Some Generic Python Ideas

I am getting a chance to work with Python again. I am finding some features I was unaware of and like.

EXAMPLE 0

Iterating over slices of an array
>>> a = [1,2,3,4,5]
>>> a[::3]
[1, 4]
>>> a[::2]
[1, 3, 5]
>>> a[::5]
[1]
>>> a[::-1]
[5, 4, 3, 2, 1] //reverse order!
>>> b =["this", a, 234.0, 2012-12-12]
>>> b[::-1]
[1988, 234.0, [1, 2, 3, 4, 5], 'this']
>>> b[::2]
['this', 234.0]

This is a generator expression. Not sure why, as it seems like another general iterating thingie.
This is from the Pydocs functional programming page for version 2.

Iterating using a list comprehension
>>> words = "This is a set of fabulant vociferications".split()
>>> words
['This', 'is', 'a', 'set', 'of', 'fabulant', 'vociferications']
>>> letters = [len(word) for word in words]
>>> letters
[4, 2, 1, 3, 2, 8, 15]


EXAMPLE 1
>>> seq1 = 'abc'
>>> seq2 = ('1,2,3,4')
>>> [(x,y) for x in seq1 for y in seq2]

[('a', '1'), ('a', ','), ('a', '2'), ('a', ','), ('a', '3'), ('a', ','), ('a', '4'), ('b', '1'), ('b', ','), ('b', '2'), ('b', ','), ('b', '3'), ('b', ','), ('b', '4'), ('c', '1'), ('c', ','), ('c', '2'), ('c', ','), ('c', '3'), ('c', ','), ('c', '4')]

EXAMPLE 2
>>> seq1 = 'abc'
>>> seq2 = (1,2,3,4)
>>> seq3 = (5,6)
>>> [(x,y,z) for x in seq1 for y in seq2 for z in seq3]
[('a', 1, 5), ('a', 1, 6), ('a', 2, 5), ('a', 2, 6), ('a', 3, 5), ('a', 3, 6), ('a', 4, 5), ('a', 4, 6), ('b', 1, 5), ('
b', 1, 6), ('b', 2, 5), ('b', 2, 6), ('b', 3, 5), ('b', 3, 6), ('b', 4, 5), ('b', 4, 6), ('c', 1, 5), ('c', 1, 6), ('c',
 2, 5), ('c', 2, 6), ('c', 3, 5), ('c', 3, 6), ('c', 4, 5), ('c', 4, 6)]


Different ternary styles can be used.
These evaluate the same.
x=(2,3)[y==3]
x = 3 if (y==3) else 2
or you can branch logic
(func1 if y==2 else func2)(arg1,arg2)
or assignment
x = (class1 if y==2 else class2)(arg1,arg2)

EXAMPLE 4
While loop with nested until conditional logic common to python, using break statement to break a loop.
>>> while True:
...     response = input()
...     if int(response) %7==0:
...             break

EXAMPLE 5
Simple inheritance
class Reader(object):
    def __init__(self,fileName):
        self.fileName = FileName
        self.line

class reader2(Reader):
    def reader(self,fileName):
        try:
            f = open(fileName)
            s = f.readline()
            line = s.strip('a')
        except IOError as e:
            print "Error: {0}:{1}".format(e.errno,e.strerror)
        return line    

EXAMPLE 6
Shows how to default a pass parm, use a regex to detect if a pass parm is a URL, or raise an exception if a required parm is missing. TODO: add an argparse example.

import sys
import re
from urllib.request import urlopen
urlregx = re.compile('(https?:\/\/)([\da-z\.-]+)\.([a-z\.]{2,6})([\/\w \.-]*)*')

def fetch_words(_url):
    """Fetch the list of words from a url or a collection

    Args:
        url: a url to a utf-8 text

    Returns:
        A list of strings

    Tests:
        use 'http://sixty-north.com/c/t.txt' as url
    """
    with urlopen(_url) as story:
        story_words = []
        for line in story:
            line_words = line.decode('utf-8').split()
            for word in line_words:
                story_words.append(word)
    return story_words


def print_items(items):
    """print the items

        Args:
            An iterable series of non-object items
    """
    for item in items:
        print(item)


def main(_url = None):
    if _url == None:
        url = 'http://textfiles.com/anarchy/JOLLYROGER/001.jrc'
    else:
        url = _url
    words = fetch_words(url)
    print_items(words)

if __name__ == '__main__':
    _a = sys.argv
    try:
        x = urlregx.match(_a[1])
        if x == None:
            print(_a[1]+' is not a valid url')
            sys.exit()
        main(_a[1])
    except IndexError: 
        print("from the commandline include a parameter like 'http://textfiles.com/anarchy/JOLLYROGER/003.jrc'")
        sys.exit()

