checking if a list is empty

[Emile van Sebille]

On 5/21/2011 7:46 AM John J Lee said...
> Gregory Ewing<greg.ewing at canterbury.ac.nz>  writes:
>
>> Hans Georg Schaathun wrote:
>>>   0 is a number as real and existent as any other,
>>> one would think that the empty list is also as real and existent as
>>> any other list.
>>
>> 0 does have some special properties, though, such as
>> being the additive identity and not having a multiplicative
>> inverse. Adding falseness as another special property isn't
>> too much of a stretch and turns out to be useful.
>>
>> Likewise, it's useful to treat empty containers as having
>> a similar special property, since they're often a base or
>> terminating case in algorithms.
>>
>> It's especially useful in a dynamic language where any
>> additional operations such as finding the length or
>> comparing with zero has a run-time cost.
>>
>> Yes, you have to learn it, but it's a small thing to
>> learn with a considerable payoff.
>
> (apologies if somebody already bikeshedded this argument in this thread)
>
> In the absence of an explicit interface declaration (have any standards
> emerged for that in Python 3, BTW?), the use of len() does give you some
> information about the interface, which sometimes makes it easier to
> change the function.
>
> I'm sure you fully understand this, but I'll spell it out.  Consider
> this function:
>
> def xyzzy(x):
>      if x:
>          print "yes"
>
>
> Let's say I've read the function, and I've seen this call site:
>
> xyzzy(["spam", "eggs"])
>
>
> Now I want to change xyzzy:
>
> def xyzzy(x):
>      if x:
>          print "probably"
>      if len(x) == 1:
>          print "definitely"
>
>
> But there may be many call sites.  Perhaps xyzzy even implements part of
> a poorly-documented external API.  So can x be None?  There's no way to
> know short of checking all call sites, which may be impossible.  It may
> not even be feasible to check the *only* call site, if you're
> implementing somebody else's poorly documented closed-source API (a
> situation I came across at work only yesterday, when that situation
> resulted in a bug report).  If it's written this way, it's clear that it
> can't be None:

qualified: "...without having been trapped or crashing" so if I found 
this function in running code it would be clear to me that, given that 
the app is running and hasn't been crashing, either it hasn't yet been 
None, or the code isn't accessed at all.

Emile



>
> def xyzzy(x):
>      if len(x) != 0:
>          print "yes"
>
>
> John