get first element from a list without exception

Question

Consider this code

description = ""
desc = hxs.select('/html/head/meta[2]/@content').extract()
if len(desc) > 0:
    description = desc[0]       
item["description"] = description

desc is a list of strings. If list is empty description is an empty string, if not it's the first elements from the list. How to make it more pythonic?

Forgot to mention that I have to use 2.7

iruvar · Accepted Answer · 2013-07-10 18:07:49Z

17

Alternately you could use make use of the fact that next supports a default

item["description"]  = next(iter(desc), "")

answered Jul 10, 2013 at 18:07

iruvar

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2 Comments

user7075574 Over a year ago

To those that are wondering what is iterable in Python

cowlinator Over a year ago

Note that this is not idempotent. (Meaning that next() will only return the first element the first time it is called).

Frédéric Hamidi · Accepted Answer · 2013-07-10 17:50:22Z

13

You can write:

desc = hxs.select("/html/head/meta[2]/@content").extract()
item["description"] = desc[0] if len(desc) > 0 else ""

As pointed out in the comments below, you can also directly evaluate the list in a boolean context:

item["description"] = desc[0] if desc else ""

answered Jul 10, 2013 at 17:50

Frédéric Hamidi

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10 Comments

Waleed Khan Over a year ago

if len(desc) > 0 could be replaced by if len(desc).

Henry Keiter Over a year ago

@WaleedKhan or just if desc, since empty lists are falsy.

tdelaney Over a year ago

Or item["description"] = desc and desc[0] or "" if this needs to work on earlier pythons that would choke on the 'else' part.

Frédéric Hamidi Over a year ago

@tdelaney, version 2.5, to be specific.

abarnert Over a year ago

@FrédéricHamidi: But you're being explicit about the wrong thing. You care about the emptiness of the list, not the length; the fact that using the length to check the emptiness might be the fastest way to do that is irrelevant. (And the fact that it might not means it may even be harmful.)

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chepner · Accepted Answer · 2013-07-10 17:59:20Z

1

You can use exception handling (although it's more verbose than using the conditional expression).

desc = hxs.select('/html/head/meta[2]/@content').extract()
try:
    description = desc[0]
except IndexError:
    description = ""

answered Jul 10, 2013 at 17:59

chepner

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2 Comments

abarnert Over a year ago

It may be more verbose, but that doesn't mean it's worse. As the docs say: "This common Python coding style assumes the existence of valid keys or attributes and catches exceptions if the assumption proves false. This clean and fast style is characterized by the presence of many try and except statements. The technique contrasts with the LBYL style common to many other languages such as C."

chepner Over a year ago

In this case, both approaches are so simple and readable that I would probably choose the conditional expression as the shorter approach :)

abarnert · Accepted Answer · 2013-07-10 18:11:13Z

0

I don't think you should actually do this in your case, but for completeness…

For an iterator that might be empty, you just use the optional default value for next:

desc = foo.finditer(bar)
item["description"] = next(desc, "")

Meanwhile, you can always use iter() to "peek" at the iterator you would get from a sequence (or other non-iterator iterable).

So:

desc = foo.findall(bar)
item["description"] = next(iter(desc), "")

I think this is less readable than just using the list as a sequence (whether you use chepner's EAFP answer or Frédéric Hamidi's LYBL) rather than just an iterable. After all, we have the sequence API for a reason.

answered Jul 10, 2013 at 18:11

abarnert

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