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