langchain-ai/langchain · warning · ValueError

char should be a single character

Error message

char should be a single character

What it means

`AsciiCanvas.draw_on` places one character at a coordinate, so `char` must be a string of length exactly 1. Passing a multi-character string (or empty string) has no single grid cell to occupy and raises `ValueError: char should be a single character`.

Source

Thrown at libs/core/langchain_core/runnables/graph_ascii.py:107

    def point(self, x: int, y: int, char: str) -> None:
        """Create a point on ASCII canvas.

        Args:
            x: x coordinate. Should be `>= 0` and `<` number of columns in
                the canvas.
            y: y coordinate. Should be `>= 0` an `<` number of lines in the
                canvas.
            char: character to place in the specified point on the
                canvas.

        Raises:
            ValueError: if char is not a single character or if
                coordinates are out of bounds.
        """
        if len(char) != 1:
            msg = "char should be a single character"
            raise ValueError(msg)
        if x >= self.cols or x < 0:
            msg = "x should be >= 0 and < number of columns"
            raise ValueError(msg)
        if y >= self.lines or y < 0:
            msg = "y should be >= 0 and < number of lines"
            raise ValueError(msg)

        self.canvas[y][x] = char

    def line(self, x0: int, y0: int, x1: int, y1: int, char: str) -> None:
        """Create a line on ASCII canvas.

        Args:
            x0: x coordinate where the line should start.
            y0: y coordinate where the line should start.
            x1: x coordinate where the line should end.
            y1: y coordinate where the line should end.
            char: character to draw the line with.

View on GitHub (pinned to e32fa9a52e)

Solutions

1. Draw multi-character content one character at a time: `for i, c in enumerate(s): canvas.draw_on(x + i, y, c)`.
2. Use a single glyph (e.g. `">"`, `"*"`) where one cell is expected.
3. Guard with `if len(char) != 1: raise/handle` before calling when char comes from user config.

Example fix

# before
canvas.draw_on(x, y, "->")

# after
for i, ch in enumerate("->"):
    canvas.draw_on(x + i, y, ch)

assert isinstance(char, str) and len(char) == 1
canvas.draw_on(x, y, char)

from typing import TypeGuard

def is_single_char(s: object) -> TypeGuard[str]:
    return isinstance(s, str) and len(s) == 1

When it happens

Trigger: Calling `canvas.draw_on(x, y, "->")`, `draw_on(x, y, "")`, or passing a label/arrow of length > 1; customizing `DrawGraph` (which itself draws arrow chars like `>` one cell at a time) with multi-char glyphs such as `"=>"` or emoji.

Common situations: Extending graph ASCII rendering with custom arrowheads or markers; drawing edge labels directly with `draw_on` instead of looping per character; passing a character variable that defaults to an empty string.

Related errors

• Canvas dimensions should be > 1
• x should be >= 0 and < number of columns
• Node with id {id} already exists
• Source node {source.id} not in graph
• Target node {target.id} not in graph

AI-assisted analysis of langchain-ai/langchain@e32fa9a52e (2026-08-14). Data as JSON: /api/errors/b1ac0a8d9eb9dc08. Report an issue: GitHub.