Beyond coverage

It looks like the­re are a lot of opi­nions or as­sump­tions about unit tes­ts an­d ­co­de co­ve­ra­ge, most of them con­fu­sing of bia­sed in se­ve­ral wa­ys. For exam­ple, I’­ve heard or read things like “this is fi­ne, it has X% co­ve­ra­ge”, or “che­ckin­g ­for co­ve­ra­ge on pu­ll re­ques­ts does­n’t hel­p”, or “dro­pping the co­ve­ra­ge le­vel is ­not an is­sue”, and many mo­re of the like.

This arti­cle ai­ms to shed so­me li­ght on the is­sue of unit tes­ting wi­th and co­de ­co­ve­ra­ge. Ho­pe­fu­lly by the end of it, we’­ll get an idea of whi­ch of the ­pre­vious sta­te­men­ts are ri­ght and whi­ch are wrong (s­poi­ler aler­t: the­y’­re all w­ron­g, but for di­ffe­rent rea­son­s).

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PyCon CZ - Day 1

The day started at 9.00, and at first it was time of check-in, etc. After that, I solved one riddle by kiwi.com, and earned a discount in flights, which was a nice way to start the conference.

Then after break­fast and so­me ne­two­rking going th­rou­gh the booths of the s­pon­sor­s, it was ti­me for the first ta­lk of the da­y: “When bugs bi­te - wh­y ­ne­glec­ting edge ca­ses can ki­ll“.

It was a great ta­lk, a ca­se for so­ftwa­re en­gi­nee­ring in ge­ne­ral (i­t’s no­t ­P­y­tho­n-s­pe­ci­fi­c, whi­ch is what makes the to­pic mo­re in­te­res­tin­g). The bes­t ­thing I liked about the ta­lk was the re­ma­rks, and the idea of shi­fting the ­min­d­set when it co­mes to de­ve­lo­p­men­t, so we should do so­me “ne­ga­ti­ve thi­nkin­g” in the form of what can go wrong wi­th this? How can this fai­l?, and su­ch. I­t’s im­por­tan­t, be­cau­se this is cri­ti­cal in or­der to make ro­bust so­ftwa­re. Mos­t ­pro­ble­ms I ha­ve seen on so­ftwa­re bugs we­re re­lated to so­me op­ti­mis­tic thi­nkin­g, and even wor­se, op­ti­mis­tic unit tes­tin­g, in the way that de­ve­lo­pers on­ly tes­t ­for ha­pp­y-­pa­th sce­na­rio­s, wi­thout thi­nking all sort of things that can go­ w­ron­g.

Afte­rwar­d­s, was the ti­me for the ta­lk about pa­ra­llel pro­ce­s­sing (poor per­so­n’s ­pa­ra­llel pro­ce­s­sing), whi­ch was fi­ne. Then I spent so­me ti­me ta­ck­ling so­me of ­the cha­llen­ges spon­sors had avai­la­ble, so I did so­me co­ding and re­cap of the e­ven­ts so fa­r.

Then I listened to a talk about wolfcrypt which is a tool for crypto in Python. The talk introduced some crypto concepts (symmetric crypto, public key, etc.), which was good. Most of the questions revolved around comparison with other tools in python (default libraries, pycrypto, etc.).

Then it was ti­me for lun­ch and do so­me mo­re co­din­g, and the next ta­lk I a­tten­ded to was ca­lled “should I mo­ck or should I no­t?“, whi­ch I liked ve­r­y ­mu­ch and ga­ve so­me food for thou­gh­t.

Then it ca­me one mo­re ta­lk, after whi­ch it was ti­me for mi­ne, so I pre­sen­te­d clean co­de in Py­thon. The­re we­re so­me in­te­res­ting ques­tion­s, and the en­ti­re ­pre­sen­ta­tion went re­la­ti­ve­ly qui­ck­l­y.

On­ce the ta­lk ses­sions we­re ove­r, the­re was one last tra­ck for li­gh­tning ta­lks, whi­ch are alwa­ys su­per en­ter­tai­nin­g.

After the fi­nish of the first da­y, I atten­ded the speake­r’s din­ne­r, whi­ch was a ­ni­ce oppor­tu­ni­ty to ne­two­rk wi­th the co­m­mu­ni­ty, whi­ch is alwa­ys grea­t.

Looking fo­rward to so­me mo­re in­te­res­ting ta­lks to­mo­rro­w, and to see the re­sul­ts of the cha­llen­ges.

The __set_name__ method for descriptors

Descriptors generally have to interact with attributes of the managed object, and this is done by inspecting __dict__ on that object (or calling getattr/setattr, but the problem is the same), and finding the key under the specific name.

For this rea­so­n, the des­crip­tor wi­ll ha­ve to know the na­me of the key to look ­fo­r, whi­ch is re­lated to the na­me of the attri­bu­te is ma­na­gin­g.

On pre­vious ver­sions of Py­thon this had to be do­ne ex­pli­ci­tl­y. If we wanted to­ wo­rk around it, the­re we­re so­me mo­re ad­van­ced wa­ys to do so. Lu­cki­l­y, afte­r PE­P-487 (a­dded in Py­thon 3.6), ­the­re are so­me enhan­ce­men­ts re­gar­ding cla­ss crea­tio­n, whi­ch al­so affec­ts ­des­crip­tor­s.

Le­t’s re­view the pro­ble­m, the pre­vious appro­aches to ta­ck­le it, and the mo­der­n way of sol­ving it.

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