Add support for configuring --num-workers with a env variable (#21407)

Fixes #21395 

Specify num workers env var and added documentation for it.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: KevinRK29

docs/source/command_line.rst

@@ -1052,6 +1052,9 @@ enabled by default starting from mypy 2.0.
     disables parallel checking. Automatic detection of the optimal number
     of workers is not supported yet.
 
+    This setting will override the ``MYPY_NUM_WORKERS`` environment
+    variable if it is set.
+
 Notes:
 
 * An import cycle is always processed as a whole by a worker process. Thus,

docs/source/config_file.rst

@@ -1018,6 +1018,8 @@ These options may only be set in the global section (``[mypy]``).
 
     Use specific number of parallel worker processes for type-checking, see
     :ref:`parallel type-checking <parallel>` for more details.
+    This setting will be overridden by the ``MYPY_NUM_WORKERS`` environment
+    variable.
 
 
 Advanced options

mypy/main.py

@@ -393,7 +393,8 @@ def infer_python_executable(options: Options, special_opts: argparse.Namespace)
 
 FOOTER: Final = """Environment variables:
   Define MYPYPATH for additional module search path entries.
-  Define MYPY_CACHE_DIR to override configuration cache_dir path."""
+  Define MYPY_CACHE_DIR to override configuration cache_dir path.
+  Define MYPY_NUM_WORKERS to override configuration num_workers value."""
 
 
 class CapturableArgumentParser(argparse.ArgumentParser):
@@ -1465,6 +1466,14 @@ def set_strict_flags() -> None:
         options.cache_dir = environ_cache_dir
     options.cache_dir = os.path.expanduser(options.cache_dir)
 
+    # Override num_workers if provided in the environment
+    environ_num_workers = os.getenv("MYPY_NUM_WORKERS", "")
+    if environ_num_workers.strip():
+        try:
+            options.num_workers = int(environ_num_workers)
+        except ValueError:
+            parser.error(f"MYPY_NUM_WORKERS must be an integer, got {environ_num_workers!r}")
+
     # Parse command line for real, using a split namespace.
     special_opts = argparse.Namespace()
     parser.parse_args(args, SplitNamespace(options, special_opts, "special-opts:"))