Python Intro - #1 - Lists

listsarecool.py

# Welcome to Jeff's first 5 minute intro to Python.

# More illustrative of a language than just the syntax, I think
# how a language handles data says a lot.  So, I'm going to talk
# mostly about that, and let's start with lists.

# Since we all know how to program, I'm going to just leave a lot to
# be picked up by example rather than by exposition.   

# --- save as 'listsarecool.py'

import string

def main():
    # lists are first class entities in Python
    colors = [ 'red', 'yellow', 'black', 'blue', 'brown', 'white' ]
    
    # they follow standard array semantics with some nice enhancements:
    firstcolor = colors[0]
    lastcolor = colors[-1]
    blacknblue = colors[2:4]
    everysecondcolor = colors[::2]
    colorsreversed = colors[::-1]
    
    print "Lists can be joined together as strings like this: {0}".format( ", ".join(colors) )
    
    # oops, I forgot one, but lists are mutable.
    colors.append('orange')
    
    # a linear search for existence of an element in a list is simple:
    if 'milk' in colors:
        print "Wow, it turns out milk is a color"
    else:
        print "Everyone knows that 'milk' is actually a number"
    
    # 'in' is also used during iteration, this is the prefered form
    for color in colors:
        print color
       
        
    # You can transform lists with list comprehensions, which wrap the iteration
    reversedcolors = [ color[::-1] for color in colorsreversed ]
    print "In reverse: {0}".format( ", ".join(reversedcolors) )
    
    # they can also do filtering:
    fiveletters = [ color for color in colors if len(color) == 5 ]
    print "These colors have 5 letters: ", ", ".join(fiveletters)
    
    # essentially, a list comprehension is syntactic sugar for map() & filter()
    # the following is the same as: 
    #     print [color[::-1] for color in colors if len(color) == 5]
    is_len_five = lambda s: len(s) == 5
    reverse_string = lambda s: s[::-1]
    fivereversed = map( reverse_string, filter(is_len_five, colors) )
    print fivereversed
    
    # btw: strings can be treated as lists of characters
    phone = '(260) 555-1212'
    justdigits = [c for c in phone if c in string.digits]
    normalized_phone = "".join(justdigits)
    print "normalized {0} -> {1}".format(phone, normalized_phone)
    
    # There are some custom classes for supporting more complex data
    # structures, but the .pop(n) method lets you treat a list as 
    # a stack or a queue easily enough.
    stack = []
    stack.append( 1 )    # it would be nice if it were named 'push' but oh well
    stack.append( 2 )
    top = stack.pop(-1)   
    assert top == 2
    
    queue = []
    queue.append( 1 )
    queue.append( 2 )
    head = queue.pop(0)
    assert head == 1
    
    
if __name__ == "__main__":
    main()
    
# --- end 'listsarecool.py'

# You can execute it thusly:    python listsarecool.py