Cache directory may be set via environment (#7016) (#7443)

This adds an environment variable MYPY_CACHE_DIR which can set the
current cache directory. This variable will override the cache_dir
setting in the configuration but can be overridden via the command line.
Fixes #7016.

Author: hetmankp

docs/source/command_line.rst

@@ -533,6 +533,9 @@ beyond what incremental mode can offer, try running mypy in
     change this folder. This flag can also be useful for controlling
     cache use when using :ref:`remote caching <remote-cache>`.
 
+    This setting will override the ``MYPY_CACHE_DIR`` environment
+    variable if it is set.
+
     Mypy will also always write to the cache even when incremental
     mode is disabled so it can "warm up" the cache. To disable
     writing to the cache, use ``--cache-dir=/dev/null`` (UNIX)

docs/source/config_file.rst

@@ -416,6 +416,8 @@ section of the command line docs.
 ``cache_dir`` (string, default ``.mypy_cache``)
     Specifies the location where mypy stores incremental cache info.
     User home directory and environment variables will be expanded.
+    This setting will be overridden by the ``MYPY_CACHE_DIR`` environment
+    variable.
 
     Note that the cache is only read when incremental mode is enabled
     but is always written to, unless the value is set to ``/dev/nul``

mypy/main.py

@@ -244,7 +244,8 @@ def infer_python_executable(options: Options,
 """  # type: Final
 
 FOOTER = """Environment variables:
-  Define MYPYPATH for additional module search path entries."""  # type: Final
+  Define MYPYPATH for additional module search path entries.
+  Define MYPY_CACHE_DIR to override configuration cache_dir path."""  # type: Final
 
 
 def process_options(args: List[str],
@@ -704,6 +705,11 @@ def add_invertible_flag(flag: str,
         for dest, value in strict_flag_assignments:
             setattr(options, dest, value)
 
+    # Override cache_dir if provided in the environment
+    environ_cache_dir = os.getenv('MYPY_CACHE_DIR', '')
+    if environ_cache_dir.strip():
+        options.cache_dir = environ_cache_dir
+
     # Parse command line for real, using a split namespace.
     special_opts = argparse.Namespace()
     parser.parse_args(args, SplitNamespace(options, special_opts, 'special-opts:'))