Add --warn-unused-configs flag (#3865)

This is handy for debugging misspellings in mypy.ini, and other mishaps due to e.g. file moves.

Author: gvanrossum

docs/source/config_file.rst

@@ -34,6 +34,11 @@ characters.
   separated by commas.  These sections specify additional flags that
   only apply to *modules* whose name matches at least one of the patterns.
 
+.. note::
+
+   The ``warn_unused_configs`` flag may be useful to debug misspelled
+   section names.
+
 Global flags
 ************
 
@@ -72,6 +77,10 @@ The following global flags may only be set in the global section
 - ``warn_unused_ignores`` (Boolean, default False) warns about
   unneeded ``# type: ignore`` comments.
 
+- ``warn_unused_configs`` (Boolean, default False) warns about
+  per-module sections in the config file that didn't match any
+  files processed in the current run.
+
 - ``strict_optional`` (Boolean, default False) enables experimental
   strict Optional checks.
 

mypy/main.py

@@ -53,6 +53,11 @@ def main(script_path: Optional[str], args: Optional[List[str]] = None) -> None:
         a = e.messages
         if not e.use_stdout:
             serious = True
+    if options.warn_unused_configs and options.unused_configs:
+        print("Warning: unused section(s) in %s: %s" %
+              (options.config_file,
+               ", ".join("[mypy-%s]" % glob for glob in options.unused_configs.values())),
+              file=sys.stderr)
     if options.junit_xml:
         t1 = time.time()
         util.write_junit_xml(t1 - t0, serious, a, options.junit_xml)
@@ -281,6 +286,8 @@ def add_invertible_flag(flag: str,
                              " from non-Any typed functions")
     add_invertible_flag('--warn-unused-ignores', default=False, strict_flag=True,
                         help="warn about unneeded '# type: ignore' comments")
+    add_invertible_flag('--warn-unused-configs', default=False, strict_flag=True,
+                        help="warn about unnused '[mypy-<pattern>]' config sections")
     add_invertible_flag('--show-error-context', default=False,
                         dest='show_error_context',
                         help='Precede errors with "note:" messages explaining context')
@@ -695,6 +702,7 @@ def parse_config_file(options: Options, filename: Optional[str]) -> None:
                     glob = glob.replace(os.altsep, '.')
                 pattern = re.compile(fnmatch.translate(glob))
                 options.per_module_options[pattern] = updates
+                options.unused_configs[pattern] = glob
 
 
 def parse_section(prefix: str, template: Options,

mypy/options.py

@@ -3,7 +3,7 @@
 import pprint
 import sys
 
-from typing import Mapping, MutableMapping, Optional, Tuple, List, Pattern, Dict
+from typing import Dict, List, Mapping, MutableMapping, Optional, Pattern, Set, Tuple
 
 from mypy import defaults
 
@@ -87,6 +87,9 @@ def __init__(self) -> None:
         # Warn about unused '# type: ignore' comments
         self.warn_unused_ignores = False
 
+        # Warn about unused '[mypy-<pattern>] config sections
+        self.warn_unused_configs = False
+
         # Files in which to ignore all non-fatal errors
         self.ignore_errors = False
 
@@ -131,6 +134,8 @@ def __init__(self) -> None:
         # Per-module options (raw)
         pm_opts = OrderedDict()  # type: OrderedDict[Pattern[str], Dict[str, object]]
         self.per_module_options = pm_opts
+        # Map pattern back to glob
+        self.unused_configs = OrderedDict()  # type: OrderedDict[Pattern[str], str]
 
         # -- development options --
         self.verbosity = 0  # More verbose messages (for troubleshooting)
@@ -164,6 +169,8 @@ def clone_for_module(self, module: str) -> 'Options':
         updates = {}
         for pattern in self.per_module_options:
             if self.module_matches_pattern(module, pattern):
+                if pattern in self.unused_configs:
+                    del self.unused_configs[pattern]
                 updates.update(self.per_module_options[pattern])
         if not updates:
             return self