go-delve/delve · error

argument of read_file was not a string

Error message

argument of read_file was not a string

What it means

read_file's sole argument must be a starlark.String path. If the first positional argument is any other Starlark value (int, list, None, bytes-like value, etc.), the builtin rejects it with this error before attempting os.ReadFile. This type check protects os.ReadFile from receiving a non-path value.

Source

Thrown at pkg/terminal/starbind/starlark.go:111

				return nil, errors.New("argument of dlv_command is not a string")
			}
			argstrs[i] = string(a)
		}
		err := env.ctx.CallCommand(strings.Join(argstrs, " "))
		if err != nil && strings.Contains(err.Error(), " has exited with status ") {
			return env.interfaceToStarlarkValue(err), nil
		}
		return starlark.None, decorateError(thread, err)
	})
	builtindoc(dlvCommandBuiltinName, "(Command)", "interrupts, continues and steps through the program.")

	env.env[readFileBuiltinName] = starlark.NewBuiltin(readFileBuiltinName, func(thread *starlark.Thread, _ *starlark.Builtin, args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
		if len(args) != 1 {
			return nil, decorateError(thread, errors.New("wrong number of arguments"))
		}
		path, ok := args[0].(starlark.String)
		if !ok {
			return nil, decorateError(thread, errors.New("argument of read_file was not a string"))
		}
		buf, err := os.ReadFile(string(path))
		if err != nil {
			return nil, decorateError(thread, err)
		}
		return starlark.String(string(buf)), nil
	})
	builtindoc(readFileBuiltinName, "(Path)", "reads a file.")

	env.env[appendFileBuiltinName] = starlark.NewBuiltin(appendFileBuiltinName, func(thread *starlark.Thread, _ *starlark.Builtin, args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
		if len(args) != 2 {
			return nil, decorateError(thread, errors.New("wrong number of arguments"))
		}
		path, ok := args[0].(starlark.String)
		if !ok {
			return nil, decorateError(thread, errors.New("first argument of append_file was not a string"))
		}
		f, err := os.OpenFile(string(path), os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o640)

View on GitHub (pinned to a23773e6c3)

Solutions

1. Ensure the argument is a quoted string or a str-typed variable: read_file("/tmp/log.txt")
2. Convert values with str() before calling: read_file(str(my_value))
3. Print or inspect the argument with print(type(x)) to confirm it is a string

Example fix

// before
path = None
contents = read_file(path)

// after
path = "/tmp/delve_state.txt"
contents = read_file(path)

if type(p) != "string":
    p = str(p)
contents = read_file(p)

def ensure_string(v):
    if type(v) == "string":
        return v
    return str(v)

# usage
contents = read_file(ensure_string(maybe_path))

When it happens

Trigger: Calling read_file with a non-string first argument, e.g. read_file(None), read_file(42), read_file(some_list), or passing the result of a function that returns a non-string value (such as a loaded Variable) as the path.

Common situations: Scripts that assume a variable holds a path string but it actually holds a parsed value, a None returned by an earlier failed call, or a numeric file descriptor. Also common when users confuse read_file with a generic file-open API expecting flags/modes.

Understand the failure class

Background: "Wrong argument type", "must be a string", "expected Array or Prism::Scope": TypeError and ArgumentError when a library receives a value of the wrong type — this error's family across 28 libraries.

Related errors

• argument of dlv_command is not a string
• first argument of append_file was not a string
• first argument of write_file was not a string
• value not loaded
• not hashable

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/8a5fc5cf41bd9b1a. Report an issue: GitHub.