'str' object has no attribute 'decode' & "write() argument must be str, not bytes"

I would like to make a simple web application using Amazon Polly.

This is my directory structure.

.
├── data.txt
├── main.py
└── templates
    └── index.html

This is my main.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
from flask import Flask
from flask import render_template
from flask import request
from flask import send_file
from boto3 import Session
import os
import csv
from io import StringIO
from contextlib import closing

app = Flask(__name__)

@app.route("/", methods=['POST', 'GET'])
def index():
    if request.method == "POST":
        ## Sessionを作成
        session = Session(aws_access_key_id='???', aws_secret_access_key='???', region_name='us-east-1')
        polly = session.client("polly")

        ## polly.mp3が存在した場合は削除
        if os.path.isfile('/tmp/output.mp3'):
            os.remove ('/tmp/output.mp3')

        ## data.txtをCSVとして読み込む
        f = open('data.txt')
        reader = csv.reader(f)

        ## CSVを1行ずつ読み込む
        for line in reader:

            ## CSVの1つめの項目で男女を切り替え
            if line[0] == "M":
                voice="Matthew"
            elif line[0] == "F":
                voice="Joanna"

            ## 音声データを作成
            response = polly.synthesize_speech(
                        Text = '<speak><amazon:domain name="conversational"><prosody rate="slow">' + line[1].decode("utf-8") + '</prosody></amazon:domain></speak>',
                        OutputFormat = "mp3",
                        VoiceId = voice, Engine='neural', TextType='ssml')

            ## polly.mp3に追加書き込み
            if "AudioStream" in response:
                with closing(response["AudioStream"]) as stream:
                    data = stream.read()
                    fw = open("/tmp/output.mp3", "a+")
                    fw.write(data)
                    fw.close()

        f.close()

        return send_file("/tmp/output.mp3",as_attachment=True)
    else:
        return render_template("index.html")

if __name__ == "__main__":
    app.run()

This is my index.html

<html>
    <body>
        <form action="/" method="POST">
            <input type="submit" value="download">
        </form>
    </body>
</html>

When I tested this code locally (Windows10, Windows Subsystems for Linux, Debian, Python3.7.6), I got this error message.

  File "main.py", line 41, in index    
    Text = '<speak><amazon:domain name="conversational"><prosody rate="slow">' + line[1].decode("utf-8") + '</prosody></amazon:domain></speak>',
AttributeError: 'str' object has no attribute 'decode'

So, I searched the Internet and found this page. 'str' object has no attribute 'decode'. Python 3 error?

I drop the .decode('utf-8') part:

Text = '<speak><amazon:domain name="conversational"><prosody rate="slow">' + line[1] + '</prosody></amazon:domain></speak>',

Then I got a new error message.

  File "main.py", line 50, in index
    fw.write(data)  
TypeError: write() argument must be str, not bytes

I searched "write() argument must be str, not bytes". A lot of websites suggest that I should put the .decode('utf-8') part.

Could you give me any information or suggestion?

Solution

As @furas commented,

open("/tmp/output.mp3", "ba+")

worked perfectly. Thank you @furas!