List
****

.. highlight:: python

Initialise
----------

::

  >>> l = []
  >>> sports = ['football', 'cricket']

Adding
------

::

  l.append(value)

Append/Extend
-------------

::

  In [5]: l1 = [1,2,3]
  In [6]: l2 = [4,5,6]
  In [7]: l1.extend(l2)
  In [8]: l1
  Out[8]: [1, 2, 3, 4, 5, 6]

Not sure if it is OK to do the following, but it appears to work::

  >>> l1 = ['peter', 'alison',]
  >>> l2 = ['barry', 'martin',]
  >>> l1
  ['peter', 'alison']
  >>> l2
  ['barry', 'martin']
  >>> l1 + l2
  ['peter', 'alison', 'barry', 'martin']
  >>>

Convert
-------

List to dictionary::

  >>> l = [(1, 'one'), (2, 'two'), (3, 'three')]
  >>> dict(l)
  {1: 'one', 2: 'two', 3: 'three'}

List to tuple::

  >>> sports = ['football', 'cricket']
  >>> tuple(sports)
  ('football', 'cricket')

Delete/Remove
-------------

::

  s = [1, 2, 3]
  s.remove(2)
  >> s
  [1, 3]

Duplicates
----------

From `StackOverflow: Remove duplicates and preserve order`_

To remove duplicate entries from a list and preserve the order::

  from collections import OrderedDict
  list(OrderedDict.fromkeys([4, 3, 4, 2, 4, 1]))

Get
---

Get the last element in the list::

  l[-1]

Note: Also see *Slicing* below...

Index
-----

::

  ['Exbourne', 'North Tawton', 'Winkleigh'].index('Winkleigh')

Looping
-------

::

  >>> for idx, season in enumerate(['Spring', 'Summer', 'Fall', 'Winter']):
  ...     print idx, season
  0 Spring
  1 Summer
  2 Fall
  3 Winter

Version 2.6 adds a ``start`` parameter (which defaults to 0).

Looping - Over More Than One List
---------------------------------

::

  questions = ['name', 'quest', 'favorite color']
  answers = ['lancelot', 'the holy grail', 'blue']
  for q, a in zip(questions, answers):
      print 'What is your %s?  It is %s.' % (q, a)

Reverse
-------

Reverses the items of l **in place**::

  l.reverse()

Slicing
-------

From `good primer for python slice notation`_::

  a[start:end] # items start through end-1
  a[start:]    # items start through the rest of the array
  a[:end]      # items from the beginning through end-1
  a[:]         # a copy of the whole array

Sorting
-------

Sorting (in place)::

  l.sort()


Sorting a ``list`` of ``dict`` (the ``dict`` contains the ``created`` key)::

  from operator import itemgetter
  return sorted(result, key=itemgetter("created"), reverse=True)

Sorting (python 3)::

  from datetime import date
  data = [
      {'expiry': date(2010, 6, 2), 'name': 'Patrick'},
      {'expiry': date(2010, 3, 1), 'name': 'Andrea'},
  ]
  sorted(data, key=lambda item: item.get('expiry'))
  >>> [{'name': 'Andrea', 'expiry': datetime.date(2010, 3, 1)}, {'name': 'Patrick', 'expiry': datetime.date(2010, 6, 2)}]

  sorted(data, key=lambda item: item.get('expiry'), reverse=True)
  >>> [{'name': 'Patrick', 'expiry': datetime.date(2010, 6, 2)}, {'name': 'Andrea', 'expiry': datetime.date(2010, 3, 1)}]

Sorting (python 3) - with a function (I prefer this)::

  def expiry_as_str(item):
      """Sort by date by converting to a string.

      Handles 'None' dates.

      """
      d = item.get('expiry', None)
      return d.strftime('%Y%m%d') if d else ''

  return sorted(result, key=expiry_as_str)


.. _`good primer for python slice notation`: http://stackoverflow.com/questions/509211/good-primer-for-python-slice-notation
.. _`StackOverflow: Remove duplicates and preserve order`: https://stackoverflow.com/a/39835527