Python a bytes-like object is required, not 'str'

Question

I am new to Python. I am running following simple web server:

from wsgiref.simple_server import make_server
from io import BytesIO

def message_wall_app(environ, start_response):
    output = BytesIO()
    status = '200 OK' # HTTP Status
    headers = [('Content-type', 'text/html; charset=utf-8')]
    start_response(status, headers)
    print(b"<h1>Message Wall</h1>",file=output)
##    if environ['REQUEST_METHOD'] == 'POST': 
##        size = int(environ['CONTENT_LENGTH'])
##        post_str = environ['wsgi.input'].read(size)
##        print(post_str,"<p>", file=output)
##    print('<form method="POST">User: <input type="text" '
##          'name="user">Message: <input type="text" '
##          'name="message"><input type="submit" value="Send"></form>', 
##           file=output)         
    # The returned object is going to be printed
    return [output.getvalue()]     

httpd = make_server('', 8000, message_wall_app)
print("Serving on port 8000...")

# Serve until process is killed
httpd.serve_forever()

Unfortunately i get following error:

Traceback (most recent call last):
  File "C:\Users\xxx\Python36\lib\wsgiref\handlers.py", line 137, in run
    self.result = application(self.environ, self.start_response)
  File "C:/xxx/Python/message_wall02.py", line 9, in message_wall_app
    print("<h1>Message Wall</h1>".encode('ascii'),file=output)
TypeError: a bytes-like object is required, not 'str'....

Please suggest what i am doing wrong. thanks.

Answer 1

You can't use print() to write to binary files. print() converts arguments to str() before writing to a text file object.

From the print() function documentation :

Print objects to the text stream file , separated by sep and followed by end . [...]

All non-keyword arguments are converted to strings like str() does and written to the stream, separated by sep and followed by end .

Bold emphasis mine . Note that the file object must be a text stream, not a binary stream.

Either write to a TextIOWrapper() object wrapping your BytesIO() object, call .write() on the BytesIO() object to write bytes objects directly, or write to a StringIO() object and encode the resulting string value at the end.