Add beginnings of error code support (#7267)

This PR adds a foundation for error codes and implements a few error
codes. It also adds support for `# type: ignore[code1, ...]` which 
ignores only specific error codes on a line.

Only a few errors include interesting error codes at this point. I'll add 
support for more error codes in additional PRs. Most errors will implicitly 
fall back to a `misc` error code.

Error codes are only shown if `--show-error-codes` is used.

The error codes look like this in mypy output:

```
t.py:3: error: "str" has no attribute "trim"  [attr-defined]
```

Error codes are intended to be short but human-readable. The name
of an error code refers to the check that produces this error. In the above
example we generate a "no attribute" error when we check whether an 
attribute is defined.

Work towards #7239.

Author: JukkaL

mypy/build.py

@@ -199,7 +199,9 @@ def _build(sources: List[BuildSource],
         reports = Reports(data_dir, options.report_dirs)
 
     source_set = BuildSourceSet(sources)
-    errors = Errors(options.show_error_context, options.show_column_numbers)
+    errors = Errors(options.show_error_context,
+                    options.show_column_numbers,
+                    options.show_error_codes)
     plugin, snapshot = load_plugins(options, errors, stdout)
 
     # Construct a build manager object to hold state during the build.

mypy/errorcodes.py

@@ -0,0 +1,50 @@
+"""Classification of possible errors mypy can detect.
+
+These can be used for filtering specific errors.
+"""
+
+from typing import List
+from typing_extensions import Final
+
+
+# All created error codes are implicitly stored in this list.
+all_error_codes = []  # type: List[ErrorCode]
+
+
+class ErrorCode:
+    def __init__(self, code: str, description: str, category: str) -> None:
+        self.code = code
+        self.description = description
+        self.category = category
+
+    def __str__(self) -> str:
+        return '<ErrorCode {}>'.format(self.code)
+
+
+ATTR_DEFINED = ErrorCode(
+    'attr-defined', "Check that attribute exists", 'General')  # type: Final
+NAME_DEFINED = ErrorCode(
+    'name-defined', "Check that name is defined", 'General')  # type: Final
+CALL_ARG = ErrorCode(
+    'call-arg', "Check number, names and kinds of arguments in calls", 'General')  # type: Final
+ARG_TYPE = ErrorCode(
+    'arg-type', "Check argument types in calls", 'General')  # type: Final
+VALID_TYPE = ErrorCode(
+    'valid-type', "Check that type (annotation) is valid", 'General')  # type: Final
+MISSING_ANN = ErrorCode(
+    'var-annotated', "Require variable annotation if type can't be inferred",
+    'General')  # type: Final
+OVERRIDE = ErrorCode(
+    'override', "Check that method override is compatible with base class",
+    'General')  # type: Final
+RETURN_VALUE = ErrorCode(
+    'return-value', "Check that return value is compatible with signature",
+    'General')  # type: Final
+ASSIGNMENT = ErrorCode(
+    'assignment', "Check that assigned value is compatible with target", 'General')  # type: Final
+
+SYNTAX = ErrorCode(
+    'syntax', "Report syntax errors", 'General')  # type: Final
+
+MISC = ErrorCode(
+    'misc', "Miscenallenous other checks", 'General')  # type: Final

mypy/errors.py

@@ -9,6 +9,8 @@
 from mypy.scope import Scope
 from mypy.options import Options
 from mypy.version import __version__ as mypy_version
+from mypy.errorcodes import ErrorCode
+from mypy import errorcodes as codes
 
 T = TypeVar('T')
 allowed_duplicates = ['@overload', 'Got:', 'Expected:']  # type: Final
@@ -45,6 +47,9 @@ class ErrorInfo:
     # The error message.
     message = ''
 
+    # The error code.
+    code = None  # type: Optional[ErrorCode]
+
     # If True, we should halt build after the file that generated this error.
     blocker = False
 
@@ -68,6 +73,7 @@ def __init__(self,
                  column: int,
                  severity: str,
                  message: str,
+                 code: Optional[ErrorCode],
                  blocker: bool,
                  only_once: bool,
                  origin: Optional[Tuple[str, int, int]] = None,
@@ -81,12 +87,23 @@ def __init__(self,
         self.column = column
         self.severity = severity
         self.message = message
+        self.code = code
         self.blocker = blocker
         self.only_once = only_once
         self.origin = origin or (file, line, line)
         self.target = target
 
 
+# Type used internally to represent errors:
+#   (path, line, column, severity, message, code)
+ErrorTuple = Tuple[Optional[str],
+                   int,
+                   int,
+                   str,
+                   str,
+                   Optional[ErrorCode]]
+
+
 class Errors:
     """Container for compile errors.
 
