Learning Python

Chapter 2

1. The basics. Here are the sort of results you should get, along with a few comments about their meaning:

                        Numbers

   >>> 2 ** 16              # 2 raised to the power 16
   65536
   >>> 2 / 5, 2 / 5.0       # integer / truncates, float / doesn't
   (0, 0.4)
   
   Strings

   >>> "spam" + "eggs"      # concatenation
   'spameggs'
   >>> S = "ham"
   >>> "eggs " + S
   'eggs ham'
   >>> S * 5                # repetition
   'hamhamhamhamham'
   >>> S[:0]                # an empty slice at the front--[0:0]
   ''
   >>> "green %s and %s" % ("eggs", S)  # formatting
   'green eggs and ham'
   
   Tuples

   >>> ('x',)[0]                        # indexing a single-item tuple
   'x'
   >>> ('x', 'y')[1]                    # indexing a 2-item tuple
   'y'
   
   Lists

   >>> L = [1,2,3] + [4,5,6]            # list operations
   >>> L, L[:], L[:0], L[-2], L[-2:]
   ([1, 2, 3, 4, 5, 6], [1, 2, 3, 4, 5, 6], [], 5, [5, 6])
   >>> ([1,2,3]+[4,5,6])[2:4]
   [3, 4]
   >>> [L[2], L[3]]                     # fetch from offsets, store in a list
   [3, 4]
   >>> L.reverse(); L                   # method: reverse list in-place
   [6, 5, 4, 3, 2, 1]
   >>> L.sort(); L                      # method: sort list in-place
   [1, 2, 3, 4, 5, 6]
   >>> L.index(4)                       # method: offset of first 4 (search)
   3
   
   Dictionaries

   >>> {'a':1, 'b':2}['b']              # index a dictionary by key
   2
   >>> D = {'x':1, 'y':2, 'z':3}
   >>> D['w'] = 0                       # create a new entry
   >>> D['x'] + D['w']
   1
   >>> D[(1,2,3)] = 4                   # a tuple used as a key (immutable)
   >>> D
   {'w': 0, 'z': 3, 'y': 2, (1, 2, 3): 4, 'x': 1}
   >>> D.keys(), D.values(), D.has_key((1,2,3))          # methods
   (['w', 'z', 'y', (1, 2, 3), 'x'], [0, 3, 2, 4, 1], 1)
   
   Empties

   >>> [[]], ["",[],(),{},None]                          # lots of nothings
   ([[]], ['', [], (), {}, None])

2. Indexing and slicing ...