some problems for an introductory python test

[Chris Angelico]

On Wed, Aug 11, 2021 at 4:18 AM Hope Rouselle <hrouselle at jevedi.xotimo> wrote:
>
> I totally agree with you but I didn't know that even numbers were like
> that in Python.  In fact, I still don't quite believe it...
>
> >>> 2.__add__(3)
> SyntaxError: invalid syntax

Yeah, that's because "2." looks like the beginning of a float.

> But then I tried:
>
> >>> (2).__add__(3)
> 5
>
> Now I do believe it! :-)  Awesome.  I had no idea.

You can also do it this way:

>>> x = 2
>>> x.__add__(3)
5

But don't teach this; it ignores several problems. Notably, a subclass
should be able to (re)define operators:

>>> class FancyNumber(int):
...     def __repr__(self): return "FancyNumber(%d)" % self
...     def __add__(self, other): return type(self)(int(self) + other)
...     def __radd__(self, other): return type(self)(other + int(self))
...
>>> FancyNumber(2) + 3
FancyNumber(5)
>>> 2 + FancyNumber(3)
FancyNumber(5)
>>> FancyNumber(2).__add__(3)
FancyNumber(5)
>>> (2).__add__(FancyNumber(3))
5

With the + operator, you always get a FancyNumber back. Explicitly
calling the dunder method fails. (General principle: Dunder methods
are for defining, not for calling, unless you're in the process of
defining a dunder.)

> (*) More opinions
>
> So, yeah, the idea of a course like that is to try to simplify the world
> to students, but it totally backfires in my opinion.  There is so much
> syntax to learn, so many little details...  We spend the entire semester
> discussing these little details.

Agreed, and if you try to teach all the syntax, you're inevitably
going to get bogged down like that.

> I posted here recently a study of the semantics of slices.  When I
> finally got it, I concluded it's not very simple.  The course introduces
> a few examples and expects students to get it all from these examples.
> I would rather not introduce slices but teach students enough to write
> procedures that give them the slices.  The slices are the fish; learning
> to write the procedures is the know-how.  (I'm fine with even teaching
> them how to write procedures to add or subtract numbers [and the rest of
> arithmetic], although none of them would find mysterious what is the
> meaning of arithmetic expressions such as 3 + 6 - 9, even taking
> precedence of operators into account.  That's syntax they already know.
> If we teach them the syntax of procedures, we could be essentially done
> with syntax.)

A language like Python is useful, not because every tiny part of it is
easy to explain, but because the language *as a whole* does what is
expected of it. Toy languages are a lot easier to explain on a
concrete level (look up Brainf*ck - it has very few operations and
each one can be very simply defined), and that might be tempting in
terms of "hey, we can teach the entire language in ten minutes", but
they're utterly useless for students.

Instead, my recommendation would be: Entice students with power.
Pretend you're teaching some aspect of magic at a wizards' school and
make sure your students feel massively OP. Sure, they might not
understand what makes those cantrips work, but they know that they DO
work. You can have students casting print calls left and right, doing
complex incantations with list comprehensions, and even defining their
own classes, all without ever worrying about the details of how it all
works. Then once the "wow" is achieved, you can delve into the details
of how things actually function, transforming progressively from
"wizardry" to "science".

And to be quite frank, a lot of code IS magic to most programmers.
Which isn't a problem. Do you REALLY need to understand how your CPU
does register renaming in order to benefit from it? Or do you need to
know every detail of the TCP packet header before using the internet?
Not likely. Those details hide away under the covers, and we make
happy use of them. There's infinite knowledge out there, and you - and
your students - are free to dip into exactly as much as you're
comfortable with.

ChrisA