Archive for September, 2010

Creating and importing a module

September 30, 2010 Leave a comment

If you have some functions that you use often, you can collect them in a module.

def f1(n):
    return n + 1

How to use it (

#!/usr/bin/env python

import mymodule

print mymodule.f1(5)      # => 6
print mymodule.__name__   # => mymodule (note that .py is missing)

It is also possible to add some testing code to a module:

#!/usr/bin/env python

def f1(n):
    return n + 1

if __name__ == "__main__":
    number = 1977
    print f1(number)   # => 1978

Now, you can still import it like in, or you can launch it as if it were a standalone script. In the latter case the test suite will be executed. If you import it, the test suite is not executed. A test suite is a good practice to be sure that the module is working as expected.

Categories: python Tags: , ,

Prison break

September 29, 2010 Leave a comment


The big boss of Montreal Prison celebrates his 50th birthday next week. For this special occasion he came up with the idea to let some prisoners go. In the prison there are 600 cells, one person in each. The cells are numbered from 1 to 600. The prison guard should go through all cells and open the doors. Then he goes to the 2nd cell and flips the lock at every second cell (that is, close the door if it was open and open it if it was closed). Then go the 3rd cell and flip the lock of every third cell, etc. This procedure should be repeated with every cell. Question: who are those lucky guys who get release, i.e. which cells remain open at the end?

Example with 8 cells:

00000000        initialization, all cells are closed
11111111        step 1, flip every lock
10101010        step 2, flip every 2nd lock
10001110        step 3, flip every 3rd lock
10011111        step 4, flip every 4th lock
10010111        step 5, flip every 5th lock
10010011        step 6, flip every 6th lock
10010001        step 7, flip every 7th lock
10010000        step 8, flip every 8th lock

Read more…

Categories: python Tags: , , ,

Reverse an integer

September 29, 2010 3 comments


Take an integer and reverse its digits. The result is also an integer. Example: 83657 becomes 75638.


#!/usr/bin/env python

def reverse_int(n):
    return int(str(n)[::-1])

n = 83657
print n                 # 83657
print reverse_int(n)    # 75638

Summary: convert the number to string, reverse the string, then convert it back to integer. Details: 83657 -> str(83657) returns "83657" which is a string -> reverse it, we get "75638" -> int("75638") converts it to an integer.


If you want to concatenate a string and an integer, first you need to convert the integer to string. Example:

n = 83657
print "The value of n is " + n          # error, won't work
print "The value of n is " + str(n)     # OK
Categories: python Tags: , ,

Fibonacci numbers

September 28, 2010 1 comment


Now an easy one. Calculate the first N Fibonacci numbers, where F0 = 0, F1 = 1, …, Fn = Fn-1 + Fn-2. Write a function that receives N as a parameter and returns a list with the first N Fibonacci numbers. Example: fib(10) would produce [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55]. Notice that F0 = 0 is included in the result.


#!/usr/bin/env python


def fib(n):
    assert(n >= 0)
    li = []
    a, b = 0, 1
    for i in range(n+1):
        a, b = b, a+b
    return li
if __name__ == "__main__":
    print fib(10)

Here we solved the problem in an iterative way, but you can do it with recursive calls too.

See for a comprehensive list how to implement it in any language.

Categories: python Tags: ,

A to Z, then Z to A

September 28, 2010 Leave a comment


Write a Python script which prints the alphabet from “a” to “z”. Reverse its source code as explained in the previous post, i.e. reverse the order of lines and reverse the order of characters in each line. This reversed script should now print the alphabet from “z” to “a”!


$ python                                        
a b c d e f g h i j k l m n o p q r s t u v w x y z
$ ./ >
$ python 
z y x w v u t s r q p o n m l k j i h g f e d c b a
$ ./ >
$ python 
a b c d e f g h i j k l m n o p q r s t u v w x y z
$ diff 

You can find the script in my previous post. As you can see, applying twice, you get back the original file.

Note that this challenge is not specific to Python. It can be solved in C++ too, for instance.

The solution is below, but first try to solve it yourself.
Read more…

Categories: python Tags: , , ,

Reverse a file

September 27, 2010 3 comments


Take a text file and reverse it the following ways: (1) reverse characters in each line, and (2) reverse the order of lines too. Let’s see an example:


#!/usr/bin/env python
print "Please, reverse me completely!"


"!yletelpmoc em esrever ,esaelP" tnirp
nohtyp vne/nib/rsu/!#

Read more…

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September 27, 2010 Leave a comment

Printing to the standard output is buffered. What to do if you want to see the output immediately?

import sys
import os

# reopen stdout file descriptor with write mode
# and 0 as the buffer size (unbuffered)
sys.stdout = os.fdopen(sys.stdout.fileno(), 'w', 0)

print "unbuffered text"


This tip is from

Update (20130206)
The solution above switches buffered mode off, but you can’t switch it back on because you lose the original sys.stdout file descriptor. I have a more sophisticated solution, available here ( as part of my jabbapylib library.

Usage #1:

# text that you want to print in unbuffered mode comes here
autoflush(False)    # back to normal

Usage #2:

# using a context manager
with AutoFlush():
    # unbuffered text comes here

# here you are back to normal

Let’s not forget the simplest and most trivial solution either:

sys.stdout.flush()    # flush out immediately