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Important Topics Continue:

4. Nesting : 
 

>>> M = [[1, 2, 3],               # A 3 × 3 matrix, as nested lists
         [4, 5, 6],               # Code can span lines if bracketed
         [7, 8, 9]]
>>> M
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
Here, we’ve coded a list that contains three other lists. The effect is to represent a
3 × 3 matrix of numbers. Such a structure can be accessed in a variety of ways:
>>> M[1]                          # Get row 2
[4, 5, 6]
>>> M[1][2]                       # Get row 2, then get item 3 within the row
6

5. Comprehensions :

>>> col2 = [row[1] for row in M]             # Collect the items in column 2
>>> col2
[2, 5, 8]
>>> M                                        # The matrix is unchanged
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

>>> [row[1] + 1 for row in M]                 # Add 1 to each item in column 2
[3, 6, 9]
>>> [row[1] for row in M if row[1] % 2 == 0]  # Filter out odd items
[2, 8]

>>> diag = [M[i][i] for i in [0, 1, 2]]      # Collect a diagonal from matrix
>>> diag
[1, 5, 9]
>>> doubles = [c * 2 for c in 'spam']        # Repeat characters in a string
>>> doubles
['ss', 'pp', 'aa', 'mm']

>>> list(range(4))                           # 0..3 (list() required in 3.X)
[0, 1, 2, 3]
>>> list(range(−6, 7, 2))                    # −6 to +6 by 2 (need list() in 3.X)
[−6, −4, −2, 0, 2, 4, 6]
>>> [[x ** 2, x ** 3] for x in range(4)]     # Multiple values, "if" filters
[[0, 0], [1, 1], [4, 8], [9, 27]]
>>> [[x, x / 2, x * 2] for x in range(−6, 7, 2) if x > 0]
[[2, 1, 4], [4, 2, 8], [6, 3, 12]]

>>> G = (sum(row) for row in M)              # Create a generator of row sums
>>> next(G)                                  # iter(G) not required here

6
>>> next(G)                                  # Run the iteration protocol next()
15
>>> next(G)
24

>>> list(map(sum, M))                        # Map sum over items in M
[6, 15, 24]
In Python 2.7 and 3.X, comprehension syntax can also be used to create sets and
dictionaries:
>>> {sum(row) for row in M}                  # Create a set of row sums
{24, 6, 15}
>>> {i : sum(M[i]) for i in range(3)}        # Creates key/value table of row sums
{0: 6, 1: 15, 2: 24}
In fact, lists, sets, dictionaries, and generators can all be built with comprehensions in
3.X and 2.7:
>>> [ord(x) for x in 'spaam']                # List of character ordinals
[115, 112, 97, 97, 109]
>>> {ord(x) for x in 'spaam'}                # Sets remove duplicates
{112, 97, 115, 109}
>>> {x: ord(x) for x in 'spaam'}             # Dictionary keys are unique
{'p': 112, 'a': 97, 's': 115, 'm': 109}
>>> (ord(x) for x in 'spaam')                # Generator of values
<generator object <genexpr> at 0x000000000254DAB0>