如何在Dash回调的值之前显示一些文本？

import dash
import dash_core_components as dcc
import dash_html_components as html
from dash.dependencies import Input, Output

app = dash.Dash(__name__)

app.layout = html.Div([
    dcc.Input(id='input', type='text', value=''),
    html.Div(id='output')
])

@app.callback(
    Output('output', 'children'),
    [Input('input', 'value')]
)
def update_output(value):
    if value:
        return html.Div([
            '输入的值是: ',
            html.Span(value)
        ])
    else:
        return '请输入一个值'

if __name__ == '__main__':
    app.run_server(debug=True)