func string_format()

in starlark/library.go [1673:1810]

• 117 lines of code
• 30 McCabe index (conditional complexity)

func string_format(_ *Thread, b *Builtin, args Tuple, kwargs []Tuple) (Value, error) {
	format := string(b.Receiver().(String))
	var auto, manual bool // kinds of positional indexing used
	buf := new(strings.Builder)
	index := 0
	for {
		literal := format
		i := strings.IndexByte(format, '{')
		if i >= 0 {
			literal = format[:i]
		}

		// Replace "}}" with "}" in non-field portion, rejecting a lone '}'.
		for {
			j := strings.IndexByte(literal, '}')
			if j < 0 {
				buf.WriteString(literal)
				break
			}
			if len(literal) == j+1 || literal[j+1] != '}' {
				return nil, fmt.Errorf("format: single '}' in format")
			}
			buf.WriteString(literal[:j+1])
			literal = literal[j+2:]
		}

		if i < 0 {
			break // end of format string
		}

		if i+1 < len(format) && format[i+1] == '{' {
			// "{{" means a literal '{'
			buf.WriteByte('{')
			format = format[i+2:]
			continue
		}

		format = format[i+1:]
		i = strings.IndexByte(format, '}')
		if i < 0 {
			return nil, fmt.Errorf("format: unmatched '{' in format")
		}

		var arg Value
		conv := "s"
		var spec string

		field := format[:i]
		format = format[i+1:]

		var name string
		if i := strings.IndexByte(field, '!'); i < 0 {
			// "name" or "name:spec"
			if i := strings.IndexByte(field, ':'); i < 0 {
				name = field
			} else {
				name = field[:i]
				spec = field[i+1:]
			}
		} else {
			// "name!conv" or "name!conv:spec"
			name = field[:i]
			field = field[i+1:]
			// "conv" or "conv:spec"
			if i := strings.IndexByte(field, ':'); i < 0 {
				conv = field
			} else {
				conv = field[:i]
				spec = field[i+1:]
			}
		}

		if name == "" {
			// "{}": automatic indexing
			if manual {
				return nil, fmt.Errorf("format: cannot switch from manual field specification to automatic field numbering")
			}
			auto = true
			if index >= len(args) {
				return nil, fmt.Errorf("format: tuple index out of range")
			}
			arg = args[index]
			index++
		} else if num, ok := decimal(name); ok {
			// positional argument
			if auto {
				return nil, fmt.Errorf("format: cannot switch from automatic field numbering to manual field specification")
			}
			manual = true
			if num >= len(args) {
				return nil, fmt.Errorf("format: tuple index out of range")
			} else {
				arg = args[num]
			}
		} else {
			// keyword argument
			for _, kv := range kwargs {
				if string(kv[0].(String)) == name {
					arg = kv[1]
					break
				}
			}
			if arg == nil {
				// Starlark does not support Python's x.y or a[i] syntaxes,
				// or nested use of {...}.
				if strings.Contains(name, ".") {
					return nil, fmt.Errorf("format: attribute syntax x.y is not supported in replacement fields: %s", name)
				}
				if strings.Contains(name, "[") {
					return nil, fmt.Errorf("format: element syntax a[i] is not supported in replacement fields: %s", name)
				}
				if strings.Contains(name, "{") {
					return nil, fmt.Errorf("format: nested replacement fields not supported")
				}
				return nil, fmt.Errorf("format: keyword %s not found", name)
			}
		}

		if spec != "" {
			// Starlark does not support Python's format_spec features.
			return nil, fmt.Errorf("format spec features not supported in replacement fields: %s", spec)
		}

		switch conv {
		case "s":
			if str, ok := AsString(arg); ok {
				buf.WriteString(str)
			} else {
				writeValue(buf, arg, nil)
			}
		case "r":
			writeValue(buf, arg, nil)
		default:
			return nil, fmt.Errorf("format: unknown conversion %q", conv)
		}
	}
	return String(buf.String()), nil
}