From c1b5284f72087cd8339534ec32066cabf1647825 Mon Sep 17 00:00:00 2001
From: LordBaryhobal <lordbaryhobal@gmail.com>
Date: Tue, 23 Jun 2026 12:27:31 +0200
Subject: [PATCH] feat(checker): type check subscript on dataframes

---
 midas/checker/frames.py | 51 +++++++++++++++++++++++++++++++++++++++++
 midas/checker/python.py | 41 ++++++++++++++++++++++++++++++++-
 2 files changed, 91 insertions(+), 1 deletion(-)
 create mode 100644 midas/checker/frames.py

diff --git a/midas/checker/frames.py b/midas/checker/frames.py
new file mode 100644
index 0000000..069143a
--- /dev/null
+++ b/midas/checker/frames.py
@@ -0,0 +1,51 @@
+from typing import Optional
+
+from midas.checker.types import ColumnType, DataFrameType
+
+
+class FrameManager:
+    @classmethod
+    def set_column(
+        cls, frame: DataFrameType, name: str, column: ColumnType
+    ) -> DataFrameType:
+        new_columns: list[DataFrameType.Column] = []
+        index: int = len(frame.columns)
+        replace: bool = False
+        for i, col in enumerate(frame.columns):
+            if col.name == name:
+                index = i
+                replace = True
+            new_columns.append(col)
+
+        new_col: DataFrameType.Column = DataFrameType.Column(
+            index=index,
+            name=name,
+            type=column,
+        )
+        if replace:
+            new_columns[index] = new_col
+        else:
+            new_columns.append(new_col)
+
+        return DataFrameType(columns=new_columns)
+
+    @classmethod
+    def set_columns(
+        cls, frame: DataFrameType, names: list[str], columns: list[ColumnType]
+    ) -> DataFrameType:
+        for name, col in zip(names, columns):
+            frame = cls.set_column(frame, name, col)
+        return frame
+
+    @classmethod
+    def get_column(cls, frame: DataFrameType, name: str) -> Optional[ColumnType]:
+        for col in frame.columns:
+            if col.name == name:
+                return col.type
+        return None
+
+    @classmethod
+    def get_columns(
+        cls, frame: DataFrameType, names: list[str]
+    ) -> list[Optional[ColumnType]]:
+        return [cls.get_column(frame, name) for name in names]
diff --git a/midas/checker/python.py b/midas/checker/python.py
index 5ea0c85..ceb9e6a 100644
--- a/midas/checker/python.py
+++ b/midas/checker/python.py
@@ -1,13 +1,14 @@
 import ast
 import logging
 from dataclasses import dataclass
-from typing import Any, Optional
+from typing import Any, Optional, cast
 
 import midas.ast.python as p
 from midas.ast.location import Location
 from midas.ast.printer import MidasPrinter
 from midas.checker.environment import Environment
 from midas.checker.evaluator import Evaluator
+from midas.checker.frames import FrameManager
 from midas.checker.operators import (
     PY_COMPARATOR_METHODS,
     PY_OPERATOR_METHODS,
@@ -647,6 +648,8 @@ class PythonTyper(
         match unfolded:
             case TupleType():
                 return self._visit_tuple_subscript(unfolded, expr)
+            case DataFrameType():
+                return self._visit_frame_subscript(unfolded, expr)
 
         operation: Optional[Type] = self.types.lookup_member(object, "__getitem__")
         if operation is None:
@@ -1252,3 +1255,39 @@ class PythonTyper(
                     expr.location, f"Invalid index type {expr.index} on {tup}"
                 )
                 return UnknownType()
+
+    def _visit_frame_subscript(
+        self, frame: DataFrameType, expr: p.SubscriptExpr
+    ) -> Type:
+        match expr.index:
+            case p.LiteralExpr(value=str() as name):
+                column: Optional[ColumnType] = FrameManager.get_column(frame, name)
+                if column is None:
+                    self.reporter.error(
+                        expr.location, f"Unknown column '{name}' on {frame}"
+                    )
+                    return UnknownType()
+                return column
+
+            case p.ListExpr(items=indices) if all(
+                isinstance(index, p.LiteralExpr) and isinstance(index.value, str)
+                for index in indices
+            ):
+                indices = cast(list[p.LiteralExpr], indices)
+                names: list[str] = [cast(str, index.value) for index in indices]
+                columns: list[ColumnType] = []
+                for name in names:
+                    column: Optional[ColumnType] = FrameManager.get_column(frame, name)
+                    if column is None:
+                        self.reporter.error(
+                            expr.location, f"Unknown column '{name}' on {frame}"
+                        )
+                        return UnknownType()
+                    columns.append(column)
+                return TupleType(items=tuple(columns))
+
+            case _:
+                self.reporter.error(
+                    expr.location, f"Invalid index type {expr.index} on {frame}"
+                )
+                return UnknownType()