Improve plugin-related error messages

Address feedback by @gvanrossum in #3517.

Author: JukkaL

mypy/build.py

@@ -16,6 +16,7 @@
 import hashlib
 import json
 import os.path
+import re
 import sys
 import time
 from os.path import dirname, basename
@@ -343,23 +344,29 @@ def load_custom_plugins(default_plugin: Plugin, options: Options, errors: Errors
     back to default_plugin.
     """
 
+    if not options.config_file:
+        return default_plugin
+
+    line = find_config_file_line_number(options.config_file, 'plugins')
+    if line == -1:
+        line = 1  # We need to pick some line number that doesn't look too confusing
+
     def plugin_error(message: str) -> None:
-        errors.report(0, 0, message)
+        errors.report(line, 0, message)
         errors.raise_error()
 
+    errors.set_file(options.config_file, None)
     custom_plugins = []
     for plugin_path in options.plugins:
-        if options.config_file:
-            # Plugin paths are relative to the config file location.
-            plugin_path = os.path.join(os.path.dirname(options.config_file), plugin_path)
-        errors.set_file(plugin_path, None)
+        # Plugin paths are relative to the config file location.
+        plugin_path = os.path.join(os.path.dirname(options.config_file), plugin_path)
 
         if not os.path.isfile(plugin_path):
-            plugin_error("Can't find plugin")
+            plugin_error("Can't find plugin '{}'".format(plugin_path))
         plugin_dir = os.path.dirname(plugin_path)
         fnam = os.path.basename(plugin_path)
         if not fnam.endswith('.py'):
-            plugin_error("Plugin must have .py extension")
+            plugin_error("Plugin '{}' does not have a .py extension".format(fnam))
         module_name = fnam[:-3]
         import importlib
         sys.path.insert(0, plugin_dir)
@@ -372,19 +379,21 @@ def plugin_error(message: str) -> None:
             assert sys.path[0] == plugin_dir
             del sys.path[0]
         if not hasattr(m, 'plugin'):
-            plugin_error('Plugin does not define entry point function "plugin"')
+            plugin_error('Plugin "{}" does not define entry point function "plugin"'.format(
+                module_name))
         try:
             plugin_type = getattr(m, 'plugin')(__version__)
         except Exception:
             print('Error calling the plugin(version) entry point of {}\n'.format(plugin_path))
             raise  # Propagate to display traceback
         if not isinstance(plugin_type, type):
             plugin_error(
-                'Type object expected as the return value of "plugin" (got {!r})'.format(
-                    plugin_type))
+                'Type object expected as the return value of "{}.plugin" (got {!r})'.format(
+                    module_name, plugin_type))
         if not issubclass(plugin_type, Plugin):
             plugin_error(
-                'Return value of "plugin" must be a subclass of "mypy.plugin.Plugin"')
+                'Return value of "{}.plugin" must be a subclass of "mypy.plugin.Plugin"'.format(
+                    module_name))
         try:
             custom_plugins.append(plugin_type(options.python_version))
         except Exception:
@@ -397,6 +406,24 @@ def plugin_error(message: str) -> None:
         return ChainedPlugin(options.python_version, custom_plugins + [default_plugin])
 
 
+def find_config_file_line_number(path: str, setting_name: str) -> int:
+    """Return the approximate location of setting_name within mypy config file.
+
+    Return -1 if can't determine the line unambiguously.
+    """
+    try:
+        results = []
+        with open(path) as f:
+            for i, line in enumerate(f):
+                if re.match(r'\s*{}\s*='.format(setting_name), line):
+                    results.append(i + 1)
+        if len(results) == 1:
+            return results[0]
+    except OSError:
+        pass
+    return -1
+
+
 # TODO: Get rid of all_types.  It's not used except for one log message.
 #       Maybe we could instead publish a map from module ID to its type_map.
 class BuildManager:

test-data/unit/check-custom-plugin.test

@@ -30,35 +30,36 @@ plugins=<ROOT>/test-data/unit/plugins/fnplugin.py,
 [[mypy]
 plugins=missing.py
 [out]
-tmp/missing.py:0: error: Can't find plugin
+tmp/mypy.ini:2: error: Can't find plugin 'tmp/missing.py'
 --' (work around syntax highlighting)
 
 [case testInvalidPluginExtension]
 # flags: --config-file tmp/mypy.ini
 [file mypy.ini]
 [[mypy]
-plugins=badext.pyi
-[file badext.pyi]
+plugins=dir/badext.pyi
+[file dir/badext.pyi]
 [out]
-tmp/badext.pyi:0: error: Plugin must have .py extension
+tmp/mypy.ini:2: error: Plugin 'badext.pyi' does not have a .py extension
 
 [case testMissingPluginEntryPoint]
 # flags: --config-file tmp/mypy.ini
 [file mypy.ini]
 [[mypy]
-plugins=<ROOT>/test-data/unit/plugins/noentry.py
+ plugins = <ROOT>/test-data/unit/plugins/noentry.py
 [out]
-<ROOT>/test-data/unit/plugins/noentry.py:0: error: Plugin does not define entry point function "plugin"
+tmp/mypy.ini:2: error: Plugin "noentry" does not define entry point function "plugin"
 
 [case testInvalidPluginEntryPointReturnValue]
 # flags: --config-file tmp/mypy.ini
 def f(): pass
 f()
 [file mypy.ini]
 [[mypy]
+
 plugins=<ROOT>/test-data/unit/plugins/badreturn.py
 [out]
-<ROOT>/test-data/unit/plugins/badreturn.py:0: error: Type object expected as the return value of "plugin" (got None)
+tmp/mypy.ini:3: error: Type object expected as the return value of "badreturn.plugin" (got None)
 
 [case testInvalidPluginEntryPointReturnValue2]
 # flags: --config-file tmp/mypy.ini
@@ -68,4 +69,4 @@ f()
 [[mypy]
 plugins=<ROOT>/test-data/unit/plugins/badreturn2.py
 [out]
-<ROOT>/test-data/unit/plugins/badreturn2.py:0: error: Return value of "plugin" must be a subclass of "mypy.plugin.Plugin"
+tmp/mypy.ini:2: error: Return value of "badreturn2.plugin" must be a subclass of "mypy.plugin.Plugin"