@@ -111,8 +128,9 @@ class Errors:
     # Path to current file.
     file = ''  # type: str
 
-    # Ignore errors on these lines of each file.
-    ignored_lines = None  # type: Dict[str, Set[int]]
+    # Ignore some errors on these lines of each file
+    # (path -> line -> error-codes)
+    ignored_lines = None  # type: Dict[str, Dict[int, List[str]]]
 
     # Lines on which an error was actually ignored.
     used_ignored_lines = None  # type: Dict[str, Set[int]]
@@ -135,10 +153,13 @@ class Errors:
     target_module = None  # type: Optional[str]
     scope = None  # type: Optional[Scope]
 
-    def __init__(self, show_error_context: bool = False,
-                 show_column_numbers: bool = False) -> None:
+    def __init__(self,
+                 show_error_context: bool = False,
+                 show_column_numbers: bool = False,
+                 show_error_codes: bool = False) -> None:
         self.show_error_context = show_error_context
         self.show_column_numbers = show_column_numbers
+        self.show_error_codes = show_error_codes
         self.initialize()
 
     def initialize(self) -> None:
@@ -197,7 +218,7 @@ def set_file(self, file: str,
         self.scope = scope
 
     def set_file_ignored_lines(self, file: str,
-                               ignored_lines: Set[int],
+                               ignored_lines: Dict[int, List[str]],
                                ignore_all: bool = False) -> None:
         self.ignored_lines[file] = ignored_lines
         if ignore_all:
@@ -226,6 +247,8 @@ def report(self,
                line: int,
                column: Optional[int],
                message: str,
+               code: Optional[ErrorCode] = None,
+               *,
                blocker: bool = False,
                severity: str = 'error',
                file: Optional[str] = None,
@@ -237,7 +260,9 @@ def report(self,
 
         Args:
             line: line number of error
+            column: column number of error
             message: message to report
+            code: error code (defaults to 'misc' for 'error' severity)
             blocker: if True, don't continue analysis after this error
             severity: 'error' or 'note'
             file: if non-None, override current file as context
@@ -267,8 +292,11 @@ def report(self,
         if end_line is None:
             end_line = origin_line
 
+        if severity == 'error' and code is None:
+            code = codes.MISC
+
         info = ErrorInfo(self.import_context(), file, self.current_module(), type,
-                         function, line, column, severity, message,
+                         function, line, column, severity, message, code,
                          blocker, only_once,
                          origin=(self.file, origin_line, end_line),
                          target=self.current_target())
@@ -293,7 +321,7 @@ def add_error_info(self, info: ErrorInfo) -> None:
                 # Check each line in this context for "type: ignore" comments.
                 # line == end_line for most nodes, so we only loop once.
                 for scope_line in range(line, end_line + 1):
-                    if scope_line in self.ignored_lines[file]:
+                    if self.is_ignored_error(scope_line, info, self.ignored_lines[file]):
                         # Annotation requests us to ignore all errors on this line.
                         self.used_ignored_lines[file].add(scope_line)
                         return
@@ -305,6 +333,16 @@ def add_error_info(self, info: ErrorInfo) -> None:
             self.only_once_messages.add(info.message)
         self._add_error_info(file, info)
 
+    def is_ignored_error(self, line: int, info: ErrorInfo, ignores: Dict[int, List[str]]) -> bool:
+        if line not in ignores:
+            return False
+        elif not ignores[line]:
+            # Empty list means that we ignore all errors
+            return True
+        elif info.code:
+            return info.code.code in ignores[line]
+        return False
+
     def clear_errors_in_targets(self, path: str, targets: Set[str]) -> None:
         """Remove errors in specific fine-grained targets within a file."""
         if path in self.error_info_map:
@@ -319,11 +357,11 @@ def clear_errors_in_targets(self, path: str, targets: Set[str]) -> None:
     def generate_unused_ignore_errors(self, file: str) -> None:
         ignored_lines = self.ignored_lines[file]
         if not self.is_typeshed_file(file) and file not in self.ignored_files:
-            for line in ignored_lines - self.used_ignored_lines[file]:
+            for line in set(ignored_lines) - self.used_ignored_lines[file]:
                 # Don't use report since add_error_info will ignore the error!
                 info = ErrorInfo(self.import_context(), file, self.current_module(), None,
                                  None, line, -1, 'error', "unused 'type: ignore' comment",
-                                 False, False)
+                                 None, False, False)
                 self._add_error_info(file, info)
 
     def is_typeshed_file(self, file: str) -> bool:
@@ -373,7 +411,7 @@ def format_messages(self, error_info: List[ErrorInfo]) -> List[str]:
         a = []  # type: List[str]
         errors = self.render_messages(self.sort_messages(error_info))
         errors = self.remove_duplicates(errors)
-        for file, line, column, severity, message in errors:
+        for file, line, column, severity, message, code in errors:
             s = ''
             if file is not None:
                 if self.show_column_numbers and line >= 0 and column >= 0:
@@ -385,6 +423,8 @@ def format_messages(self, error_info: List[ErrorInfo]) -> List[str]:
                 s = '{}: {}: {}'.format(srcloc, severity, message)
             else:
                 s = message
+            if self.show_error_codes and code:
+                s = '{}  [{}]'.format(s, code.code)
             a.append(s)
         return a
 
@@ -420,18 +460,16 @@ def targets(self) -> Set[str]:
                    for info in errs
                    if info.target)
 
-    def render_messages(self, errors: List[ErrorInfo]) -> List[Tuple[Optional[str], int, int,
-                                                                     str, str]]:
+    def render_messages(self,
+                        errors: List[ErrorInfo]) -> List[ErrorTuple]:
         """Translate the messages into a sequence of tuples.
 
-        Each tuple is of form (path, line, col, severity, message).
+        Each tuple is of form (path, line, col, severity, message, code).
         The rendered sequence includes information about error contexts.
         The path item may be None. If the line item is negative, the
         line number is not defined for the tuple.
         """
-        result = []  # type: List[Tuple[Optional[str], int, int, str, str]]
-        # (path, line, column, severity, message)
-
+        result = []  # type: List[ErrorTuple]
         prev_import_context = []  # type: List[Tuple[str, int]]
         prev_function_or_member = None  # type: Optional[str]
         prev_type = None  # type: Optional[str]
@@ -455,7 +493,7 @@ def render_messages(self, errors: List[ErrorInfo]) -> List[Tuple[Optional[str],
                     # Remove prefix to ignore from path (if present) to
                     # simplify path.
                     path = remove_path_prefix(path, self.ignore_prefix)
-                    result.append((None, -1, -1, 'note', fmt.format(path, line)))
+                    result.append((None, -1, -1, 'note', fmt.format(path, line), None))
                     i -= 1
 
             file = self.simplify_path(e.file)
@@ -467,27 +505,27 @@ def render_messages(self, errors: List[ErrorInfo]) -> List[Tuple[Optional[str],
                     e.type != prev_type):
                 if e.function_or_member is None:
                     if e.type is None:
-                        result.append((file, -1, -1, 'note', 'At top level:'))
+                        result.append((file, -1, -1, 'note', 'At top level:', None))
                     else:
                         result.append((file, -1, -1, 'note', 'In class "{}":'.format(
-                            e.type)))
+                            e.type), None))
                 else:
                     if e.type is None:
                         result.append((file, -1, -1, 'note',
                                        'In function "{}":'.format(
-                                           e.function_or_member)))
+                                           e.function_or_member), None))
                     else:
                         result.append((file, -1, -1, 'note',
                                        'In member "{}" of class "{}":'.format(
-                                           e.function_or_member, e.type)))
+                                           e.function_or_member, e.type), None))
             elif e.type != prev_type:
                 if e.type is None:
-                    result.append((file, -1, -1, 'note', 'At top level:'))
+                    result.append((file, -1, -1, 'note', 'At top level:', None))
                 else:
                     result.append((file, -1, -1, 'note',
-                                   'In class "{}":'.format(e.type)))
+                                   'In class "{}":'.format(e.type), None))
 
-            result.append((file, e.line, e.column, e.severity, e.message))
+            result.append((file, e.line, e.column, e.severity, e.message, e.code))
 
             prev_import_context = e.import_ctx
             prev_function_or_member = e.function_or_member
@@ -518,10 +556,9 @@ def sort_messages(self, errors: List[ErrorInfo]) -> List[ErrorInfo]:
             result.extend(a)
         return result
 
-    def remove_duplicates(self, errors: List[Tuple[Optional[str], int, int, str, str]]
-                          ) -> List[Tuple[Optional[str], int, int, str, str]]:
+    def remove_duplicates(self, errors: List[ErrorTuple]) -> List[ErrorTuple]:
         """Remove duplicates from a sorted error list."""
-        res = []  # type: List[Tuple[Optional[str], int, int, str, str]]
+        res = []  # type: List[ErrorTuple]
         i = 0
         while i < len(errors):
             dup = False