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4.1 `if` Statements

Perhaps the most well-known statement type is the `if` statement. For example:

      >>> #  [Code which sets 'x' to a value...]
      >>> if x < 0:
      ...      x = 0
      ...      print 'Negative changed to zero'
      ... elif x == 0:
      ...      print 'Zero'
      ... elif x == 1:
      ...      print 'Single'
      ... else:
      ...      print 'More'
      ...

There can be zero or more `elif` parts, and the `else` part is optional. The keyword ``elif`' is short for `else if', and is useful to avoid excessive indentation. An `if` ... `elif` ... `elif` ... sequence is a substitute for the switch or case statements found in other languages.