REFS
http://stackoverflow.com/questions/101268/hidden-features-of-python#112303
https://github.com/gregmalcolm/python_koans/wiki

http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-189-a-gentle-introduction-to-programming-using-python-january-iap-2011/
https://docs.python.org/2/howto/functional.html

http://www.python-course.eu/lambda.php

FURTHER
How to do this in Python
    [BsonSerializer(typeof(DynamicMongoBsonSerializer))]
    public class DynamicMongoEntity : MongoEntityBase
    {
        public DynamicMongoEntity()
        {
            FieldsAsDynamic = new BsonDocument().ConvertBsonDocumentToDynamicObject();
        }

        /// <summary>
        /// BsonDocument representation of the FieldsAsDynamic property
        ///
        /// GETTER -> Will parse the BsonExtraElements document to json if it hasnt already
        /// SETTER -> Will set the local holder to the updated dynamic object and then deserize the dynamic json back into a BsonDocument for the BsonExtraElements
        /// </summary>
        public override BsonDocument CatchAll
        {
            get { return BsonSerializer.Deserialize<BsonDocument>(JsonConvert.SerializeObject(FieldsAsDynamic)); }
            set { FieldsAsDynamic = value.ConvertBsonDocumentToDynamicObject(); }
        }

        /// <summary>
        /// a c# dynamic representation of the BsonExtraElements document
        /// </summary>
        [BsonIgnore]
        [JsonIgnore]
        public dynamic FieldsAsDynamic { get; set; }

        /// <summary>
        /// This will find a dynamic mongo object in a give collection based on a key and value
        /// </summary>
        /// <param name="collection"></param>
        /// <param name="key"></param>
        /// <param name="value"></param>
        /// <returns></returns>
        public static DynamicMongoEntity FindByDynamicField(MongoCollection collection, string key, object value)
        {
            var dynamicQuery = Query.EQ(key, BsonValue.Create(value));

            var dynamicObject = collection.FindOneAs<DynamicMongoEntity>(dynamicQuery);

            return dynamicObject;
        }

        public static List<DynamicMongoEntity> FindAllByDynamicField(MongoCollection collection, string key, object value)
        {
            var dynamicQuery = Query.EQ(key, BsonValue.Create(value));

            var dynamicObjectList = collection.FindAs<DynamicMongoEntity>(dynamicQuery).ToList();

            return dynamicObjectList;
        }

        public static List<DynamicMongoEntity> FindAllByDynamicFields(MongoCollection collection, string[] keys, object[] values, Func<IEnumerable<IMongoQuery>, IMongoQuery> operatorFunc )
        {
            if (keys.Length != values.Length)
                throw new ArgumentException("Expected equal number of keys and values.");

            IEnumerable<IMongoQuery> queryExpressions = keys.Select((key, i) => Query.EQ(key, BsonValue.Create(values[i]))).ToList();

            var dynamicQuery = operatorFunc(queryExpressions);

            var dynamicObjectList = collection.FindAs<DynamicMongoEntity>(dynamicQuery).ToList();

            return dynamicObjectList;
        }

        public static IMongoQuery QueryDynamicField(string key, object value, Func<string, BsonValue, IMongoQuery> func)
        {
            return func(key, BsonValue.Create(value));
        }
    }

EXCEPTION HANDLING

#! python
# code page

import sys

"""module for exception testing"""

def convert(s):
        """converts to int"""
        x = -1.0
        try:
                x = int(s)
        except (ValueError,TypeError) as e:
                print("Conversion error: {}"\
                      .format(str(e)),file=sys.stderr)
        except:
                print("Life sucks then you die")
        return x

ITERATION EXAMPLES
iterable = ['Summer','Spring','Autumn','Winter']
iterator = iter(iterable)
for each in iterator:
...     next(iterator)Where next and iter are built ins and iterable and iterator are instance variables