Use .adr-dir for --adr-path if it exists

Use whatever the user passed if they passed anything, then check for
.adr-dir before falling back to `doc/adr`.

Fixes mrwilson#25

README.md

@@ -35,14 +35,18 @@ $ python setup.py install
 Usage: adr-viewer [OPTIONS]
 
 Options:
-  --adr-path TEXT  Directory containing ADR files.  [default: doc/adr/]
-  --output TEXT    File to write output to.  [default: index.html]
-  --serve          Serve content at http://localhost:8000/
-  --port INT       Custom server port [default: 8000]
-  --help           Show this message and exit.
+  --adr-path TEXT      Directory containing ADR files; pass explicitly, read
+                       .adr-dir if it exists or uses doc/adr/
+  --output TEXT        File to write output to.  [default: index.html]
+  --serve              Serve content at http://localhost:8000/
+  --port INTEGER       Change port for the server  [default: 8000]
+  --template-dir TEXT  Template directory.
+  --help               Show this message and exit.
 ```
 
 The default for `--adr-path` is `doc/adr/` because this is the default path generated by `adr-tools`.
+If `.adr-dir` exists in the directory in which `adr-viewer` was executed, it will be read and used for the `--adr-path`,
+following the `adr-tools` convention for overrides.
 
 ## Supported Record Types
 

adr_viewer/__init__.py

@@ -103,13 +103,39 @@ def generate_content(path, template_dir_override=None):
     return render_html(config, template_dir_override)
 
 
+CONVENTIONAL_ADR_DIR = 'doc/adr/'
+DEFAULT_ADR_DIR_FILE = '.adr-dir'
+
+
+def resolve_adr_dir(maybe_dir=None, adr_dir_file=DEFAULT_ADR_DIR_FILE):
+    """
+    If passed something, blindly use it. Otherwise, resolve based on
+    conventions in the ADR tooling ecosystem.
+    """
+    def lookup():
+        adr_dir = CONVENTIONAL_ADR_DIR
+        if os.path.exists(adr_dir_file):
+            with open(adr_dir_file, 'r') as file:
+                adr_dir = file.read().strip()
+        return adr_dir
+
+    return maybe_dir if maybe_dir else lookup()
+
+
 @click.command()
-@click.option('--adr-path',      default='doc/adr/',   help='Directory containing ADR files.',         show_default=True)
+@click.option('--adr-path',
+              default=None,
+              help=f"""
+                Directory containing ADR files; pass explicitly,
+                read {DEFAULT_ADR_DIR_FILE} if it exists or uses {CONVENTIONAL_ADR_DIR}
+              """,
+              show_default=True)
 @click.option('--output',        default='index.html', help='File to write output to.',                show_default=True)
 @click.option('--serve',         default=False,        help='Serve content at http://localhost:8000/', is_flag=True)
 @click.option('--port',          default=8000,         help='Change port for the server',              show_default=True)
 @click.option('--template-dir',  default=None,         help='Template directory.',                     show_default=True)
 def main(adr_path, output, serve, port, template_dir):
+    adr_path = resolve_adr_dir(adr_path)
     content = generate_content(adr_path, template_dir)
 
     if serve:

adr_viewer/test_adr_viewer.py

@@ -1,4 +1,6 @@
-from adr_viewer import parse_adr_to_config, render_html
+import tempfile
+
+from adr_viewer import parse_adr_to_config, render_html, resolve_adr_dir
 
 
 def test_should_extract_title_from_record():
@@ -85,4 +87,20 @@ def test_should_render_html_with_collapsible_index():
 def test_should_ignore_invalid_files():
     config = parse_adr_to_config('test/adr/0003-bad-formatting.md')
 
-    assert config is None
+    assert config is None
+
+
+def test_resolve_adr_path_default():
+    assert resolve_adr_dir() == "doc/adr/"
+
+
+def test_resolve_adr_path_explicit():
+    assert resolve_adr_dir("docs/adrs") == "docs/adrs"
+
+
+def test_resolve_adr_path_adrdir():
+    test_adr_dir_content = "just a test  "
+    with tempfile.NamedTemporaryFile('w') as tmpfile:
+        tmpfile.write(test_adr_dir_content)
+        tmpfile.flush()
+        assert resolve_adr_dir(None, adr_dir_file=tmpfile.name) == test_adr_dir_content.strip()