Generalized Set Functions

Let’s look at a more useful example of special argument-matching modes at work. At the end of Chapter 16, we wrote a function that returned the intersection of two sequences (it picked out items that appeared in both). Here is a version that intersects an arbitrary number of sequences (one or more) by using the varargs matching form *args to collect all the passed-in arguments. Because the arguments come in as a tuple, we can process them in a simple for loop. Just for fun, we’ll code a union function that also accepts an arbitrary number of arguments to collect items that appear in any of the operands:

def intersect(*args):
    res = []
    for x in args[0]:                  # Scan first sequence
        for other in args[1:]:         # For all other args
            if x not in other: break   # Item in each one?
        else:                          # No: break out of loop
            res.append(x)              # Yes: add items to end
    return res

def union(*args):
    res = []
    for seq in args:                   # For all args
        for x in seq:                  # For all nodes
            if not x in res:
                res.append(x)          # Add new items to result
    return res

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Because these are tools worth reusing (and they’re too big to retype interactively), we’ll store the functions in a module file called inter2.py (if you’ve forgotten how modules and imports work, see the introduction in Chapter 3, or stay tuned for in-depth coverage in Part V). In both functions, the arguments passed in at the call come in as the args tuple. As in the original intersect, both work on any kind of sequence. Here, they are processing strings, mixed types, and more than two sequences:

% python
>>> from inter2 import intersect, union
>>> s1, s2, s3 = "SPAM", "SCAM", "SLAM"

>>> intersect(s1, s2), union(s1, s2)           # Two operands
(['S', 'A', 'M'], ['S', 'P', 'A', 'M', 'C'])

>>> intersect([1,2,3], (1,4))                  # Mixed types
[1]

>>> intersect(s1, s2, s3)                      # Three operands
['S', 'A', 'M']

>>> union(s1, s2, s3)
['S', 'P', 'A', 'M', 'C', 'L']

Note

I should note that because Python now has a set object type (described in Chapter 5), none of the set-processing examples in this book are strictly required anymore; they are included only as demonstrations of coding techniques. Because it’s constantly improving, Python has an uncanny way of conspiring to make my book examples obsolete over time!