If the condition of a if statement is not fulfilled no statement is executed.

>>> seq = 'ATGACGATAG'
>>> if 'n' in seq:
	print "sequence contains undefined characters"
>>>
      

An alternative sequence of statements, that will be executed if the condition is not fulfilled, can be specified with the else statement.

>>> seq = 'ATGACGATAG'
>>> if 'n' in seq:
...    print "sequence contains undefined bases"
... else:
       print "sequence contains only defined bases"
sequence contains only defined bases
      

Here the if and else are followed by a block containing the statements to execute depending on the truth value of the condition. In this case exactly one of them is executed, which is illustrated in Figure 7.4