Improve legibility on light-background terminals

This fixes the problem of light-gray text being rendered on a light
background.

Users can now choose a different Pygments style/theme using the
PY_DEVTOOLS_STYLE environment variable.

If PY_DEVTOOLS_STYLE is not set, we attempt auto-detection using the
COLORFGBG environment variable, as is commonly done by some tools on
Linux and macOS.

- Dark-background terminals use the "vim" style, as before.
- Light-background terminals now use the "sas" style.
- If auto-detection fails, we use the "default" style, which is legible
  on most terminals.

A list of available styles can be obtained by running:

    import pygments
    list(pygments.styles.get_all_styles())

devtools/prettier.py

@@ -4,7 +4,7 @@
 from collections import OrderedDict
 from collections.abc import Generator
 
-from .utils import DataClassType, LaxMapping, SQLAlchemyClassType, env_true, isatty
+from .utils import DataClassType, LaxMapping, SQLAlchemyClassType, bg_dark_or_light, env_true, isatty
 
 try:
     from functools import cache
@@ -32,6 +32,8 @@
 MISSING = object()
 PRETTY_KEY = '__prettier_formatted_value__'
 
+PYGMENTS_STYLE = os.getenv('PY_DEVTOOLS_STYLE', {'dark': 'vim', 'light': 'sas', None: 'default'}[bg_dark_or_light()])
+
 
 def fmt(v: 'Any') -> 'Any':
     return {PRETTY_KEY: v}
@@ -50,7 +52,7 @@ def get_pygments() -> 'Tuple[Any, Any, Any]':
     except ImportError:  # pragma: no cover
         return None, None, None
     else:
-        return pygments, PythonLexer(), Terminal256Formatter(style='vim')
+        return pygments, PythonLexer(), Terminal256Formatter(style=PYGMENTS_STYLE)
 
 
 # common generator types (this is not exhaustive: things like chain are not include to avoid the import)

devtools/utils.py

@@ -3,6 +3,7 @@
 
 __all__ = (
     'isatty',
+    'bg_dark_or_light',
     'env_true',
     'env_bool',
     'use_highlight',
@@ -111,6 +112,26 @@ def use_highlight(highlight: 'Optional[bool]' = None, file_: 'Any' = None) -> bo
     return isatty(file_)
 
 
+def bg_dark_or_light() -> str | None:
+    """
+    Returns:
+        'dark' if the terminal background is dark,
+        'light' if the terminal background is light, or
+        None if the terminal background color is unknown.
+    """
+    colorfgbg = os.environ.get('COLORFGBG', '')
+    try:
+        _, bg_str = colorfgbg.split(';')
+        bg = int(bg_str)
+    except ValueError:
+        return None
+    if 0 <= bg <= 6 or bg == 8:
+        return 'dark'
+    elif bg == 7 or 9 <= bg <= 15:
+        return 'light'
+    return None
+
+
 def is_literal(s: 'Any') -> bool:
     import ast
 

docs/plugins.py

@@ -51,7 +51,7 @@ def gen_example_html(markdown: str):
 def gen_examples_html(m: re.Match) -> str:
     sys.path.append(str(THIS_DIR.resolve()))
 
-    os.environ.update(PY_DEVTOOLS_HIGHLIGHT='true', PY_DEVTOOLS_WIDTH='80')
+    os.environ.update(PY_DEVTOOLS_HIGHLIGHT='true', PY_DEVTOOLS_STYLE='vim', PY_DEVTOOLS_WIDTH='80')
     conv = Ansi2HTMLConverter()
     name = THIS_DIR / Path(m.group(1))
 

tests/conftest.py

@@ -5,3 +5,4 @@
 
 def pytest_sessionstart(session):
     os.environ.pop('PY_DEVTOOLS_HIGHLIGHT', None)
+    os.environ['PY_DEVTOOLS_STYLE'] = 'vim'