Fall back to FORCE_COLOR environment variable if MYPY_FORCE_COLOR is …

…not present (#13814)

Fixes #13806

This PR adds support for a FORCE_COLOR environment variable. If both
MYPY_FORCE_COLOR and FORCE_COLOR are present, mypy will continue to use
MYPY_FORCE_COLOR over FORCE_COLOR. However, if only FORCE_COLOR is set,
mypy will use that environment variable in much the same way it
currently uses MYPY_FORCE_COLOR.

MYPY_FORCE_COLOR appears to be undocumented and untested currently, so
this PR doesn't add any tests. However, @hugovk has tested this change
manually and using GitHub Actions, and reports that it appears to work
as expected.

mypy/dmypy/client.py

@@ -19,7 +19,7 @@
 from mypy.dmypy_os import alive, kill
 from mypy.dmypy_util import DEFAULT_STATUS_FILE, receive
 from mypy.ipc import IPCClient, IPCException
-from mypy.util import check_python_version, get_terminal_width
+from mypy.util import check_python_version, get_terminal_width, should_force_color
 from mypy.version import __version__
 
 # Argument parser.  Subparsers are tied to action functions by the
@@ -653,7 +653,7 @@ def request(
     args["command"] = command
     # Tell the server whether this request was initiated from a human-facing terminal,
     # so that it can format the type checking output accordingly.
-    args["is_tty"] = sys.stdout.isatty() or int(os.getenv("MYPY_FORCE_COLOR", "0")) > 0
+    args["is_tty"] = sys.stdout.isatty() or should_force_color()
     args["terminal_width"] = get_terminal_width()
     bdata = json.dumps(args).encode("utf8")
     _, name = get_status(status_file)

mypy/util.py

@@ -519,6 +519,10 @@ def parse_gray_color(cup: bytes) -> str:
     return gray
 
 
+def should_force_color() -> bool:
+    return bool(int(os.getenv("MYPY_FORCE_COLOR", os.getenv("FORCE_COLOR", "0"))))
+
+
 class FancyFormatter:
     """Apply color and bold font to terminal output.
 
@@ -531,8 +535,7 @@ def __init__(self, f_out: IO[str], f_err: IO[str], hide_error_codes: bool) -> No
         if sys.platform not in ("linux", "darwin", "win32", "emscripten"):
             self.dummy_term = True
             return
-        force_color = int(os.getenv("MYPY_FORCE_COLOR", "0"))
-        if not force_color and (not f_out.isatty() or not f_err.isatty()):
+        if not should_force_color() and (not f_out.isatty() or not f_err.isatty()):
             self.dummy_term = True
             return
         if sys.platform == "win32":