eTechTips » Python

Python: if/else Statements

ecdown — Fri, 20 Jan 2012 16:25:47 +0000

There comes a time when you are writing a program and you need to branch based on input to a program.
This is the format of the if/else statement in Python.

if  :
  
else:

There may be times when you have more than one value set to test against. In that case you can write a many if/else
statements but you can chain the if/else statements with an elif. So the chain becomes if/elif/else.

if x < 5:
  
elif x > 5 and x < 10:
  
else:

Python: Allow for empty if else blocks

ecdown — Fri, 20 Jan 2012 16:15:28 +0000

I was toying with a prime number finder and was creating a set of if/else statements for a horribly failed attempt
and wanted to skip to the next number on any of the if statements that were true. I didn’t want to put an empty print
statement as that would disrupt the output I was expecting from the program. I found the pass statement.
Here is an example from my not very successful attempt at finding primes:

if inp > 2 and inp % 2 == 0:
  pass
elif inp > 3 and inp % 3 == 0:
  pass
elif inp > 7 and inp % 7 == 0:
  pass
elif inp > 9 and inp % 9 == 0:
  pass
else:
  print inp, " is a prime"

Yes, this does not accurately find the primes, but it would skip any number that is divisible by 2,3,5,7, or 9. And
anything that does not match this will be listed as a prime, this is incorrect and would be made to add new elif
statements as each prime was found.

How to exit python script

ecdown — Tue, 23 Aug 2011 13:43:50 +0000

When I was trying to terminate a python script after a failure, I tried to find the nicest way to exit. The sys.exit() command provided by importing the sys library was what I found.

The function can take an optional numeric argument, usually in the range of 0-127, and with no argument it returns 0. The return code is passed back to the Operating System.

import sys
sys.exit(1)

When exit returns a non-zero value, it is considered an error.

Gallery of Hello World!

ecdown — Thu, 04 Aug 2011 15:14:39 +0000

Here is a list of the Hello World Programs for different languages:

#include
int main()
{
printf("Hello World!");
}

C++

#include
int main()
{
cout << "Hello World!" << endl;
}

public class HelloWorld
  public static void Main()
  {
    System.Console.WriteLine("Hello World!");
  }

Java

class HelloWorld {
static void main(String[] args)
{
System.out.println("Hello World!");
}

SHELL

echo "Hello World"

Python

print "Hello World!\n"

Ruby

puts "Hello World!"

Perl

print "Hello World!\n";

PHP

This is the simplest form of Hello World for most of these languages.

Python: AttributeError: ‘getopt’ module has no attribute ‘GetoptError’

ecdown — Wed, 28 Jul 2010 16:31:22 +0000

So I was figuring out how to parse command line arguments with python and using the getopt module with the following code


#!/usr/bin/env python
import sys
import getopt

def main(argv):
   infile = "some.xml"
   try:
      opts,args = getopt.getopt(argv,"hi:d",["help","infile="]
   except getopt.GetoptError:
      usage()
      sys.exit(2)

if __name__ == "__main__":
    main(sys.argv[1:])

And I was getting the following error:

AttributeError: ‘getopt’ module has no attribute ‘GetoptError’

So my first problem was that I had named my script getopt.py and after renaming the script, I got the same error. Well you now need to remove the getopt.pyc file that was generated from your earlier running of the poorly named script.