Python idiom for taking the single item from a list

Post by orangecat » Tue, Sep 28, 2010, 12:51 AM UTC

jemfinch wrote: post #76

Scriptor wrote: post #73

Earlier quoted context omitted.

It is certainly okay for some functions to be complex. Some may rely on simpler functions, but the one you'd use can still do a complex task in general. A function that squares a number is simple, one that computes the standard deviation is definitely more complex.

> A function that squares a number is simple, one that computes the standard deviation is definitely more complex. I'm talking about the complexity difference between this: def stddev(pop): total = 0 count = 0 for x in pop: total += x count += 1 mean = total / float(count) variance = 0 for x in pop: variance += (x - mean)**2 return math.sqrt(variance) and this: def stddev(pop): return math.sqrt(variance(pop)) def var…

    def stddev(pop):
        mean = sum(pop) / float(len(pop))
        variance = sum( (x-mean)**2 for x in pop)
        return math.sqrt(variance)

Post by statictype » Tue, Sep 28, 2010, 12:59 AM UTC

jemfinch wrote: post #58

j_baker wrote: post #47

Personally, I think this is a bit on the "clever" side. Plus, the error message you get isn't as easy to understand as if you used an assert statement. I'd probably just do something like this: def get_single(l): assert l and len(l) == 1 return l[0] Then you get the best of both worlds: readability and a concise one-liner.

> assert l and len(l) == 1 This is redundant: if a list's length is 1, then it's true in a boolean context. Also, please stop naming your lists 'l'. On a vast array of fonts, it differs only in a few pixels from '1'. Use "L" instead :)

Post by robrenaud » Tue, Sep 28, 2010, 2:45 AM UTC

mccutchen wrote: post #72

viraptor wrote: post #67

Earlier quoted context omitted.

You don't need to do that for a set. sets support `len()`. Also set supports `pop()`, so the code is very similar to the list one: assert s and len(s)==1 return s.pop() Or if you want to stay in the immutable land: assert s and len(s)==1 return tuple(s)[0]

Both of those still fail for other non-list iterables, like generators.

Post by jemfinch » Tue, Sep 28, 2010, 4:17 AM UTC

statictype wrote: post #83

jemfinch wrote: post #58

Earlier quoted context omitted.

> assert l and len(l) == 1 This is redundant: if a list's length is 1, then it's true in a boolean context. Also, please stop naming your lists 'l'. On a vast array of fonts, it differs only in a few pixels from '1'. Use "L" instead :)

This is redundant: if a list's length is 1, then it's true in a boolean context. In a boolean context, the list is true if the length is non-zero. This example and the one the article is about is for the case where you know the list to have exactly one element. Not zero and not more than one.

Post by spoondan » Tue, Sep 28, 2010, 5:49 AM UTC

torrentabuse wrote: post #27

Proposed style guide: When you want to get the (n+1)th item from a list, do: item = stuff[n] To get the first item, do: item = stuff[0] Unless the list has one element, then do: (item, ) = stuff At least you'll never be accused of consistency.