fix: stdin handling for empty submissions (#9556)

Light2Dark · web-flow · commit 9d7a8fe3bbdc · 2026-05-21T10:20:31.000-07:00
## 📝 Summary  - Pressing Enter with no input in marimo's stdin box was silently swallowed by the frontend, so any CLI calling `input("Press Enter to continue:")` (e.g. `logfire`, `click.prompt`, `rich.prompt`, `pdb`) would hang forever. - Once unblocked, a second bug surfaced: `builtins.input()` raised `EOFError` on blank submissions because `ThreadSafeStdin.readline()` returned `""` — which CPython's `input()` interprets as EOF rather than a blank line. ### Fix **Frontend** (`ConsoleOutput.tsx`) - `StdInput` now submits on Enter even when empty (history skip preserved). - `StdInputWithResponse` renders past responses with a `>` chevron, sky-11 color, and an `(empty)` placeholder so user-typed responses are visually distinct from stdout — even when the prompt half is empty (which happens when CLIs `print()` the prompt themselves before calling `input()`). <img width="501" height="191" alt="image" src="https://github.com/user-attachments/assets/59999aad-09fe-4174-9408-b009981130be" /> **Backend** (`marimo/_messaging/types.py`, `streams.py`, `_pyodide/streams.py`) - `Stdin.readline()` now appends `"\n"` so blank submissions surface as `"\n"` rather than `""`. `readlines()` simplified to `[readline()]`. - Lifted `readline`/`readlines` to the `Stdin` base class — both subclasses had identical implementations. - `_readline_with_prompt` unchanged: still returns the bare response, so `input_override` continues to satisfy Python's `input()` contract. ## 📋 Pre-Review Checklist  - [x] For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on [Discord](https://marimo.io/discord?ref=pr), or the community [discussions](https://github.com/marimo-team/marimo/discussions) (Please provide a link if applicable). - [x] Any AI generated code has been reviewed line-by-line by the human PR author, who stands by it. - [x] Video or media evidence is provided for any visual changes (optional).  ## ✅ Merge Checklist - [x] I have read the [contributor guidelines](https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md). - [ ] Documentation has been updated where applicable, including docstrings for API changes. - [x] Tests have been added for the changes made.
diff --git a/frontend/src/components/editor/notebook-cell.tsx b/frontend/src/components/editor/notebook-cell.tsx
@@ -740,6 +740,7 @@ const EditableCellComponent = ({
             <ConsoleOutput
               consoleOutputs={cellRuntime.consoleOutputs}
               stale={consoleOutputStale}
+              interrupted={cellRuntime.interrupted}
               // Empty name if serialization triggered
               cellName={cellRuntime.serialization ? "_" : cellData.name}
               onRefactorWithAI={handleRefactorWithAI}
@@ -1204,6 +1205,7 @@ const SetupCellComponent = ({
             <ConsoleOutput
               consoleOutputs={cellRuntime.consoleOutputs}
               stale={consoleOutputStale}
+              interrupted={cellRuntime.interrupted}
               // Don't show name
               cellName={"_"}
               onRefactorWithAI={handleRefactorWithAI}
diff --git a/frontend/src/components/editor/output/console/ConsoleOutput.tsx b/frontend/src/components/editor/output/console/ConsoleOutput.tsx
@@ -85,6 +85,7 @@ interface Props {
   className?: string;
   consoleOutputs: WithResponse<OutputMessage>[];
   stale: boolean;
+  interrupted: boolean;
   debuggerActive: boolean;
   onRefactorWithAI?: OnRefactorWithAI;
   onClear?: () => void;
@@ -111,6 +112,7 @@ const ConsoleOutputInternal = (props: Props): React.ReactNode => {
   const {
     consoleOutputs: rawConsoleOutputs,
     stale,
+    interrupted,
     cellName,
     cellId,
     onSubmitDebugger,
@@ -280,6 +282,7 @@ const ConsoleOutputInternal = (props: Props): React.ReactNode => {
                 output={output.data}
                 response={output.response}
                 isPassword={isPassword}
+                interrupted={interrupted}
               />
             );
           }
@@ -359,9 +362,9 @@ const StdInput = (props: {
           if (e.key === "Enter" && !e.shiftKey) {
             if (value) {
               addToHistory(value);
-              onSubmit(value);
-              setValue("");
             }
+            onSubmit(value);
+            setValue("");
             e.preventDefault();
             e.stopPropagation();
           }
@@ -382,12 +385,27 @@ const StdInputWithResponse = (props: {
   output: string;
   response?: string;
   isPassword?: boolean;
+  interrupted?: boolean;
 }) => {
+  const { output, response, isPassword, interrupted } = props;
+  const hasResponse = response != null && response !== "";
+  const wasInterruptedWithoutResponse = interrupted && !hasResponse;
+
   return (
     <div className="flex gap-2 items-center">
-      {renderText(props.output)}
-      {!props.isPassword && (
-        <span className="text-(--sky-11)">{props.response}</span>
+      {renderText(output)}
+      {!isPassword && !wasInterruptedWithoutResponse && (
+        <span
+          className="inline-flex items-center gap-1 text-(--sky-11)"
+          aria-label="stdin response"
+        >
+          <ChevronRightIcon className="w-4 h-4 shrink-0 opacity-70" />
+          {hasResponse ? (
+            response
+          ) : (
+            <span className="italic opacity-70">(empty)</span>
+          )}
+        </span>
       )}
     </div>
   );
diff --git a/frontend/src/components/editor/output/console/__tests__/ConsoleOutput.test.tsx b/frontend/src/components/editor/output/console/__tests__/ConsoleOutput.test.tsx
@@ -28,6 +28,7 @@ describe("ConsoleOutput integration", () => {
     cellName: "test_cell",
     consoleOutputs: [] as WithResponse<OutputMessage>[],
     stale: false,
+    interrupted: false,
     debuggerActive: false,
     onSubmitDebugger: () => {
       // noop
@@ -59,6 +60,7 @@ describe("ConsoleOutput pdb history", () => {
     cellName: "test_cell",
     consoleOutputs: [] as WithResponse<OutputMessage>[],
     stale: false,
+    interrupted: false,
     debuggerActive: false,
     onSubmitDebugger: vi.fn(),
   };
@@ -118,6 +120,82 @@ describe("ConsoleOutput pdb history", () => {
     expect(newInput).toHaveValue("next");
   });
 
