Python Programming for Biology: Bioinformatics and Beyond

Global variables

If a variable is defined at a module level and if it is desired to change it inside a function,

then Python requires a ‘global’ statement in the function to get hold of the variable from

outside.  For  example,  suppose  we  have  a  counter  that  is  incremented  inside  a  function,

then we could do

counter = 0

def someFunction(argument):

global counter

counter += 1

performOperation(argument)

In  some  ways  it  is  good  that  Python  has  this  arduous  way  of  modifying  variables

defined  at  module  level,  because  it  is  generally  not  a  good  idea  to  code  this  way.  Often

you would avoid using global entirely by passing variables in as arguments and collecting

them as output:

counter = 0

def someFunction(argument, counterVal):

counterVal += 1

performOperation(argument)

return counterVal

counter = someFunction(input, counter)