为什么在Python 3.4的循环中使用socket.connect（）会停止工作？

Question

I'm just starting to learn programming python, and have been following a tutorial for creating a simple port scanner in order to learn about programming sockets. I'm able to make a successful connection to localhost when I manually enter all the code for a single iteration, however if I take the same code, and apply it within a for loop utilizing try/except, I immediately get exceptions for every port in the range, even when I know that some of the ports are open. I believe that I've isolated the problem to socket.connect() because I've entered code below that that I know never gets executed.

I can enter the following code, and get a successful return:

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.settimeout(10)
port = 22
s.connect(('127.0.0.1', port))
s.send(b'test')
banner = s.recv(1024)
print(banner)
s.close()

returns:

b'SSH-2.0-OpenSSH_6.2\r\n'

Process finished with exit code 0

However, as soon as I take that code and move it into a for loop with the port number as the iterator, it stops working.

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.settimeout(10)
for port in range(1,26):
    print("[+]Attempting to connect to : " + str(port))
    try:
      s.connect(('127.0.0.1', port))
      s.send(b'test')
      banner = s.recv(1024)
      s.close()
      if banner:
        print("Port " + port + "is Open: " + banner)
    except: print("[+]Port " + str(port) + " is closed")  

returns:

[+]Attempting to connect to : 1
[+]Port 1 is closed
[+]Attempting to connect to : 2
[+]Port 2 is closed
[+]Attempting to connect to : 3
[+]Port 3 is closed
....ETC....ETC....ETC....
[+]Attempting to connect to : 24
[+]Port 24 is closed
[+]Attempting to connect to : 25
[+]Port 25 is closed

Even though I KNOW port 22 is open and listening on localhost. (ie I am able to ssh to 127.0.0.1 without issue). I have tried everything I can think of to no avail, including changing the data type of port to an int manually by using the internal int() function, I've tried the socket.connect_ex object, etc. I've also put code right below the socket.connect statement just to see if it shows up, which it never does.

Answer 1

The Zen of Python states:

Errors should never pass silently.
Unless explicitly silenced.

Only you have not silenced the error but instead just replaced it with a message that is non-descriptive of what actually happened:

>>> "Port" + 1
Traceback (most recent call last):
  File "<pyshell#15>", line 1, in <module>
    "Port "+1
TypeError: Can't convert 'int' object to str implicitly

is what you will get if opening port 1 worked, but after you close a socket you can't connect to anything else:

>>> a = socket.socket()
>>> a.close()
>>> a.connect(("www.python.com",80))
Traceback (most recent call last):
  File "<pyshell#18>", line 1, in <module>
    a.connect(("www.python.com",80))
OSError: [Errno 9] Bad file descriptor

So you need to create a new socket inside the loop for it to work properly but most importantly: you need to limit the errors you catch:

try:
    #if this is the only line you expect to fail, then it is the only line in the try
    s.connect(('127.0.0.1', port))
except ConnectionError: 
    #if a ConnectionError is the only one you expect, it is the only one you catch
    print("[+]Port " + str(port) + " is closed")
else: #if there was no error
    s.send(b'test')
    banner = s.recv(1024)
    s.close()
    if banner:
        print("Port " + port + "is Open: " + banner)

then you will see the actual errors you are getting instead of guessing what went wrong which is also against The Zen of Python :

In the face of ambiguity, refuse the temptation to guess.