1. Sequences

Copy a list 

listC = list(listA) / listA[:]

def sum_nums(nums):
"""Returns the sum of the numbers in NUMS.
>>> sum_nums([6, 24, 1984])
2014
>>> sum_nums([-32, 0, 32])
0
""" 
if (nums == []):
    return 0
else:
    return nums[0] + sum_nums( nums[1:] )

When recursively processing lists, the base case is often the empty list and the recursive case is often all-but-the-first items.

If a recursive function needs to keep track of more state than the arguments of the original function, you may need a helper function.

 2. Data abstraction 

A data abstraction lets us manipulate compound values as units, without needing to worry about the way the values are stored. 拆分compound

A pair abstraction

If we needed to frequently manipulate "pairs" of values in our program, we could use a pair data abstraction.

pair(a, b)	constructs a new pair from the two arguments.
first(pair)	returns the first value in the given pair.
second(pair)	returns the second value in the given pair.

Rational numbers

Constructor	rational(n,d)	constructs a new rational number.
Selectors	numer(rat)	returns the numerator of the given rational number.
	denom(rat)	returns the denominator of the given rational number.

Reducing to lowest terms

from math import gcd def rational(n, d):

"""Construct a rational that represents n/d in lowest terms."""

        g = gcd(n, d)

                return [n//g, d//g]