llnl.util.tty.color._force_color: init in global scope (#44036)

Currently SPACK_COLOR=always is not respected in the build process on
macOS, because the global `_force_color` is re-evaluated in global scope
during module setup, where it is always `None`.

So, move global init bits from main.py to the module itself.

lib/spack/llnl/util/tty/color.py

@@ -59,6 +59,7 @@
 
 To output an @, use '@@'.  To output a } inside braces, use '}}'.
 """
+import os
 import re
 import sys
 from contextlib import contextmanager
@@ -101,9 +102,29 @@ def __init__(self, message):
 # Mapping from color arguments to values for tty.set_color
 color_when_values = {"always": True, "auto": None, "never": False}
 
-# Force color; None: Only color if stdout is a tty
-# True: Always colorize output, False: Never colorize output
-_force_color = None
+
+def _color_when_value(when):
+    """Raise a ValueError for an invalid color setting.
+
+    Valid values are 'always', 'never', and 'auto', or equivalently,
+    True, False, and None.
+    """
+    if when in color_when_values:
+        return color_when_values[when]
+    elif when not in color_when_values.values():
+        raise ValueError("Invalid color setting: %s" % when)
+    return when
+
+
+def _color_from_environ() -> Optional[bool]:
+    try:
+        return _color_when_value(os.environ.get("SPACK_COLOR", "auto"))
+    except ValueError:
+        return None
+
+
+#: When `None` colorize when stdout is tty, when `True` or `False` always or never colorize resp.
+_force_color = _color_from_environ()
 
 
 def try_enable_terminal_color_on_windows():
@@ -164,19 +185,6 @@ def _err_check(result, func, args):
             debug("Unable to support color on Windows terminal")
 
 
-def _color_when_value(when):
-    """Raise a ValueError for an invalid color setting.
-
-    Valid values are 'always', 'never', and 'auto', or equivalently,
-    True, False, and None.
-    """
-    if when in color_when_values:
-        return color_when_values[when]
-    elif when not in color_when_values.values():
-        raise ValueError("Invalid color setting: %s" % when)
-    return when
-
-
 def get_color_when():
     """Return whether commands should print color or not."""
     if _force_color is not None:

lib/spack/spack/main.py

@@ -427,7 +427,7 @@ def make_argument_parser(**kwargs):
     parser.add_argument(
         "--color",
         action="store",
-        default=os.environ.get("SPACK_COLOR", "auto"),
+        default=None,
         choices=("always", "never", "auto"),
         help="when to colorize output (default: auto)",
     )
@@ -622,7 +622,8 @@ def setup_main_options(args):
     # with color
     color.try_enable_terminal_color_on_windows()
     # when to use color (takes always, auto, or never)
-    color.set_color_when(args.color)
+    if args.color is not None:
+        color.set_color_when(args.color)
 
 
 def allows_unknown_args(command):