Allow using a custom python function in tab_title_template

Makes it easier to do complex processing

docs/changelog.rst

@@ -140,6 +140,9 @@ Detailed list of changes
   controlled by :opt:`cursor_blink_interval` and
   :opt:`cursor_stop_blinking_after`. (:pull:`8551`)
 
+- Allow using a custom python function to draw tab titles in the tab bar, see
+  :opt:`tab_title_template`
+
 - Wayland: Fix incorrect window size calculation when transitioning from
   full screen to non-full screen with client side decorations (:iss:`8826`)
 

kitty/options/definition.py

@@ -1511,6 +1511,14 @@ use :code:`{sup.index}`. All data available is:
 :code:`tab.progress_percent`
     If a command running in a window reports the progress for a task, show this progress as a percentage
     from all windows in the tab, averaged. Empty string is no progress is reported.
+:code:`custom`
+    This will call a function named :code:`draw_title(data)` from the file :file:`tab_bar.py` placed in
+    the kitty config directory. The function will be passed a dictionary of data, the same data that
+    can be used in this template. It can then perform arbitrarily complex processing and return a string.
+    For example: :code:`tab_title_template "{custom}"` will use the output of the function as the tab title.
+    Any print statements in the :code:`draw_title()` will print to the STDOUT of the kitty process, useful
+    for debugging.
+
 
 Note that formatting is done by Python's string formatting machinery, so you can
 use, for instance, :code:`{layout_name[:2].upper()}` to show only the first two

kitty/tab_bar.py

@@ -303,6 +303,7 @@ def draw_title(draw_data: DrawData, screen: Screen, tab: TabBarData, index: int,
         prefix += '{activity_symbol}'
     if prefix:
         template = '{fmt.fg.red}' + prefix + '{fmt.fg.tab}' + template
+    eval_locals['custom'] = load_custom_draw_title(eval_locals)
     try:
         title = eval(compile_template(template), {'__builtins__': safe_builtins}, eval_locals)
     except Exception as e:
@@ -486,15 +487,24 @@ def draw_tab_with_powerline(
 
 
 @run_once
-def load_custom_draw_tab() -> DrawTabFunc:
+def load_custom_draw_tab_module() -> dict[str, Any]:
     import runpy
     import traceback
     try:
-        m = runpy.run_path(os.path.join(config_dir, 'tab_bar.py'))
-        func: DrawTabFunc = m['draw_tab']
+        return runpy.run_path(os.path.join(config_dir, 'tab_bar.py'))
+    except FileNotFoundError:
+        return {}
     except Exception as e:
         traceback.print_exc()
-        log_error(f'Failed to load custom draw_tab function with error: {e}')
+        log_error(f'Failed to load custom tab_bar.py module with error: {e}')
+        return {}
+
+
+@run_once
+def load_custom_draw_tab() -> DrawTabFunc:
+    m = load_custom_draw_tab_module()
+    func: DrawTabFunc | None = m.get('draw_tab')
+    if func is None:
         return draw_tab_with_fade
 
     @wraps(func)
@@ -512,6 +522,24 @@ def load_custom_draw_tab() -> DrawTabFunc:
     return draw_tab
 
 
+class CustomDrawTitleFunc:
+
+    def __init__(self, data: dict[str, Any], implementation: Callable[[dict[str, Any]], str] | None = None):
+        self._implementation = implementation
+        self._data = data.copy()
+
+    def __str__(self) -> str:
+        if self._implementation is None:
+            return ''
+        return str(self._implementation(self._data))
+    __repr__ = __str__
+
+
+def load_custom_draw_title(data: dict[str, Any]) -> CustomDrawTitleFunc:
+    m = load_custom_draw_tab_module()
+    return CustomDrawTitleFunc(data, m.get('draw_title'))
+
+
 class CellRange(NamedTuple):
     start: int
     end: int