how to replace a certain substring in python

# Function to replace all occurrences of AB with C 
  
def replaceABwithC(input, pattern, replaceWith): 
    return input.replace(pattern, replaceWith) 
  
# Driver program 
if __name__ == "__main__": 
    input   = 'helloABworld'
    pattern = 'AB'
    replaceWith = 'C'
    print replaceABwithC(input,pattern,replaceWith) 

OUTPUT:
  'helloCworld'