17.8 Sorting Sequences

Another staple of many systems is sorting: ordering items in a collection according to some constraint. The script in Example 17-24 defines a simple sort routine in Python, which orders a list of objects on a field. Because Python indexing is generic, the field can be an index or key -- this function can sort lists of either sequences or mappings.

Example 17-24. PP2E\Dstruct\Classics\sort1.py

def sort(list, field):
    res = []                                     # always returns a list
    for x in list:
        i = 0
        for y in res:
            if x[field] <= y[field]: break       # list node goes here?
            i = i+1
        res[i:i] = [x]                           # insert in result slot
    return res 

if __name__ == '__main__':
    table = [ {'name':'john', 'age':25}, {'name':'doe', 'age':32} ]
    print sort(table, 'name') 
    print sort(table, 'age')  
    table = [ ('john', 25), ('doe', 32) ]
    print sort(table, 0)
    print sort(table, 1)

Here is this module's self-test code in action:

C:\...\PP2E\Dstruct\Classics>python sort1.py
[{'age': 32, 'name': 'doe'}, {'age': 25, 'name': 'john'}]
[{'age': 25, 'name': 'john'}, {'age': 32, 'name': 'doe'}]
[('doe', 32), ('john', 25)]
[('john', 25), ('doe', 32)]

17.8.1 Adding Comparison Functions

Since functions can be passed in like any other object, we can easily allow for an optional comparison function. In the next version (Example 17-25), the second argument takes a function that should return true if its first argument should be placed before its second. A lambda is used to provide an ascending-order test by default. This sorter also returns a new sequence that is the same type as the sequence passed in, by applying the slicing techniques used in earlier sections: if you sort a tuple of nodes, you get back a tuple.

Example 17-25. PP2E\Dstruct\Classics\sort2.py

def sort(seq, func=(lambda x,y: x <= y)):             # default: ascending
    res = seq[:0]                                     # return seq's type
    for j in range(len(seq)):
        i = 0
        for y in res:
            if func(seq[j], y): break
            i = i+1
        res = res[:i] + seq[j:j+1] + res[i:]          # seq can be immutable
    return res 

if __name__ == '__main__':
    table = ({'name':'doe'}, {'name':'john'})
    print sort(list(table),  (lambda x, y: x['name'] > y['name']))
    print sort(tuple(table), (lambda x, y: x['name'] <= y['name']))
    print sort('axbyzc')

This time, the table entries are ordered per a field comparison function passed in:

C:\...\PP2E\Dstruct\Classics>python sort2.py
[{'name': 'john'}, {'name': 'doe'}]
({'name': 'doe'}, {'name': 'john'})
abcxyz

This version also dispenses with the notion of a field altogether and lets the passed-in function handle indexing if needed. That makes this version much more general; for instance, it's also useful for sorting strings.