Python Programming for Biology: Bioinformatics and Beyond

Return values

There can be more than one return statement in a function, although typically there is only

one,  at  the  bottom.  Consider  the  following  function  to  get  some  text  to  say  whether  a

number  is  positive  or  negative.  It  has  three  return  statements:  the  first  two  return

statements  pass  back  text  strings  but  the  last,  which  would  be  reached  if  the  input  value

were zero, has no explicit return value and thus passes back the Python None object. Any

function code after this final return is ignored.

def getSign(value):

if value > 0:

return 'Positive'

elif value < 0:

return 'Negative'

---

return

print('Hello world') # This line is ignored

print getSign(33.6) # Result: 'Positive'

print getSign(-7) # Result: 'Negative'

print getSign(0) # Result: None

All of the examples of functions so far have returned only single values; however, it is

possible  to  pass  back  more  than  one  value  via  the  return  statement.  In  the  following

example we define a function that takes two arguments and passes back two values. The

return values are really passed back inside a single tuple, which can be caught as a single

collection  of  values.  However,  if  required  the  returned  values  can  be  extracted

immediately to give a clean syntax.

def calcFunc(x, y):

w = x*x - y*y

z = x*x + y*y

return w, z

values = calcFunc(1.414, 2.0) # Grab output as a whole tuple

print(values) # Result: (-2.000604, 5.999396)

a, b = calcFunc(1.414, 2.0) # Grab individual values

print a # Result: -2.000604

print b # Result: 5.999396

Input arguments

In  the  previous  section  we  saw  that  a  function  can  take  a  specified  number  of  input

arguments.  However,  compared  to  many  programming  languages  Python  is  somewhat

flexible in the way it accepts arguments.