Only apply ANSI_COLORS_DISABLED when present and not an empty string

Author: hugovk

README.md

@@ -98,7 +98,8 @@ In order of precedence:
 
 1. Calling `colored` or `cprint` with a truthy `no_color` disables colour.
 2. Calling `colored` or `cprint` with a truthy `force_color` forces colour.
-3. Setting the `ANSI_COLORS_DISABLED` environment variable to any value disables colour.
+3. Setting the `ANSI_COLORS_DISABLED` environment variable to any non-empty value
+   disables colour.
 4. Setting the [`NO_COLOR`](https://no-color.org/) environment variable to any non-empty
    value disables colour.
 5. Setting the [`FORCE_COLOR`](https://force-color.org/) environment variable to any

src/termcolor/termcolor.py

@@ -120,7 +120,7 @@ def _can_do_colour(
         return True
 
     # Then check env vars:
-    if "ANSI_COLORS_DISABLED" in os.environ:
+    if os.environ.get("ANSI_COLORS_DISABLED"):
         return False
     if os.environ.get("NO_COLOR"):
         return False

tests/test_termcolor.py

@@ -215,11 +215,14 @@ def test_environment_variables_force_color_empty_string(
     [
         # Set only env vars
         (None, None, ["ANSI_COLORS_DISABLED=1"], False),
+        (None, None, ["ANSI_COLORS_DISABLED="], False),
         (None, None, ["NO_COLOR=1"], False),
         (None, None, ["NO_COLOR="], False),
         (None, None, ["FORCE_COLOR=1"], True),
         (None, None, ["FORCE_COLOR="], False),
         (None, None, ["ANSI_COLORS_DISABLED=1", "NO_COLOR=1", "FORCE_COLOR=1"], False),
+        (None, None, ["ANSI_COLORS_DISABLED=1", "FORCE_COLOR=1"], False),
+        (None, None, ["ANSI_COLORS_DISABLED=", "FORCE_COLOR=1"], True),
         (None, None, ["NO_COLOR=1", "FORCE_COLOR=1"], False),
         (None, None, ["NO_COLOR=1", "FORCE_COLOR="], False),
         (None, None, ["TERM=dumb"], False),