+  it("should submit an empty string when Enter is pressed with no input", () => {
+    // Many CLIs prompt "Press Enter to continue" and expect "" back.
+    const onSubmitDebugger = vi.fn();
+    const outputs: WithResponse<OutputMessage>[] = [
+      stdinPrompt("Press Enter to continue: "),
+    ];
+
+    renderWithProvider(
+      <ConsoleOutput
+        {...defaultProps}
+        consoleOutputs={outputs}
+        onSubmitDebugger={onSubmitDebugger}
+      />,
+    );
+
+    const input = screen.getByTestId("console-input");
+    fireEvent.keyDown(input, { key: "Enter" });
+
+    expect(onSubmitDebugger).toHaveBeenCalledWith("", 0);
+  });
+
+  it("should not record empty submissions in input history", () => {
+    const onSubmitDebugger = vi.fn();
+    const outputs1: WithResponse<OutputMessage>[] = [stdinPrompt("(Pdb) ")];
+
+    const { rerender } = renderWithProvider(
+      <ConsoleOutput
+        {...defaultProps}
+        consoleOutputs={outputs1}
+        onSubmitDebugger={onSubmitDebugger}
+      />,
+    );
+
+    let input = screen.getByTestId("console-input");
+    fireEvent.change(input, { target: { value: "step" } });
+    fireEvent.keyDown(input, { key: "Enter" });
+
+    const outputs2: WithResponse<OutputMessage>[] = [
+      stdinPrompt("(Pdb) ", "step"),
+      stdinPrompt("(Pdb) "),
+    ];
+    rerender(
+      <TooltipProvider>
+        <ConsoleOutput
+          {...defaultProps}
+          consoleOutputs={outputs2}
+          onSubmitDebugger={onSubmitDebugger}
+        />
+      </TooltipProvider>,
+    );
+
+    // Submit an empty value; this should NOT enter the history stack.
+    input = screen.getByTestId("console-input");
+    fireEvent.keyDown(input, { key: "Enter" });
+
+    const outputs3: WithResponse<OutputMessage>[] = [
+      stdinPrompt("(Pdb) ", "step"),
+      stdinPrompt("(Pdb) ", ""),
+      stdinPrompt("(Pdb) "),
+    ];
+    rerender(
+      <TooltipProvider>
+        <ConsoleOutput
+          {...defaultProps}
+          consoleOutputs={outputs3}
+          onSubmitDebugger={onSubmitDebugger}
+        />
+      </TooltipProvider>,
+    );
+
+    // ArrowUp should jump back to "step", skipping the empty submission.
+    input = screen.getByTestId("console-input");
+    fireEvent.keyDown(input, { key: "ArrowUp" });
+    expect(input).toHaveValue("step");
+  });
+
   it("should navigate through multiple history entries across remounts", () => {
     const onSubmitDebugger = vi.fn();
 
