[CONTRIBUTING.md] Add some more coding conventions (#21939)

CONTRIBUTING.md

@@ -366,3 +366,67 @@ duration = float_or_none(video.get('durationMs'), scale=1000)
 view_count = int_or_none(video.get('views'))
 ```
 
+### Inline values
+
+Extracting variables is acceptable for reducing code duplication and improving readability of complex expressions. However, you should avoid extracting variables used only once and moving them to opposite parts of the extractor file, which makes reading the linear flow difficult.
+
+#### Example
+
+Correct:
+
+```python
+title = self._html_search_regex(r'<title>([^<]+)</title>', webpage, 'title')
+```
+
+Incorrect:
+
+```python
+TITLE_RE = r'<title>([^<]+)</title>'
+# ...some lines of code...
+title = self._html_search_regex(TITLE_RE, webpage, 'title')
+```
+
+### Collapse fallbacks
+
+Multiple fallback values can quickly become unwieldy. Collapse multiple fallback values into a single expression via a list of meta values.
+
+#### Example
+
+Good:
+
+```python
+description = self._html_search_meta(
+    ['og:description', 'description', 'twitter:description'],
+    webpage, 'description', default=None)
+```
+
+Unwieldy:
+
+```python
+description = (
+    self._og_search_description(webpage, default=None)
+    or self._html_search_meta('description', webpage, default=None)
+    or self._html_search_meta('twitter:description', webpage, default=None))
+```
+
+### Trailing parentheses
+
+Always move trailing parentheses after the last argument.
+
+#### Example
+
+Correct:
+
+```python
+    lambda x: x['ResultSet']['Result'][0]['VideoUrlSet']['VideoUrl'],
+    list)
+```
+
+Incorrect:
+
+```python
+    lambda x: x['ResultSet']['Result'][0]['VideoUrlSet']['VideoUrl'],
+    list,
+)
+```
+