool]) -> N
 
         elif applied is False:
             logger.verbose(
-                f"Adjust rule not applied to '{node.name}'",
+                f"fmf node '{node.name}' not modified by adjust rule",
                 tmt.utils.format_value(rule, key_color='cyan'),
                 color='red',
                 level=3,
                 topic=tmt.log.Topic.ADJUST_DECISIONS,
             )
 
         else:
+            assert before is not None
+
             logger.verbose(
-                f"Adjust rule applied to '{node.name}'",
-                tmt.utils.format_value(rule, key_color='cyan'),
+                f"fmf node '{node.name}' modified by adjust rule",
+                tmt.utils.templates.render_diff(
+                    NODE_ADJUST_DIFF_TEMPLATE,
+                    before.data,
+                    node.data,
+                    RULE=tmt.utils.format_value(rule, key_color='cyan'),
+                ),
                 color='green',
                 level=3,
                 topic=tmt.log.Topic.ADJUST_DECISIONS,

tmt/policy.py

@@ -7,7 +7,7 @@
 from tmt.container import Extra, MetadataContainer, metadata_field
 from tmt.log import Logger, Topic
 from tmt.utils import FieldValueSource, Path, ShellScript
-from tmt.utils.templates import render_template
+from tmt.utils.templates import render_diff, render_template
 
 if TYPE_CHECKING:
     from tmt.base import Core, Test
@@ -17,8 +17,7 @@
 
 #: A template showing changes made by an instruction.
 KEY_DIFF_TEMPLATE = """
-{{ OLD_VALUE | to_yaml | prefix('- ') | style(fg='red') | trim }}
-{{ NEW_VALUE | to_yaml | prefix('+ ') | style(fg='green') | trim }}
+{{ DIFF | join('\n') }}
 
 Field value source changed from {{ OLD_VALUE_SOURCE.value | style(fg='red') }} to {{ NEW_VALUE_SOURCE.value | style(fg='green') }}
 """  # noqa: E501
@@ -128,11 +127,11 @@ def set_key(
                 current_value_source = obj._field_value_sources[key] = FieldValueSource.POLICY
 
                 logger.info(
-                    f"Modified '{obj.name}'",
-                    render_template(
+                    f"fmf node '{obj.name}' modified by policy",
+                    render_diff(
                         KEY_DIFF_TEMPLATE,
-                        OLD_VALUE={key: old_value_exported},
-                        NEW_VALUE={key: current_value_exported},
+                        {key: old_value_exported},
+                        {key: current_value_exported},
                         OLD_VALUE_SOURCE=old_value_source,
                         NEW_VALUE_SOURCE=current_value_source,
                     ),

tmt/utils/templates.py

@@ -5,9 +5,11 @@
 custom filters.
 """
 
+import difflib
 import re
 import shlex
 import textwrap
+from collections.abc import Iterable
 from re import Match
 from typing import (
     TYPE_CHECKING,
@@ -563,3 +565,50 @@ def render_template_file_into_file(
     )
 
     output_filepath.append_text('\n')
+
+
+def render_diff(
+    template: str,
+    left: Any,
+    right: Any,
+    **variables: Any,
+) -> str:
+    """
+    Render a template showing diff between two data structures.
+
+    Both objects are turned into their YAML representation, and then
+    both representations are compared for lines missing and added:
+
+    * lines unique to the ``left`` object will be prefixed with a
+      ``-`` string, and "negatively" colorized (red by default).
+    * lines unique to the ``right`` object will be prefixed with a
+      ``+`` string, and "positively" colorized (green by default).
+
+    :param template: template to render. Variable ``DIFF`` will contain
+        colorized lines of the diff.
+    :param left: object on the left side of the comparison.
+    :param right: object on the right side of the comparison
+    """
+
+    from tmt.utils.themes import style
+
+    def _render_diff() -> Iterable[str]:
+        """
+        Compare two objects and render "missing" and "added" lines.
+        """
+
+        for line in difflib.ndiff(
+            dict_to_yaml(left).splitlines(), dict_to_yaml(right).splitlines()
+        ):
+            # See https://docs.python.org/3/library/difflib.html#difflib.Differ for "markers"
+            # at beginnings of lines
+
+            # '- ' - unique in "before"
+            if line.startswith('- '):
+                yield style(line, fg='red')
+
+            # '+ ' - unique in "after"
+            elif line.startswith('+ '):
+                yield style(line, fg='green')
+
+    return render_template(template, DIFF=_render_diff(), **variables)