@@ -192,6 +270,41 @@ describe("ConsoleOutput pdb history", () => {
     fireEvent.keyDown(input, { key: "ArrowDown" });
     expect(input).toHaveValue("");
   });
+
+  it("should distinguish an interrupted prompt from a bare-Enter submission", () => {
+    // After interrupt, cell.ts coerces pending stdin prompts to response: "".
+    // We must render that case differently from a real bare-Enter response,
+    // so the user isn't told they "submitted" a blank value.
+    const interruptedOutputs: WithResponse<OutputMessage>[] = [
+      stdinPrompt("Press Enter to continue: ", ""),
+    ];
+
+    const { rerender } = renderWithProvider(
+      <ConsoleOutput
+        {...defaultProps}
+        consoleOutputs={interruptedOutputs}
+        interrupted={true}
+      />,
+    );
+
+    // No response chunk should be rendered for an interrupted pending prompt.
+    expect(screen.queryByLabelText("stdin response")).not.toBeInTheDocument();
+
+    // Same outputs, but the cell isn't interrupted -- this is a real
+    // bare-Enter submission, so we should render the (empty) placeholder.
+    rerender(
+      <TooltipProvider>
+        <ConsoleOutput
+          {...defaultProps}
+          consoleOutputs={interruptedOutputs}
+          interrupted={false}
+        />
+      </TooltipProvider>,
+    );
+
+    expect(screen.getByLabelText("stdin response")).toBeInTheDocument();
+    expect(screen.getByText("(empty)")).toBeInTheDocument();
+  });
 });
 
 describe("ConsoleOutput debounced clearing", () => {
@@ -219,6 +332,7 @@ describe("ConsoleOutput debounced clearing", () => {
     cellName: "test_cell",
     consoleOutputs: [] as WithResponse<OutputMessage>[],
     stale: false,
+    interrupted: false,
     debuggerActive: false,
     onSubmitDebugger: vi.fn(),
   };
diff --git a/frontend/src/components/editor/renderers/vertical-layout/vertical-layout.tsx b/frontend/src/components/editor/renderers/vertical-layout/vertical-layout.tsx
@@ -405,6 +405,7 @@ const VerticalCell = memo(
           <ConsoleOutput
             consoleOutputs={consoleOutputs}
             stale={outputStale}
+            interrupted={interrupted}
             cellName={name}
             onSubmitDebugger={() => null}
             cellId={cellId}
diff --git a/frontend/src/components/scratchpad/scratchpad.tsx b/frontend/src/components/scratchpad/scratchpad.tsx
@@ -289,6 +289,7 @@ export const ScratchPad: React.FC = () => {
                 consoleOutputs={consoleOutputs}
                 className="overflow-auto"
                 stale={false}
+                interrupted={false}
                 cellName={DEFAULT_CELL_NAME}
                 onSubmitDebugger={Functions.NOOP}
                 cellId={cellId}
diff --git a/marimo/_messaging/streams.py b/marimo/_messaging/streams.py
@@ -466,20 +466,6 @@ def _readline_with_prompt(
 
         return self._stream.input_queue.get()
 
-    def readline(self, size: int | None = -1) -> str:  # type: ignore[override]
-        # size only included for compatibility with sys.stdin.readline API;
-        # we don't support it.
-        del size
-        return self._readline_with_prompt(prompt="")
-
-    def readlines(self, hint: int | None = -1) -> list[str]:  # type: ignore[override]
-        # Just an alias for readline.
-        #
-        # hint only included for compatibility with sys.stdin.readlines API;
-        # we don't support it.
-        del hint
-        return self._readline_with_prompt(prompt="").split("\n")
-
 
 @contextlib.contextmanager
 def redirect(standard_stream: Stdout | Stderr) -> Iterator[None]:
diff --git a/marimo/_messaging/types.py b/marimo/_messaging/types.py
@@ -63,7 +63,7 @@ def _ensure_plain_str(s: str) -> str:
 
 
 # These streams are not stoppable by users (we don't implement stop).
-class Stdout(io.TextIOBase):
+class Stdout(io.TextIOBase, abc.ABC):
     name = "stdout"
 
     @abc.abstractmethod
@@ -81,7 +81,7 @@ def _stop(self) -> None:
         """Tear down resources, if any."""
 
