Relying on the behaviour of empty container in conditional statements

[Bruno Desthuilliers]

horizon5 wrote:
> Hi,
> 
> my collegues and I recently held a coding style review.
> All of the code we produced is used in house on a commerical project.
> One of the minor issues I raised was the common idiom of specifing:
> 
> <pre>
> if len(x) > 0:
>     do_something()
> </pre>
> Instead of using the language-defined bahviour, as stated by PEP8:
> 
> <pre>
>     - For sequences, (strings, lists, tuples), use the fact that empty
>       sequences are false.
> 
>       Yes: if not seq:
>            if seq:
> 
>       No: if len(seq)
>           if not len(seq)
> </pre>
> 
> Without wishing to start a flame war, what are other's opinions on
> this, is using "len" safer? If so why?

I fail to see why it would be safer. But I clearly see a drawback to
explicitely testing length of objects : it doesn't work on unsized
objects (like None or 0 or False etc), so it makes code less generic
(wether this is a problem or not in a given context depends of course on
the context).

> Arguments that have been presented for using <code>len(x) > 0</code> to
> test emptiness of a container include:
>   - It's safer

cf above

>   - Not relying on weird behaviour of the language

It's not a "weird behaviour", it's a well defined and documented
behaviour. And it's not specific to Python.

>   - Explicit is better than implicit (as stated by 'this' module, Zen
> of Python)

Given that this behaviour is well defined and documented, using "if
[not] seq" is perfectly explicit.

> My own feeling is that I am willing to work with the behaviours defined
> by Python, 

and use the common Python idiom.

> and treat the use of len in these cases as excessive
> duplication (this is however, quite a minor point i agree).

It's also not idiomatic and less generic.

> Note that I have much more experience with the language (6-7 years),
> whilst the majority of my collegues have about 1-2 years experience.

Do they still write code like the following ?

if someBooleanExpression == True:
   return True
else:
   return False

If yes, time to look for another place to work IMHO. Else, what do they
think of the above snippet, vs:

return someBooleanExpression

Do they think the first one is safer and/or more explicit ?-)

-- 
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb at xiludom.gro'.split('@')])"