[Tutor] Example 1

[Magnus Lycka]

At 22:22 2002-12-10 -03-30, Adam Vardy wrote:
>I can follow the meaning of the following function. Should be simple,
>so, want to explain?
Sure!

>def union(*args):

I assume you understand the basic "def" statement. The odd thing here
is the * before args. Python has some special features in that regard.

Typically, you do

def f(x,y):
     return x + y

But sometimes you don't know how many arguments to expect.
Then you can use the *-form, like this:

 >>> def f(x,y,*more):
...     print "x=", x, "y=", y, "the rest=", more
...
 >>> f(1,2)
x= 1 y= 2 the rest= ()
 >>> f(1,2,3)
x= 1 y= 2 the rest= (3,)
 >>> f(1,2,3,4,5,6,7)
x= 1 y= 2 the rest= (3, 4, 5, 6, 7)

This means that you can construct more flexible functions in python.
You might feel that it's not needed in Python (in contrast to C for
instance), since you might as well pass a list or a tuple as an argument
to a function, but sometimes this is a more convenient approach.

There is more to learn that we don't need to explain the stuff below, see
http://www.python.org/doc/current/tut/node6.html#SECTION006700000000000000000

>     res = []

This is what we return below. An empty list that will append something
to before we get to the return line. Let's see below:

>     for seq in args:

We loop through our function call arguments, however many they are.

 From the variable name "seq" we can make a guess that the next line
of code validates: Every argument in the function call should be a
sequence, i.e. something we can loop over with a for-loop.

In other words, we expect that the function call looks something like
"union([1,4,6],(5,6,4),[2,3,4])" or perhaps "union('strings','are',
'also','sequences')".

>         for x in seq:

For each sequence, loop over the elements (x) in the sequence.

>             if not x in res:

If the sequence element (x) is not already in the result list...

>                 res.append(x)

...put it there

>     return res

And finally return a list containing only one copy each of
every element that was in any of the sequences

So...
 >>> union([1,4,6],(5,6,4),[2,3,4])
[1, 4, 6, 5, 2, 3]
 >>> x = union('strings','are','also','sequences')
 >>> print "".join(x)
stringaeloquc

OK?


-- 
Magnus Lycka, Thinkware AB
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