The spacing in indentation is very important to understand because it will cause an error if the indentation is incorrect. If you look at the following example you can see this. A single space on the second print returns a fault. The reason for this is because there is no conditional statement before the indentation and Python uses the spacing for the structure of the language.

>>> car = 0
>>> if car == 0:
...     print "Yes"
...      print "NO"
...
  File "<input>", line 3
    print "NO"
   ^
IndentationError: unexpected indent

Looking at the example on the left we will do a very simple conditional. We have two different IP addresses and want to validate if the last octect of the IP and compare them. In this case we use the string split method the break the IP address into a list based on the dot.

ip1 = "10.1.1.1"
ip2 = "10.1.1.2"

ip1list = ip1.split(".")
ip2list = ip2.split(".")

if ip1list[3] < ip2list[3]:
    print "yes"

Another construct in Python is the else condition. In this case you can view this as the alternative if a condition is not meet. In Python this is done via the else and the elif operators.

The else operator is simple to understand. The condition that first is met in the if statement is evaluated. Once that is done, if the expression turns out to be false it will hit the else construct.

ip1 = "10.1.1.1"
ip2 = "10.1.1.2"

ip1list = ip1.split(".")
ip2list = ip2.split(".")

if int(ip1list[3]) > int(ip2list[3]):
    print "yes"
else:
    print "no"

The elif operator control makes it possible to do another evaluation if the original expression returns false. Looking at this example we can see that we have a list of IP addresses. The program splits the string on the . and returns the last octect with index of 3 to the string.

Then the if and elif operations happens on the values of the last digits of the IP address

allip = ["10.1.1.10", "10.1.1.9", "10.1.1.15"]

ip1 = allip[0].split(".")[3]
ip2 = allip[1].split(".")[3]
ip3 = allip[2].split(".")[3]

if int(ip1) > int(ip2):
    print "ip1 is bigger than ip2"
elif int(ip2) > int(ip3):
    print "ip2 is bigger than ip3"

Since Python doesn't have a switch operator ( common in many other languages ) python would use the if elif to replace that functionality is the if and elif operators.

In this case we are simulating if the user picked an option from a menu. These options come into the if/elif tree until all possible options are hit. Once the proper match is met then the code related to that condition is executed ( in this case a print statement )

options = ["insert", "update", "delete","trim"]

selected_option=options[2]

if selected_option=="insert":
    print "user selected insert"
elif selected_option=="update":
    print "user selected update"
elif selected_option=="delete":
    print "user selected delete"
elif selected_option=="trim":
    print "user selected trim"

% python test.py
user selected delete