feat: add because rule rationales

BACKLOG.md

@@ -11,8 +11,8 @@ This backlog collects product and maintenance ideas from project research.
 
 ## P1 - Adoption Workflow
 
-- Add an `.archignore` or similar file, modeled after `.gitignore`, for files that should never be analyzed.
-- Add a `.because(...)` API so rules can carry user-facing rationale into failure messages and generated architecture documentation.
+- [x] Add an `.archignore` or similar file, modeled after `.gitignore`, for files that should never be analyzed.
+- [x] Add a `.because(...)` API so rules can carry user-facing rationale into failure messages and generated architecture documentation.
 - Add configuration-file support for common rules, while keeping the fluent Python API as the primary interface.
 - Add support for monorepo and multi-package Python projects.
 

README.md

@@ -137,6 +137,44 @@ options = CheckOptions(
 violations = rule.check(options)
 ```
 
+### Excluding Files With `.archignore`
+
+Add a `.archignore` file to your project root to permanently exclude generated or
+irrelevant files from architecture checks and file-based metrics:
+
+```gitignore
+# Generated code
+generated/
+
+# Migration scripts
+migrations/*.py
+
+# A single root-level file
+/legacy_adapter.py
+```
+
+Patterns support comments, blank lines, glob syntax, root-relative paths, path
+patterns, and directory patterns with a trailing `/`.
+
+### Explaining Rules With `.because(...)`
+
+Attach a rationale to a rule so failing assertions explain why the rule exists:
+
+```python
+rule = (
+    project_files("src/")
+    .in_folder("**/controllers/**")
+    .should_not()
+    .depend_on_files()
+    .in_folder("**/database/**")
+    .because("controllers should stay thin and delegate persistence")
+)
+
+assert_passes(rule)
+```
+
+When the rule fails, the rationale is included in the assertion message.
+
 ## 🐹 Use Cases
 
 Here is an overview of common use cases.

scripts/check_release_metadata.py

@@ -17,7 +17,7 @@ def read_project_version() -> str:
     content = PYPROJECT.read_text(encoding="utf-8")
     match = re.search(r'^version = "([^"]+)"$', content, re.MULTILINE)
     if match is None:
-        raise RuntimeError("Could not find project.version in pyproject.toml")
+        raise RuntimeError("Could not find [project].version in pyproject.toml")
     return match.group(1)
 
 

src/archunitpython/common/__init__.py

@@ -1,6 +1,10 @@
 from archunitpython.common.assertion.violation import EmptyTestViolation, Violation
 from archunitpython.common.error.errors import TechnicalError, UserError
-from archunitpython.common.fluentapi.checkable import Checkable, CheckOptions
+from archunitpython.common.fluentapi.checkable import (
+    Checkable,
+    CheckOptions,
+    RuleRationaleMixin,
+)
 from archunitpython.common.logging.types import LoggingOptions
 from archunitpython.common.types import Filter, Pattern, PatternMatchingOptions
 
@@ -11,6 +15,7 @@
     "UserError",
     "Checkable",
     "CheckOptions",
+    "RuleRationaleMixin",
     "LoggingOptions",
     "Pattern",
     "Filter",

src/archunitpython/common/extraction/extract_graph.py

@@ -27,6 +27,8 @@
     "*.egg-info",
 ]
 
+_ARCHIGNORE_FILE = ".archignore"
+
 
 def clear_graph_cache(options: CheckOptions | None = None) -> None:
     """Clear the cached dependency graphs."""
@@ -58,7 +60,7 @@ def extract_graph(
         project_path = os.getcwd()
 
     project_path = os.path.abspath(project_path)
-    excludes = list(exclude_patterns) if exclude_patterns is not None else list(_DEFAULT_EXCLUDE)
+    excludes = _resolve_exclude_patterns(project_path, exclude_patterns)
     ignore_type_checking_imports = bool(
         options and options.ignore_type_checking_imports
     )
@@ -94,6 +96,34 @@ def _build_cache_key(
     )
 
 
+def _resolve_exclude_patterns(
+    project_path: str,
+    exclude_patterns: list[str] | None,
+) -> list[str]:
+    """Resolve exclude patterns (explicit or defaults) plus any .archignore patterns."""
+    excludes = list(exclude_patterns) if exclude_patterns is not None else list(_DEFAULT_EXCLUDE)
+    excludes.extend(_load_archignore_patterns(project_path))
+    return excludes
+
+
+def _load_archignore_patterns(project_path: str) -> list[str]:
