[Python-ideas] "maybe import"?

[Nick Coghlan]

On 28 December 2013 22:27, Steven D'Aprano <steve at pearwood.info> wrote:
> So don't think of this as something you are going to use. Think of this
> as an investment for future Python users who are starting with 3.5 and
> don't care about backwards compatibility with Python 3.3 and 3.4 (let
> alone older versions).
>
> On that basis, I'm going to vote:
>
> +1 on "import preferred or fallback as name".
>
> +0 on allowing "import module or None", to cover the idiom:
>
>     try:
>         import module
>     except ImportError:
>         module = None
>
> which may not be encouraged, but it is occasionally useful.
>
> -0 on "from preferred or fallback import name", simply because I'm not
> sure how to extend it to multiple names, so I'd rather leave it out.
>
> -1 on Amber's original suggestion "maybe import name", as the
> benefit is too little to justify introducing a new keyword.

I used to be a fan of the "or" based variants, but I eventually went
off the idea. My reasoning went as follows:

1. For a single file script, you're only doing the import once, so
there's no repetition, and hence little to be gained from the more
concise syntax (and the cost of a new thing for readers to learn)

2. For a multi file package or application, it's trivial to factor out
the compatibility checking to a helper module, so there's *still* no
need for repetition. A compatibility module also lets you build an
adaptation layer if the APIs aren't *quite* the same (for example,
RHEL5 has the ancient pre-simplejson JSON module, so naive "import
json" code will mistakenly think it has the 2.6+ stdlib version - a
helper module can detect oddball discrepancies like that one and work
around them, but naive syntax would do the wrong thing if a similar
situation arose again in the future).

Essentially, the niche that the syntax variants can cover is too small
to be worth making the already horrendously complex import statement
syntax and semantics even *more* complicated, when you can either
spell it out explicitly in the single file case, or use a helper
module to centralise the selection logic in the multiple file case.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia