stubgen: generate valid dataclass stubs

Fixes #12441

mypy/stubgen.py

@@ -101,6 +101,7 @@
     OverloadedFuncDef,
     Statement,
     StrExpr,
+    TempNode,
     TupleExpr,
     TypeInfo,
     UnaryExpr,
@@ -650,6 +651,7 @@ def __init__(
         self.defined_names: set[str] = set()
         # Short names of methods defined in the body of the current class
         self.method_names: set[str] = set()
+        self.processing_dataclass = False
 
     def visit_mypy_file(self, o: MypyFile) -> None:
         self.module = o.fullname  # Current module being processed
@@ -699,6 +701,9 @@ def visit_overloaded_func_def(self, o: OverloadedFuncDef) -> None:
                 self.clear_decorators()
 
     def visit_func_def(self, o: FuncDef) -> None:
+        if self.analyzed and self.processing_dataclass and o.info.names[o.name].plugin_generated:
+            # Skip methods generated by the @dataclass decorator
+            return
         if (
             self.is_private_name(o.name, o.fullname)
             or self.is_not_in_all(o.name)
@@ -890,6 +895,9 @@ def visit_class_def(self, o: ClassDef) -> None:
         if not self._indent and self._state != EMPTY:
             sep = len(self._output)
             self.add("\n")
+        decorators = self.get_class_decorators(o)
+        for d in decorators:
+            self.add(f"{self._indent}@{d}\n")
         self.add(f"{self._indent}class {o.name}")
         self.record_name(o.name)
         base_types = self.get_base_types(o)
@@ -921,6 +929,7 @@ def visit_class_def(self, o: ClassDef) -> None:
         else:
             self._state = CLASS
         self.method_names = set()
+        self.processing_dataclass = False
 
     def get_base_types(self, cdef: ClassDef) -> list[str]:
         """Get list of base classes for a class."""
@@ -967,6 +976,21 @@ def get_base_types(self, cdef: ClassDef) -> list[str]:
             base_types.append(f"{name}={value.accept(p)}")
         return base_types
 
+    def get_class_decorators(self, cdef: ClassDef) -> list[str]:
+        decorators: list[str] = []
+        p = AliasPrinter(self)
+        for d in cdef.decorators:
+            if self.is_dataclass(d):
+                decorators.append(d.accept(p))
+                self.import_tracker.require_name(get_qualified_name(d))
+                self.processing_dataclass = True
+        return decorators
+
+    def is_dataclass(self, expr: Expression) -> bool:
+        if isinstance(expr, CallExpr):
+            expr = expr.callee
+        return self.get_fullname(expr) == "dataclasses.dataclass"
+
     def visit_block(self, o: Block) -> None:
         # Unreachable statements may be partially uninitialized and that may
         # cause trouble.
@@ -1323,8 +1347,14 @@ def get_init(
                 # Final without type argument is invalid in stubs.
                 final_arg = self.get_str_type_of_node(rvalue)
                 typename += f"[{final_arg}]"
+        elif self.processing_dataclass:
+            # attribute without annotation is not a dataclass field, don't add annotation.
+            return f"{self._indent}{lvalue} = ...\n"
         else:
             typename = self.get_str_type_of_node(rvalue)
+        if self.processing_dataclass and not (isinstance(rvalue, TempNode) and rvalue.no_rhs):
+            # dataclass field with default value, keep the initializer.
+            return f"{self._indent}{lvalue}: {typename} = ...\n"
         return f"{self._indent}{lvalue}: {typename}\n"
 
     def add(self, string: str) -> None:

test-data/unit/stubgen.test

@@ -3317,3 +3317,73 @@ def gen2() -> _Generator[_Incomplete, _Incomplete, _Incomplete]: ...
 
 class X(_Incomplete): ...
 class Y(_Incomplete): ...
+
+
+[case testDataclass]
+import dataclasses
+import dataclasses as dcs
+from dataclasses import dataclass
+from dataclasses import dataclass as dc
+
+@dataclasses.dataclass
+class X:
+    a: int
+    b: str = "hello"
+    non_field = None
+
+@dcs.dataclass
+class Y: ...
+
+@dataclass
+class Z: ...
+
+@dc
+class W: ...
+
+[out]
+import dataclasses
+import dataclasses as dcs
+from dataclasses import dataclass, dataclass as dc
+
+@dataclasses.dataclass
+class X:
+    a: int
+    b: str = ...
+    non_field = ...
+
+@dcs.dataclass
+class Y: ...
+@dataclass
+class Z: ...
+@dc
+class W: ...
+
+[case testDataclassWithKeywords]
+from dataclasses import dataclass
+
+@dataclass(init=False)
+class X: ...
+
+[out]
+from dataclasses import dataclass
+
+@dataclass(init=False)
+class X: ...
+
+[case testDataclassWithExplicitGeneratedMethodsOverrides_semanal]
+from dataclasses import dataclass
+
+@dataclass
+class X:
+    a: int
+    def __init__(self, a: int, b: str = ...) -> None: ...
+    def __post_init__(self) -> None: ...
+
+[out]
+from dataclasses import dataclass
+
+@dataclass
+class X:
+    a: int
+    def __init__(self, a: int, b: str = ...) -> None: ...
+    def __post_init__(self) -> None: ...