+    """Load .archignore patterns from a project root, if present."""
+    archignore_path = os.path.join(project_path, _ARCHIGNORE_FILE)
+    try:
+        with open(archignore_path, "r", encoding="utf-8", errors="replace") as f:
+            lines = f.readlines()
+    except OSError:
+        return []
+
+    patterns: list[str] = []
+    for line in lines:
+        pattern = line.strip()
+        if not pattern or pattern.startswith("#"):
+            continue
+        patterns.append(pattern)
+    return patterns
+
+
 def _extract_graph_uncached(
     project_path: str,
     exclude_patterns: list[str],
@@ -105,6 +135,7 @@ def _extract_graph_uncached(
 
     edges: list[Edge] = []
     py_files_set = set(py_files)
+    normalized_py_files = {_normalize(f) for f in py_files_set}
 
     for file_path in py_files:
         # Add self-referencing edge (ensures the file appears as a node)
@@ -129,10 +160,8 @@ def _extract_graph_uncached(
             )
             if resolved and resolved != _normalize(file_path):
                 # Check if the resolved path is in our project
-                if not is_external and resolved not in {
-                    _normalize(f) for f in py_files_set
-                }:
-                    is_external = True
+                if not is_external and resolved not in normalized_py_files:
+                    continue
 
                 edges.append(
                     Edge(
@@ -154,29 +183,65 @@ def _normalize(path: str) -> str:
 def _find_python_files(root: str, exclude: list[str]) -> list[str]:
     """Recursively find all .py files, excluding specified patterns."""
     py_files: list[str] = []
+    root = os.path.abspath(root)
     for dirpath, dirnames, filenames in os.walk(root):
         # Filter out excluded directories in-place
         dirnames[:] = [
             d
             for d in dirnames
-            if not _should_exclude(d, exclude)
+            if not _should_exclude_path(os.path.join(dirpath, d), root, exclude, is_dir=True)
         ]
 
         for filename in filenames:
-            if filename.endswith(".py") and not _should_exclude(filename, exclude):
-                full_path = os.path.join(dirpath, filename)
+            full_path = os.path.join(dirpath, filename)
+            if filename.endswith(".py") and not _should_exclude_path(
+                full_path, root, exclude, is_dir=False
+            ):
                 py_files.append(os.path.abspath(full_path))
 
     return py_files
 
 
-def _should_exclude(name: str, patterns: list[str]) -> bool:
-    """Check if a name matches any exclude pattern."""
+def _should_exclude_path(
+    path: str,
+    root: str,
+    patterns: list[str],
+    *,
+    is_dir: bool,
+) -> bool:
+    """Check if a path matches any exclude pattern."""
     import fnmatch
 
-    for pattern in patterns:
-        if fnmatch.fnmatch(name, pattern):
+    rel_path = _normalize(os.path.relpath(path, root))
+    name = os.path.basename(path)
+
+    for raw_pattern in patterns:
+        pattern = raw_pattern.strip().replace("\\", "/")
+        if not pattern or pattern.startswith("#"):
+            continue
+
+        pattern = pattern.removeprefix("./")
+        anchored = pattern.startswith("/")
+        if anchored:
+            pattern = pattern[1:]
+
+        dir_only = pattern.endswith("/")
+        if dir_only:
+            pattern = pattern.rstrip("/")
+            if not is_dir:
+                continue
+
+        if not pattern:
+            continue
+
+        if "/" in pattern or anchored:
+            if fnmatch.fnmatch(rel_path, pattern):
+                return True
+            if is_dir and rel_path == pattern:
+                return True
+        elif fnmatch.fnmatch(name, pattern):
             return True
+
     return False
 
 

src/archunitpython/common/fluentapi/__init__.py

@@ -1,3 +1,7 @@
-from archunitpython.common.fluentapi.checkable import Checkable, CheckOptions
+from archunitpython.common.fluentapi.checkable import (
+    Checkable,
+    CheckOptions,
+    RuleRationaleMixin,
+)
 
-__all__ = ["Checkable", "CheckOptions"]
+__all__ = ["Checkable", "CheckOptions", "RuleRationaleMixin"]

src/archunitpython/common/fluentapi/checkable.py

@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 from dataclasses import dataclass