 
-class Stderr(io.TextIOBase):
+class Stderr(io.TextIOBase, abc.ABC):
     name = "stderr"
 
     @abc.abstractmethod
@@ -99,9 +99,32 @@ def _stop(self) -> None:
         """Tear down resources, if any."""
 
 
-class Stdin(io.TextIOBase):
+class Stdin(io.TextIOBase, abc.ABC):
     name = "stdin"
 
+    @abc.abstractmethod
+    def _readline_with_prompt(
+        self, prompt: str = "", password: bool = False
+    ) -> str:
+        """Send a prompt to the frontend and return the user's bare response.
+
+        Subclasses implement the transport. Returns the user-typed text
+        without a trailing newline (matches Python's input() contract).
+        """
+
+    # `size` and `hint` are accepted for sys.stdin API compatibility but
+    # ignored: marimo's stdin is a single-prompt pseudofile.
+
+    def readline(self, size: int | None = -1) -> str:  # type: ignore[override]
+        del size
+        # Trailing "\n" is required so a blank submission doesn't look like
+        # EOF to builtin input() (used by rich, click, getpass, pdb, etc.).
+        return self._readline_with_prompt(prompt="") + "\n"
+
+    def readlines(self, hint: int | None = -1) -> list[str]:  # type: ignore[override]
+        del hint
+        return [self.readline()]
+
     def _stop(self) -> None:
         """Tear down resources, if any."""
 
diff --git a/marimo/_pyodide/streams.py b/marimo/_pyodide/streams.py
@@ -179,17 +179,3 @@ def _readline_with_prompt(
     def _get_response(self) -> str:
         loop = asyncio.get_event_loop()
         return loop.run_until_complete(self.stream.input_queue.get())
-
-    def readline(self, size: int | None = -1) -> str:  # type: ignore[override]
-        # size only included for compatibility with sys.stdin.readline API;
-        # we don't support it.
-        del size
-        return self._readline_with_prompt(prompt="")
-
-    def readlines(self, hint: int | None = -1) -> list[str]:  # type: ignore[override]
-        # Just an alias for readline.
-        #
-        # hint only included for compatibility with sys.stdin.readlines API;
-        # we don't support it.
-        del hint
-        return self._readline_with_prompt(prompt="").split("\n")
diff --git a/tests/_messaging/test_streams.py b/tests/_messaging/test_streams.py
@@ -25,7 +25,7 @@ async def test_readline_installed(
         await k.run(
             [exec_req.get("import sys; output = sys.stdin.readline()")]
         )
-        assert k.globals["output"] == ""
+        assert k.globals["output"] == "\n"
 
     @staticmethod
     async def test_readlines_installed(
@@ -34,7 +34,19 @@ async def test_readlines_installed(
         await k.run(
             [exec_req.get("import sys; output = sys.stdin.readlines()")]
         )
-        assert k.globals["output"] == [""]
+        assert k.globals["output"] == ["\n"]
+
+    @staticmethod
+    async def test_builtin_input_with_empty_response(
+        k: Kernel, exec_req: ExecReqProvider
+    ) -> None:
+        # Bypasses the cell-scoped input override, exercising the same
+        # readline()-based path that rich/click/getpass/pdb hit. Must not
+        # raise EOFError on an empty submission.
+        await k.run(
+            [exec_req.get("import builtins; output = builtins.input()")]
+        )
+        assert k.globals["output"] == ""
 
     @staticmethod
     async def test_getpass_installed(
diff --git a/tests/_messaging/test_types.py b/tests/_messaging/test_types.py
@@ -146,12 +146,19 @@ def test_not_stoppable(self) -> None:
 
 
 class TestStdin:
+    class MockStdin(Stdin):
+        def _readline_with_prompt(
+            self, prompt: str = "", password: bool = False
+        ) -> str:
+            del prompt, password
+            return ""
+
     def test_stdin_name(self) -> None:
-        stdin = Stdin()
+        stdin = self.MockStdin()
         assert stdin.name == "stdin"
 
     def test_not_stoppable(self) -> None:
-        stdin = Stdin()
+        stdin = self.MockStdin()
         assert not hasattr(stdin, "stop")
 
 
diff --git a/tests/_pyodide/test_pyodide_streams.py b/tests/_pyodide/test_pyodide_streams.py
