checking if a list is empty

[Steven D'Aprano]

On Fri, 06 May 2011 14:49:24 -0500, harrismh777 wrote:

> Terry Reedy wrote:
>>>> (2) if not li:
>>>
>>> This is fine.
>>
>> This is the intended way. Anything in addition is extra noise and
>> wasted calculation. In other words, let Python do the boilerplate work
>> for you.
> 
>     I agree, but I don't like it.
> 
> ... if not li  says nothing about what li is supposed to 'be' 

Yes, and? Neither does:

isempty(li)  # li could be anything that polymorphic isempty can deal with
li.isempty()  # li could be anything with a "isempty" method
len(li) == 0  # li could be anything with a length (list, dict, set, ...)


That's duck-typing for you, and it is generally a strength rather than a 
weakness. For those times when it is a weakness, that's where we have 
naming conventions, type-testing, and (in some languages) static types.


> and implies in any case that li does not exist

It does nothing of the sort. If li doesn't exist, you get a NameError.


> or worse is some kind of boolean.

Only if you're still thinking in some language that isn't Python.

Boolean tests in Python are polymorphic. bool() exists mainly to provide 
a canonical representation of true and false values, not because you are 
limited to using booleans in truth-testing. Far from it: it is often very 
useful to do something like this:

settings = local_settings or global_settings or builtin_settings

where the first non-empty *_settings wins.


>     li is in fact an empty list [] and will identify as such, and of
> course (as a list object) has all of the attributes and methods of a
> list...
> 
>     Why not have a list method that makes this more explicit:

For the same reason that we don't have an int method that makes zero 
testing more explicit:

n = 0
n.iszero()  # returns True


Because sometimes you can have too much explicitness.


-- 
Steven