list comprehensions: sorting?

[Clark C . Evans]

On Thu, Sep 06, 2001 at 09:52:08AM +0200, Alex Martelli wrote:
| > As an SQL junkie... I've gotten addicted to
| > list comprehensions over a list of tuples...
| >
| >  [process(x) for x in mylist if x[4]
| >
| > I read the above as something like...
| >
| >  SELECT process(x) FROM mylist AS x WHERE x[4]
| 
| Nice framing, and makes your wish quite understandable.
| A list comprehension can have one or more occurrences
| of 'for' and zero or more of 'if', but you might mostly
| frame those as joins:-).

A small article describing this analogy would have 
helped me pick-up list comprehensions fairly quickly.  

|     temp = []
|     for x in list:
|         if x[4]:
|             temp.append(process(x))
|     return temp
| 
| where would the new 'sortdesc' keyword fit in this model?

Let's consider...

   [process(x) for x in mylist if x[4] sortdesc x[3] ]

So here is an extended "model"...

has_sortby = 1
is_desc = 1
temp = []
for x in mylist:
    if x[4]:
        val = process(x)
        if has_sortby:
            val = (x[3],val)
        temp.append(val)
if not(has_sortby):
    return temp
else:
    temp.sort()
    if is_desc: temp.reverse()
    ret = []
    for x in temp: ret.append(x[1])
    return ret

Does this "fit" in as an extension of the model? 

|     map(process, sorted_by(2,
|         [x for x in mylist if x[4]], descend=1))

It works... but it's a bit ugly.  I'd like a
more "intuitive" syntax... and without having
to roll my own sort function.  I would think
that sorting lists as part of a list comprehension
would be a common activity... I certainly do it
often enough.

| def sorted_by(col, alist, descend=0):
|     temp = [(x[col],x) for x in alist]
|     temp.sort()
|     if descend: temp.reverse()
|     return temp

Thanks... I'm going to use this solution; but
it'd be cool if list comprehensions were extended
a bit to allow for sorting...  thank you so much
for your response Alex.

Best,

Clark