Relying on the behaviour of empty container in conditional statements

[Daniel Dittmar]

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:
[...]
> Without wishing to start a flame war, what are other's opinions on
> this, is using "len" safer? If so why?

All objects evaluate to a boolean, but not all objects support len.
So if someone passes x = iter ([]) to your method, then len (x) will 
result in an exception, whereas 'if x:' will happily (and wrongly) 
answer true.

On the other hand, maybe you should be able to accept iterators, and 
then the test would look different anyway.

> 
> My own feeling is that I am willing to work with the behaviours defined
> by Python, and treat the use of len in these cases as excessive
> duplication (this is however, quite a minor point i agree).

I find

if parameters:

perfectly acceptable, as it allows parameters to be None as well.
On the other hand, I can't stand interpreting integers as booleans, so I 
wouldn't write

if count:

Everyone has their personal quirks.

> 
> 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.
> 

I guess they're coming from Java and not from Perl :-)

Daniel