Python Features and Surprises

Today we walked through some examples of "Python surprises" -- things that work differently than one might expect. Here are some fun examples:

• The sorted() function returns a sorted list. The sort() method sorts a list in place. We've seen these before -- but the subtle difference is worth the reminder.
• List indexes -- negative indexes index backward from the end, with -1 (negative one) representing the last element of the array, -2 the second to last, etc. As a result, one needs to walk beyond the first element to get an IndexError:

  ---

  #!/usr/bin/python
  
  numbers = [1, 2, 3, 4, 5]
  
  print numbers
  try:
    for index in range(0, 10):
      print numbers[index]
  except IndexError:
    print "We counted forward up to index " + str(index) + ", at which time " \
          + "we got  an IndexError."
  
  
  print ""
  print ""
  
  
  print numbers
  try:
    for index in range(-1, -10, -1):
      print numbers[index]
  except IndexError:
    print "We counted backward up to index " + str(index) + ", at which time " \
          + "we got  an IndexError."
  
             

  ---

• Python provides a way of defining short, anonymous functions called lambdas. They are short in the sense that they, literally, map from a variable to a value without any other parameters. They are anyonymous in the sense that they don't have names.

  But, let's not read too much into the anonymous part. Although they technically don't have names -- we can assign them to variables, which serve as names. Check out the example below:

  ---

  square = lambda x: x**2
  print square (4)
           

  ---

• Even more interesting, we can use functions to parameterize lambdas. Notice the example below, where the lambda is defined using an additional variable provided via a function:

  ---

  def power(n): return lambda x: x**n
  power3 = power(3)
  
  print power3(2)
           

  ---

• We've used variable argument functions before, but we haven't really dwelled upon them or looked at how to write them. The basic idea is that we can have a list of usual positional arguments, followed by a list of arguments that are given default values, making them optional when called. Since we don't know, when called, which of the arguments are included, and which are not, we need to specify any optional arguments in the form "argumentname=value". An example is below:

  ---

  def printRecord(name, role="Student", status="Active"):
    print name + ": " + role + "(" + status + ")"
  
  printRecord("Jane Blogs")
  printRecord("John Blogs", status="Graduated")
  printRecord("Asa Frank", role="TA")
  printRecord("Gregory Kesden", role="Instructor")
  
                  

  ---

• Python has a feature called List comprehensions designed to make it easier to initialize lists. Comprehensions enable lists to be initialized using loops to generate lists, and then using if statements to filter them. Check out the examples below which show the initialization of a list with one loop, the filting of a similar comprehension using an if, the initialization of a list using multiple loops, and the filtering of the same. together they illustrate the basic idea of specifying the result, specifying the loop(s) that generate it, and then specifying an if to filter it.

  ---

  numbers = [ number for number in range(10) ]
  print numbers
  
  evens = [ number for number in range(10) if ((number%2) == 0)]
  print evens
  
  adjectives = ["Fast", "Slow", "Big", "Small" ]
  nouns = [ "Car", "Boat", "Truck", "Plane" ]
  combinations = [ (adjective, noun) for adjective in adjectives \
                                     for noun in nouns ]
  print combinations
  
  boatCombinations = [ (adjective, noun) for adjective in adjectives \
                                          for noun in nouns  \
                                          if (noun == "Boat") ]
  print boatCombinations
  
           

  ---