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Comment by onion2k | Hacker News Reader

Every time I've written something that uses a port I've added a check to see if the code returns an EADDRINUSE error when the listener connects, and offered the user an alternative port if it does. I always assumed everyone did that but apparently not. Maybe I should make an NPM package...

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Fun trick I learned: binding to port 0 causes Linux to pick a random, unused, ephemeral port for you. That's what I've started using in my EADDRINUSE fallback.

Nice

...it boggles my mind that listening on port 0 and then reporting the actual bound port is not the default. It takes only a couple minutes more to properly write (and another couple of minutes to write the code to properly consume the reported port) but it offers tremendous convenience.

In the case of Flask: It would be incredibly inconvenient to restart your development server and have it show up on a random port every single time.

The only time fixed port numbers are inconvenient is when they conflict, but otherwise they're much easier to work with.

Yes, "worse is better", the ultimate reason of why inferior technical decisions get universally adopted :/

Of course, one day the technical inferiority finally shows itself (e.g., the ports do conflict), and then people start to heroically overcome the obstacles they've created for themselves, showing outstanding feats of ingenuity. Just google "port allocator"! Every company inevitably writes their own home-grown one because everyone's deployment/integration pipeline is unique.

You could always launch once using that scheme, stash the port that you’re offered, and re-use that until it stops working.

It’s likely that the port will be available indefinitely.

> ...it boggles my mind that listening on port 0 and then reporting the actual bound port is not the default.

This already works by just telling flask to use port 0. Binding to port 0 is standard behaviour (literally, part of the standard) and like most other software of that type the first thing Flask does is report the URL it’s running on.

> but it offers tremendous convenience.

Except when it does not as is the case here. If you’re working on your web stuff having to copy/paste a URL from your term every time you restart your server.

> Binding to port 0 is standard behaviour (literally, part of the standard)

I've seen a couple of programs that explicitly check and refuse to start when asked to listen on port 0 (for example, to this day "ssh -D 127.0.0.1:0 remote-host" refuses to start).

> If you’re working on your web stuff having to copy/paste a URL from your term every time you restart your server.

Don't the developer's standard server-launching scripts (re)launch browser after they start the server? IIRC, npm start and VisualStudio+ASP.NET do exactly that.

It should be a Pip package instead!

(The main non-IANA compliant user of the port is Flask, a Python-based web framework.)

No one would want to use a Python package written by a JS developer. :)

No one would see any difference, any perceived difference in quality is a meme

I don't know, if you'd never done any Python, and perhaps particularly if you learnt JS in 'callback hell' days, you might not realise what was wrong with:

    def doSomething(theArgs, cb):
        food, bard = 0, 0
        try:
            doFoo(theArgs["foo"], lambda: food = 1)
            doBar(theArgs["bar"], lambda: bard = 1)
        except Exception:
            cb(food + bard, "didn't work")
        else:
            cb(food + bard, None)

or whatever.

I can get by in JS, I use it professionally even now, but it's not good quality; I know that. (Trivial example, I discovered `??` recently. I've definitely written a lot of null/undefined checks without it, or even naive ternaries that assume non-null/undef values aren't falsey (`thing ? thing : 123` vs. `thing ?? 123`) - which is kind of like seeing `dict.get(key, None)` in python.)

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