Refactored color_style() and no_style() to improve testability. Refs #23663.

This includes the following improvements:

- The type of the style object is now called 'Style' rather than 'dummy'.
- The new make_style() function allows generating a Style object directly
  from a config string. Before the only way to get a style object was
  through the environ and it also required that the terminal supported
  colors which isn't necessarily the case when testing.
- The output of no_style() is now cached with @lru_cache.
- The output of no_style() now has the same set of attributes as the
  other Style objects. Previously it allowed anything to pass through
  with __getattr__.

django/core/management/color.py

@@ -5,17 +5,18 @@ Sets up the terminal color scheme.
 import os
 import sys
 
+from django.utils import lru_cache
 from django.utils import termcolors
 
 
 def supports_color():
     """
-    Returns True if the running system's terminal supports color, and False
-    otherwise.
+    Returns True if the running system's terminal supports color,
+    and False otherwise.
     """
     plat = sys.platform
-    supported_platform = plat != 'Pocket PC' and (plat != 'win32' or
-                                                  'ANSICON' in os.environ)
+    supported_platform = plat != 'Pocket PC' and (plat != 'win32' or 'ANSICON' in os.environ)
+
     # isatty is not always implemented, #6223.
     is_a_tty = hasattr(sys.stdout, 'isatty') and sys.stdout.isatty()
     if not supported_platform or not is_a_tty:
@@ -23,34 +24,49 @@ def supports_color():
     return True
 
 
-def color_style():
-    """Returns a Style object with the Django color scheme."""
-    if not supports_color():
-        style = no_style()
-    else:
-        DJANGO_COLORS = os.environ.get('DJANGO_COLORS', '')
-        color_settings = termcolors.parse_color_setting(DJANGO_COLORS)
-        if color_settings:
-            class dummy:
+def make_style(config_string=''):
+    """
+    Create a Style object from the given config_string.
+
+    If config_string is empty django.utils.termcolors.DEFAULT_PALETTE is used.
+    """
+    class Style(object):
         pass
-            style = dummy()
+
+    style = Style()
+
+    color_settings = termcolors.parse_color_setting(config_string)
+
     # The nocolor palette has all available roles.
     # Use that palette as the basis for populating
     # the palette as defined in the environment.
     for role in termcolors.PALETTES[termcolors.NOCOLOR_PALETTE]:
+        if color_settings:
             format = color_settings.get(role, {})
-                setattr(style, role, termcolors.make_style(**format))
+            style_func = termcolors.make_style(**format)
+        else:
+            style_func = lambda x: x
+        setattr(style, role, style_func)
+
     # For backwards compatibility,
     # set style for ERROR_OUTPUT == ERROR
     style.ERROR_OUTPUT = style.ERROR
-        else:
-            style = no_style()
+
     return style
 
 
+@lru_cache.lru_cache(maxsize=None)
 def no_style():
-    """Returns a Style object that has no colors."""
-    class dummy:
-        def __getattr__(self, attr):
-            return lambda x: x
-    return dummy()
+    """
+    Returns a Style object with no color scheme.
+    """
+    return make_style('nocolor')
+
+
+def color_style():
+    """
+    Returns a Style object from the Django color scheme.
+    """
+    if not supports_color():
+        return no_style()
+    return make_style(os.environ.get('DJANGO_COLORS', ''))

tests/admin_scripts/tests.py

@@ -1392,6 +1392,22 @@ class CommandTypes(AdminScriptTestCase):
         self.assertOutput(out, "Prints the CREATE TABLE, custom SQL and CREATE INDEX SQL statements for the\ngiven model module name(s).")
         self.assertEqual(out.count('optional arguments'), 1)
 
+    def test_color_style(self):
+        style = color.no_style()
+        self.assertEqual(style.ERROR('Hello, world!'), 'Hello, world!')
+
+        style = color.make_style('nocolor')
+        self.assertEqual(style.ERROR('Hello, world!'), 'Hello, world!')
+
+        style = color.make_style('dark')
+        self.assertIn('Hello, world!', style.ERROR('Hello, world!'))
+        self.assertNotEqual(style.ERROR('Hello, world!'), 'Hello, world!')
+
+        # Default palette has color.
+        style = color.make_style('')
+        self.assertIn('Hello, world!', style.ERROR('Hello, world!'))
+        self.assertNotEqual(style.ERROR('Hello, world!'), 'Hello, world!')
+
     def test_command_color(self):
         class Command(BaseCommand):
             requires_system_checks = False