-from typing import Protocol
+from typing import Protocol, TypeVar
 
 from archunitpython.common.assertion.violation import Violation
 from archunitpython.common.logging.types import LoggingOptions
@@ -19,6 +19,28 @@ class CheckOptions:
     ignore_type_checking_imports: bool = False
 
 
+T = TypeVar("T", bound="RuleRationaleMixin")
+
+
+class RuleRationaleMixin:
+    """Mixin for checkable rules that can carry a human-readable rationale."""
+
+    _because_reason: str | None = None
+
+    def because(self: T, reason: str) -> T:
+        """Attach a rationale explaining why the rule exists."""
+        reason = reason.strip()
+        if not reason:
+            raise ValueError("Rule rationale must not be empty.")
+        self._because_reason = reason
+        return self
+
+    @property
+    def because_reason(self) -> str | None:
+        """Return the rationale attached with because(), if any."""
+        return self._because_reason
+
+
 class Checkable(Protocol):
     """Protocol for any architecture rule that can be checked.
 

src/archunitpython/files/fluentapi/files.py

@@ -14,7 +14,7 @@
 
 from archunitpython.common.assertion.violation import EmptyTestViolation, Violation
 from archunitpython.common.extraction.extract_graph import extract_graph
-from archunitpython.common.fluentapi.checkable import CheckOptions
+from archunitpython.common.fluentapi.checkable import CheckOptions, RuleRationaleMixin
 from archunitpython.common.pattern_matching import matches_all_patterns
 from archunitpython.common.projection.edge_projections import (
     per_external_edge,
@@ -338,7 +338,7 @@ def _check_empty_test(
     return None
 
 
-class CycleFreeFileCondition:
+class CycleFreeFileCondition(RuleRationaleMixin):
     """Checkable that verifies no cycles exist among filtered files."""
 
     def __init__(self, project_path: str | None, filters: list[Filter]) -> None:
@@ -366,7 +366,7 @@ def check(self, options: CheckOptions | None = None) -> list[Violation]:
         return gather_cycle_violations(cycles)
 
 
-class DependOnFileCondition:
+class DependOnFileCondition(RuleRationaleMixin):
     """Checkable that verifies file dependency rules."""
 
     def __init__(
@@ -393,7 +393,7 @@ def check(self, options: CheckOptions | None = None) -> list[Violation]:
         )
 
 
-class DependOnExternalModuleCondition:
+class DependOnExternalModuleCondition(RuleRationaleMixin):
     """Checkable that verifies external module dependency rules."""
 
     def __init__(
@@ -425,7 +425,7 @@ def check(self, options: CheckOptions | None = None) -> list[Violation]:
         )
 
 
-class MatchPatternFileCondition:
+class MatchPatternFileCondition(RuleRationaleMixin):
     """Checkable that verifies files match/don't match patterns."""
 
     def __init__(
@@ -452,7 +452,7 @@ def check(self, options: CheckOptions | None = None) -> list[Violation]:
         )
 
 
-class CustomFileCheckableCondition:
+class CustomFileCheckableCondition(RuleRationaleMixin):
     """Checkable that evaluates a custom condition on files."""
 
     def __init__(

src/archunitpython/metrics/extraction/extract_class_info.py

@@ -5,7 +5,10 @@
 import ast
 import os
 
-from archunitpython.common.extraction.extract_graph import _DEFAULT_EXCLUDE, _find_python_files
+from archunitpython.common.extraction.extract_graph import (
+    _find_python_files,
+    _resolve_exclude_patterns,
+)
 from archunitpython.metrics.common.types import (
     ClassInfo,
     EnhancedClassInfo,
@@ -33,7 +36,7 @@ def extract_class_info(
         project_path = os.getcwd()
 
     project_path = os.path.abspath(project_path)
-    excludes = exclude_patterns if exclude_patterns is not None else _DEFAULT_EXCLUDE
+    excludes = _resolve_exclude_patterns(project_path, exclude_patterns)
     py_files = _find_python_files(project_path, excludes)
 
     classes: list[ClassInfo] = []
@@ -53,7 +56,7 @@ def extract_enhanced_class_info(
         project_path = os.getcwd()
 
     project_path = os.path.abspath(project_path)
-    excludes = exclude_patterns if exclude_patterns is not None else _DEFAULT_EXCLUDE
+    excludes = _resolve_exclude_patterns(project_path, exclude_patterns)
     py_files = _find_python_files(project_path, excludes)
 
     results: list[FileAnalysisResult] = []