Mystery Python code from G+

mystery_code.py

#/usr/bin/env python

from __future__ import print_function
import random

list1 = list('abc') # We can expand strings into lists with this:
list2 = list1[:]    # Makes a (shallow) copy of the list

letter_counts = dict()

for letter in list1:
    # Compact way to initialize counts for each of our possible letters
    # Dynamically adjusts if you change your list1 initialization above, too.
    letter_counts[letter] = 0
    
def main(source, target=letter_counts):
    '''Randomly choose from source and update count in target (dictionary)
    
       source must be a mutable list-like object supporting the .remove() method
       target must be a dictionary-like object with valid keys (letters) initialized
       to integer values.   
    '''
    # Defining parameters and calling with arguments eliminates the need for global variables
    # ... defining target with a default is possible, perhaps even reasonable.
    
    results = 'No letters'  # (Default from the bottom "else" in the original example)
    # (Well written functions often bind some value(s) to a variable named "results" and
    # end with the statement: return results
    
    # This prevents having multiple exit points scattered throughout a function which
    # makes code easier to read and much easier to reason about and maintain.
    
    if not source:
        return results
        # random.choice() would raise an "IndexError" if we passed an empty value
        # to our "source" parameter; we can either opt for early (harmless) return
        # ... or raise our own exception.
        
        # Early returns are sometimes a reasonable exception to the 
    
    selection = random.choice(source)
    if selection not in target:
        raise RuntimeError('The impossible has occurred?')
    ## Previous is just for pedagogical purposes!
    
    if target[selection] == 1:
        # An example of dynamically testing the value count associated with your selection
        results = 'Only found exactly one %s in %s' % (selection, source)
        source.remove(selection)
    else:
        results = selection
        target[selection] += 1    
    return results

if __name__ == '__main__':
    print(main(list2))  # Invoking such that it uses the default for "target"

   

In response to:

https://plus.google.com/u/0/107541430877608655699/posts/DsJiLgjMZYn

... some very rudimentary suggestions on Python coding.