pylint woes

[Steven D'Aprano]

On Mon, 9 May 2016 07:04 am, DFS wrote:

> On 5/8/2016 11:51 AM, Steven D'Aprano wrote:
>> On Mon, 9 May 2016 12:25 am, DFS wrote:
>>
>>>>> for j in range(len(nms)):
>>>>>      cSQL = "INSERT INTO ADDRESSES VALUES (?,?,?,?,?)"
>>>>>      vals = nms[j],street[j],city[j],state[j],zipcd[j]
>>
>> Why are you assigning cSQL to the same string over and over again?
> 
> I like it in cloxe proximity to the vals statement.

The line immediately above the loop is in close proximity. Is that not close
enough?


>> Sure, assignments are cheap, but they're not infinitely cheap. They still
>> have a cost. Instead of paying that cost once, you pay it over and over
>> again, which adds up.
> 
> Adds up to what?

Potentially a significant waste of time. You know what they say about
financial waste: "a million here, a million there, and soon we're talking
about real money".

The first point is that this is a micro-pessimisation: even though it
doesn't cost much individually, its still a needless expense that your
program keeps paying. Its like friction on your code.

Python is neither the fastest nor the slowest language available. It is
often "fast enough", but it is also an easy language to write slow code in.
If you were programming in C, the compiler would almost surely see that the
assignment was to a constant, and automatically and silently hoist it
outside the loop. That's one reason why C is so fast: it aggressively
optimizes your code. (Too aggressively, in my opinion, but that's another
story.) But the Python compiler isn't that sophisticated. It's up to us,
the programmers, to be mindful of the friction we add to our code, because
if we aren't mindful of it, we can easily end up with needlessly slow code.

But as I said, that's not the major problem with doing the assignment in the
loop. It's more about readability and your reader's expectations than the
extra time it costs.


>> Worse, it is misleading. I had to read that code snippet three or four
>> times before I realised that cSQL was exactly the same each time.
> 
> You had to read 5 words three or four times?   Seriously?

Yes, seriously, because when most people read code they skim it, speed
reading, looking for salient points of interest. They don't point their
finger under each word and read it aloud syllable by syllable like a
pre-schooler with reading difficulties. (At least I don't, I can't speak
for others.) So the first couple of times I glanced at it, it just looked
like any other assignment without the details registering.

Then the next couple of times I thought that it must be *me* making the
mistake, I must be reading it wrong. Maybe there's something I missed? I
read code with the default assumption that it is more or less sensible.

Over the various posts and replies to posts, I had probably glanced at that
line a dozen times, and then read it more carefully three or four times,
before I was sure I had read what I thought I had read.


[...]
> I like mine best of all:
> 
> ziplists = zip(names,streets,cities,states,zipcodes)
> for name,street,city,state,zipcode in ziplists:

Using a temporary, single-use variable is okay, but it's often unnecessary.

But be aware that there is a particular risk with loops. You may be tempted
to think you can re-use ziplists to iterate over it twice:

data = zip(names, streets, cities, states, zipcodes)
for a, b, c in data:
    do_stuff()
...
# later
for x, y, z in data:
    do_something_else()

and that's perfectly fine, *but* there is a risk that if the for loop data
being iterated over is an iterator, it will have been exhausted by the
first loop and the second loop won't run at all.

In Python 2, zip() returns a list, but in Python 3, it returns an iterator.
So beware of using such temp variables unless you know what you're doing.


>>> I like the first one better.  python is awesome, but too many options
>>> for doing the same thing also makes it difficult.  For me, anyway.
>>
>>
>> That's the difference between a master and an apprentice. The apprentice
>> likes to follow fixed steps the same way each time. The master craftsman
>> knows her tools backwards, and can choose the right tool for the job, and
>> when the choice of tool really doesn't matter and you can use whatever
>> happens to be the closest to hand.
> 
> "her tools"... you're a woman?

What makes you think I was talking about myself? I was talking about people
in general -- when we are beginners, its only natural that (like most
beginners) we're more comfortable with a limited amount of choice. Its hard
to remember what option to use when there's only one, let alone when
there's ten. But as we progress to mastery of the language, we'll come to
understand the subtle differences between options, when they matter, and
when they don't.

If I had said "his tools", would you have thought I was talking specifically
about a man?

Does it matter if I'm a woman? Would that make my advice better or worse?




-- 
Steven