Lab 2 Material / September 4th, 2008

Short note on homework & associated tests

Eternal optimism is good, but if another person comes up to me and says

assert list(x) == list(x)

is failing!! but it shouldn't! My code works! I will (metaphorically) slap them silly if they haven't broken the code down like this:

a = list(x)
print a
b = list(x)
print b
assert a == b

Just a warning.

Handing in your homework

You'll be using Subversion to hand in your homework.

For homework #1, this involves:

  • checking out a (blank!) working copy of your subversion repository:<username>

  • creating a directory 'homework1' in your working copy

  • adding that directory to the list of files to commit

  • putting your code in a file named '', in that directory

  • adding the file to the list of files to commit

  • committing your changes

Under UNIX, here are the commands:

% svn checkout<username>
% cd <username>
% mkdir homework1
% svn add homework1
% cd homework1
% cp /some/other/path/homework-file ./
% svn add
% svn commit -m "my first homework"

(If you have a different SVN username than your login account, you may need to specify 'svn checkout --username <username> ...'.)

Under Windows, you can use TortoiseSVN (installed on all the lab Windows machines) to do this with a GUI; we can run through it in class.

Once you've done the commit, check out a new copy and make sure that the file is there and correct. (You can also browse your repository online, at the same URL.) A convenient way to do this checkout under UNIX is

svn checkout<username> TEST

This puts the new 'working copy' in the 'TEST/' directory.

For homework2, just put all the files in the 'homework2/' top-level subversion directory, with filenames as given in the homework handout.

For office hours and e-mailing me questions, you can use subversion ('svn' for short) to send me the files you want to ask me about: check in whatever you're working on and tell me the URL, e.g.<username>/homework1/

For those who want to use git, please just start by using subversion to hand in the homework. I'll set up git repos soon, sorry :).

Homework review

We'll do a brief homework review on Tuesday in class, once I've had a chance to look at your homework and send you suggestions for changes/fixes. (I'll try to do that by Friday evening.)

Scripts vs importable files

First, see the Wiki page Creating and running Python scripts for basic info.

Scripts are Python files that do something more than just define stuff -- they contain execution instructions. E.g.,
  print 'hello, world'

The name of a script file can be anything that's a valid filename. On UNIX, this is anything that doesn't contain a forward slash ('/'). On Windows, it's something similar. In general I suggest making things cross-OS-friendly and typing-friendly, e.g. no spaces, backslashes, or funny characters; 'my-favorite-script-name' is fine. There's no requirement that the filename end in '.py' although if it does than various operating systems may let you double-click on it to run it.

A neat UNIX/Mac OS X trick is to put this magic code:

#! /usr/bin/env python2.5

at the top of your script file, and then do 'chmod +x scriptfile'. This will make it a shell-executable script than be run simply by doing


Modules are importable Python files. We can talk later about the precise rules that import uses, but things in the same directory as a script are generally importable by Python. However, module files must end in '.py' and have to be valid Python identifiers, e.g.

import modulefile


import module_file

will find '' or '' appropriately, but you can't do

import module-file

to get '' because that's not a legal Python name ('-' isn't allowed in variable names.)

Generally I recommend writing modules so that they are both modules and scripts; the scripts do some basic testing of the code in the module. So, for example,
 def hello(world):
   return 'hello ' + world

 assert hello('world') == 'hello world'

will define 'hello' and also run a test on it: it can be imported as a module,

import hello
print hello.hello('steve')

or run as a script:

% python

You may not always want stuff to be run on import, especially if you have time-consuming tests, so you can use this trick (documented on the Wiki page, too) to differentiate between code that should be executed on import vs on command-line execution:
 def hello(world):
    return 'hello ' + world

 if __name__ == '__main__':
    assert hello('eva') == 'hello eva'

The 'assert' command will only be run when the file is executed as a script by doing


This kind of thing is really convenient when you're writing and testing code: put your test code at the bottom of the file and then just run the module every time you make a change (F5 in IDLE).

To recap, here's my recommended format for Python modules:
  #! /usr/bin/env python2.5

  # ... define functions/classes/etc. ...

  if __name__ == '__main__':
     # ... define tests of functions/classes etc ...
     #   OR
     # ... define script-like behavior ...

Two more command-line thoughts

If you use the '-i' flag to the Python interpreter when running a script, you'll be left at the Python prompt after the script runs:

% python -i scriptfile
... scriptfile runs ...

This Python prompt will have all your functions defined for interactive testing, etc. Very useful.

If you want to write an actual script that takes in arguments, use 'sys.argv'. It is a list of all of the arguments to your script:
  print sys.argv

so that

% python arg1 arg2 arg3 arg4

will print out arg1 arg2 arg3 arg4

You'll need this for the homework.

TCP/IP socket stuff

Connecting out

Creating a socket:

sock = socket.socket()

Connecting that socket to a remote server:

remote_address = ''
port = 25
sock.connect((remote_address, port))

Reading from the socket:

data = sock.recv(4096)

You will get up to bufsize=4096 bytes, and if there is nothing available, 'sock.recv' will block (wait) until either there is something available, or the socket closes, or you hit CTRL-C.

Writing to the socket:

data = sock.sendall('HELO')

Here's a sample SMTP (mail) session:

>>> import socket
>>> sock = socket.socket()

Connect to the server:

>>> sock.connect(('', 25))
>>> sock.recv(4096)                   #doctest: +ELLIPSIS
'220 ESMTP Exim 4.63 ...\r\n'

Say hello:

>>> sock.sendall("HELO\n")
>>> sock.recv(4096)                   #doctest: +ELLIPSIS
'250 Hello [...]\r\n'

Identify the sender address:

>>> sock.sendall("MAIL FROM:\n")
>>> sock.recv(4096)
'250 OK\r\n'

Identify the recipient address:

>>> sock.sendall("RCPT TO:\n")
>>> sock.recv(4096)
'250 Accepted\r\n'

OK, greetings are over; set up to send the actual message:

>>> sock.sendall('DATA\n')
>>> sock.recv(4096)
'354 Enter message, ending with "." on a line by itself\r\n'

Aaaaaaaaaaaaand send!

>>> sock.sendall('foo bar baz\n.\n')
>>> sock.recv(4096)       #doctest: +ELLIPSIS
'250 OK...\r\n'

Be nice & 'quit' before closing.

>>> sock.sendall('quit\n')
>>> sock.close()

Allowing connections in

Create a socket:

import socket
sock = socket.socket()

If 'interface' is a blank string, connections are allowed in to all IP addresses on this host; if it's non-blank, e.g. e.g. '', then only connections to that network interface are allowed.

interface = ''
port = 5555

sock.bind( (interface, port) )

Allow up to 5 "backlogged" connections:


Handle incoming connections until they die:

while 1:
   (client_sock, client_address) = sock.accept()

    while 1:
        data = client_sock.recv(4096)
        if not data:

        print 'got data:', data

For example, if you run this in IDLE, you can try using your Web browser to connect to


to see what's printed out on your screen by both sides...

Two final notes -- 'bind' is sometimes a bit sticky, and you may have to wait for a while (10-20 seconds) before rebinding a socket after hitting CTRL-C. One way around this is just to change the port number.

Also, if you can find a 'telnet' program (installed on many UNIXes by default; not available on Windows by default -- enable like this) then you can talk directly to TCP ports; just do

telnet 25

to connect to port 25 on