go-delve/delve · error

first argument of append_file was not a string

Error message

first argument of append_file was not a string

What it means

append_file's first argument must be a starlark.String containing the file path. If args[0] is any other Starlark type (int, list, None, etc.), the builtin returns this error before calling os.OpenFile. The message explicitly says 'first argument' because the second argument may legitimately be non-string (it is converted with toBytes).

Source

Thrown at pkg/terminal/starbind/starlark.go:127

		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)
		if err != nil {
			return nil, decorateError(thread, err)
		}
		defer f.Close()
		_, err = f.Write(toBytes(args[1]))
		return starlark.None, decorateError(thread, err)
	})
	builtindoc(appendFileBuiltinName, "(Path, Text)", "append text to the specified file.")

	env.env[writeFileBuiltinName] = starlark.NewBuiltin(writeFileBuiltinName, 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 write_file was not a string"))

View on GitHub (pinned to a23773e6c3)

Solutions

1. Quote or stringify the path: append_file(str(path_value), text)
2. Check the variable actually holds a string before the call: print(path)
3. Build paths with string concatenation instead of containers: append_file("/tmp/" + name, text)

Example fix

// before
parts = ["/tmp", "out.log"]
append_file(parts, "line\n")

// after
path = "/tmp/out.log"
append_file(path, "line\n")

if type(path) != "string":
    fail("append_file path must be a string: " + str(path))
append_file(path, text)

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

# usage
append_file(as_path(raw_path), text)

When it happens

Trigger: Calling append_file with a non-string first argument, e.g. append_file(123, "data"), append_file(None, "data"), or passing a Path-like/list value produced elsewhere in the script.

Common situations: Scripts that build paths via string operations that accidentally yield a list or None (e.g. a failed split or lookup), or users passing a numeric constant instead of a quoted path.

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
• argument of read_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/e7bffc848f9a1e85. Report an issue: GitHub.