[Python-Dev] Emotional responses to PEPs 484 and 526

[Stephen J. Turnbull]

Guido van Rossum writes:

 > I just spoke to someone who noted that [PEP 526] is likely to evoke
 > an outsize emotional response. (Similar to what happened with PEP 484.)

Emotional, yes, but I resent the "outsize" part.  Although that word
to the wise is undoubtedly enough, i.e., tl:dr if you like, let me
explain why I have one foot in each camp.

Compare Nick's version of "scientific code with SI units":

  from circuit_units import A, V, Ohm, seconds

  delta: A
  for delta in [-500n, 0, 500n]:
      input: A = 2.75u + delta
      wait(seconds(1u))
      expected: V = Ohm(100k)*input
      tolerance: V = 2.2m
      fails = check_output(expected, tolerance)
      print('%s: I(in)=%rA, measured V(out)=%rV, expected V(out)=%rV, diff=%rV.' % (
          'FAIL' if fails else 'pass',
          input, get_output(), expected, get_output() - expected
      ))

with

  from circuit_units import VoltType, uA, mV, kOhm, u_second

  expected: VoltType

  for delta in [-0.5*uA, 0*uA, 0.5*uA]:
      input = 2.75*uA + delta
      wait(1*u_second)                 
      expected = (100*kOhm)*input
      tolerance = 2.2*mV                   
      fails = check_output(expected, tolerance)
      print('%s: I(in)=%rA, measured V(out)=%rV, expected V(out)=%rV, diff=%rV.' % (
          'FAIL' if fails else 'pass',
          input, get_output(), expected, get_output() - expected
      ))

In Nick's version, literals like 500n ("500 nano-whatevers") require
Ken Kundert's proposed syntax change.  I left that in because it
streamlines the expressions.  I wrote the latter because I really
disliked Nick's version, streamlined with "SI scale syntax" or not.
Nick didn't explicitly type the *_output functions, so I didn't
either.[1]  I assume they're annotated in their module of definition.

The important point about the second version is that if we accept the
hypothesis that the pseudo-literals like '[0.5*uA, 0*uA, 0.5*uA]' are
good enough to implicitly type the variables they're assigned to (as
they are in this snippet), mypy will catch "unit errors" (which the
circuit_units module converts into TypeErrors) in the expressions.  I
think that this hypothesis is appropriate in the context of the thread.

Therefore, I think Nick's version was an abuse of variable annotation.
I don't mean to criticize Nick, as he was trying to make the best of
an unlikely proposal.  But if Nick can fall into this trap[2], I think
the fears of many that type annotations will grow like fungus on code
that really doesn't need them, and arguably is better without them,
are quite reasonable.

The point here is *not* that Nick's version is "horrible" (as many of
the "emotional" type refuseniks might say), whatever that might mean.
I can easily imagine that the snippet above is part of the embedded
software for air traffic control or medical life support equipment,
and a belt and suspenders approach ("make units visible to reviewers"
+ "mypy checking" + "full coverage in unit tests") is warranted.  Ie,
Nick's version is much better than mine in that context because the
hypothesis that "implicit declaration" is good enough is invalid.  But
in the context of discussion of how to make measurement units visible
and readable in a Python program, he grabbed an inappropriate tool
because it was close to hand.

My version does everything the OP asked for, it furthermore makes mypy
into a units checker, and it does so in a way that any intermediate
Python programmer (and many novices as well) can immediately grasp.
If Nick had been given the constraint that novices should be able to
read it, I suspect he would have written the same snippet I did.  Nick?


Footnotes: 
[1]  I think both versions could have better variable naming along
with a few other changes to make them more readable, but those aspects
were dictated by an earlier post.  I don't have the nerve to touch
Nick's code, so left those as is.

[2]  Or perhaps he did it intentionally, trying to combine two cool
ideas, variable type annotations and SI units, in one example.  If so,
I think that this combination was unwise in context.