can only concatenate str (not “bytes”) to str

问题

import socket
import os

user_url = input("Enter url: ")

host_name = user_url.split("/")[2]
mysock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
mysock.connect((host_name, 80))
cmd = 'GET ' + user_url + ' HTTP/1.0\r\n\r\n'.encode()
mysock.send(cmd)

while True:
    data = mysock.recv(512)
    if len(data) < 1:
        break
     print(data.decode(),end='\n')

mysock.close()

For some reason im gettin this error

Enter url: http://data.pr4e.org/romeo.txt

 7 mysock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
 8 mysock.connect((host_name, 80))
 9 cmd = 'GET ' + user_url + ' HTTP/1.0\r\n\r\n'.encode()
 TypeError: can only concatenate str (not "bytes") to str

Any ideas what im doing wrong with it?Encoding and decoding seems right to me, and i've trasnfered it using \n before .encode(). This is for a class

回答1:

A str is an abstract sequence of Unicode code points; a bytes is a sequence of 8-bit numbers. Python 3 made the distinction between the two very clear and does not allow you to combine them implicitly. A str may have several valid encodings, and a bytes object may or may not be the encoding of a valid Unicode string. (Or, the bytes could be the encoding of multiple different str objects depending on the encoding used to create it.)

'GET ' and user_url are str objects, while ' HTTP/1.0\r\n\r\n'.encode() is a bytes object. You want to encode the entire concatenated string instead.

cmd = 'GET {} HTTP/1.0\r\n\r\n'.format(user_url).encode()

Or perhaps written to show the steps more clearly,

cmd = 'GET {} HTTP/1.0\r\n\r\n'.format(user_url)  # still a str
mysock.send(cmd.encode())  # send the encoding of the str

回答2:

The problem is that you're encoding before concatenating:

'GET ' + user_url + ' HTTP/1.0\r\n\r\n'.encode()

You have to concatenate first, and then encode the entire thing:

('GET ' + user_url + ' HTTP/1.0\r\n\r\n').encode()

来源：https://stackoverflow.com/questions/55033372/can-only-concatenate-str-not-bytes-to-str