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Ralsina.Me — Roberto Alsina's website

Python is Not a Configuration File Format

There is a large thread in red­dit about us­ing Python as con­fig­u­ra­tion file for­mat. I want to be clear about that:


If you need to ask if it's a good idea, then you don't have a good rea­son. If you are sure that you have a good rea­son, then maybe you have a good rea­son.

There are many rea­sons for it, but I will ex­plore just two, then of­fer a sug­ges­tion.

Python is read­-on­ly, and con­fig­ur­ing is not pro­gram­ming.

Sure, it's easy to use python as a con­fig file. You just im­port the thing, and there you go, all the da­ta is there. But now your con­fig­u­ra­tion syn­tax is a ­gen­er­al pur­pose lan­guage that can do things like pop up di­alogs when you parse it.

Your con­fig can now de­pend on the whole in­ter­net, the hard­ware, the weath­er, and in­ter­ac­tive in­put. Pow­er­ful? Sure. Good idea? May­be, some­times. But your ap­pli­ca­tion is now not able to con­fig­ure it­self.

If your ap­pli­ca­tion wants to store any kind of set­ting, it won't be able to. So ­most in­ter­ac­tive, desk­top app­s, just should not use python for this, ev­er.

But what about non-in­ter­ac­tive tool­s? Well, us­ing python means that oth­er tool­s ­can't write to the con­fig file, ei­ther, which makes the tool less pow­er­ful. The pow­er to have tools use tools is one of the cor­ner­stones of mod­ern com­put­ing, and you just cut your app off that ecosys­tem. De­pend­ing on what lan­guage the tool us­es it may not even be able to parse your con­fig file.

And what hap­pens when some­one is told "use this con­fig frag­ment to achieve X"? Well, odds are, if the re­cip­i­ent has done any­thing that takes ad­van­tage of us­ing python as a con­fig for­mat, then the frag­ment will not work. It would be like do­ing copy­/­paste from ran­dom code in github in­to your own pro­gram and ex­pect­ing it to work.

So, you can't write to it from the ap­p, you can't get con­fig­u­ra­tion tip­s from the in­ter­net, you can't use oth­er tools to mod­i­fy con­fig files, and other tools have a hard time pars­ing your files.

Al­so, it means that to han­dle the gen­er­al case of con­fig­ur­ing your ap­p, y­ou need a pro­gram­mer. That is al­most cer­tain­ly overkil­l. Very few app­s need that kind of thing. If your app can on­ly be con­fig­ured by pro­gram­mer­s, y­ou may have failed at mak­ing a good app (ex­cep­tions ex­ist).

And what's the ad­vice? Well, the ad­vice is "don't do that" and the corol­lary is "­con­fig­ure us­ing data, not code". use IN­Is, or XM­L, or YAM­L, or JSON, or plain ­text files, or what­ev­er. But not code.

PS: My lat­est pro­jec­t, Niko­la us­es python as a con­fig­u­ra­tion lan­guage. I thought I had a good rea­son. I did­n't.


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