Can't concat str to bytes

def post_app(environ, start_response):
    html = form
 
    if environ['REQUEST_METHOD'] == 'POST':
        post_env = environ.copy()
        post_env['QUERY_STRING'] = ''
        post = cgi.FieldStorage(
            fp=environ['wsgi.input'],
            environ=post_env,
            keep_blank_values=True
        )
           
        a = post['a'].value.encode('utf-8')
        b = post['b'].value.encode('utf-8')
        c = post['c'].value.encode('utf-8')
 
        if not a or not b or not c:
            html = b' Empty one of the input' + b'!'
        else:
            square = Triangle(float(a), float(b), float(c)).square()
            if type(square) is float or type(square) is complex :
                html = b'Square of treangle is ' + str(square) + b'!'
            else:
                html = square
        print(html)
    start_response('200 OK', [('Content-Type', 'text/html')])
    return [html]
 
 
 
 
servak = make_server('localhost', 8080, post_app)
print("Serving HTTP on port 8080...")
servak.serve_forever()

>>> b'Square of treangle is ' + str(100) + b'!'
Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    b'Square of treangle is ' + str(100) + b'!'
TypeError: can't concat str to bytes
>>>

>>> 'Square of treangle is ' + str(100) + '!'
'Square of treangle is 100!'
>>>

>>> bytes('Square of treangle is ' + str(100) + '!', encoding="utf-8")
b'Square of treangle is 100!'
>>>

>>> s='Square of treangle is ' + str(100) + '!'
>>> s.encode()
b'Square of treangle is 100!'
>>>

>>> square = 100
>>> f'Square of treangle is {square}!'.encode()
b'Square of treangle is 100!'
>>>