(#9) change __getitem__ to use self._n instead of self._buf // 4

[margaret]

• self._buf is incorrect since there are headers
• __setitem__ uses len(self), make this consistent with __getitem__ and use self._n directly)
• adafruit_dotstar __getitem__ applies incorrect cap to range input #9

[margaret]

More context
setup for self._buf is like this

:param int n: The number of dotstars in the chain
...
self._n = n
self.start_header_size = 4
# Supply one extra clock cycle for each two pixels in the strip.
self.end_header_size = n // 16
if n % 16 != 0:
    self.end_header_size += 1
self._buf = bytearray(n * 4 + self.start_header_size + self.end_header_size)

so if you had one dotstar, length of self._buf would be 1*4 + 4 + 0 and self._buf // 4 would be 2 instead of the expected 1 in

    def __getitem__(self, index):
        if isinstance(index, slice):
            out = []
            for in_i in range(*index.indices(self._n)): # used to be self._buf // 4
                out.append(
                    tuple(self._buf[in_i * 4 + (3 - i) + self.start_header_size] for i in range(3)))
            return out

[margaret]

Tested this manually with the pycon Gemma M0 ✅

(Test with current master of this library) code.py (modified from the default pycon open session file) is

# Gemma IO demo
# Welcome to CircuitPython 2.2.4 :)

from adafruit_hid.keyboard import Keyboard
from adafruit_hid.keycode import Keycode
from digitalio import DigitalInOut, Direction, Pull
from analogio import AnalogIn, AnalogOut
from touchio import TouchIn
import adafruit_dotstar as dotstar
import microcontroller
import board
import time

# One pixel connected internally!
dot = dotstar.DotStar(board.APA102_SCK, board.APA102_MOSI, 1, brightness=0.2)

tests = [
  '[0]',
  '[1]',
  '[-1]',
  '[:1]',
  '[1:]',
  '[0:]'
]
for test_idx in tests:
  teststr = 'dot'+test_idx
  print(teststr + ' ' + '-'*80)
  try:
    print(eval(teststr))
  except IndexError as e:
    print('Error:')
    print(e)
  print()

This prints

Auto-reload is on. Simply save files over USB to run them or enter REPL to disable.
code.py output:
dot[0] --------------------------------------------------------------------------------
(0, 0, 0)

dot[1] --------------------------------------------------------------------------------
Error:


dot[-1] --------------------------------------------------------------------------------
(0, 0, 0)

dot[:1] --------------------------------------------------------------------------------
[(0, 0, 0)]

dot[1:] --------------------------------------------------------------------------------
Error:
bytearray index out of range

dot[0:] --------------------------------------------------------------------------------
Error:
bytearray index out of range

Compared to this from CPython
Python 3.6.1 (v3.6.1:69c0db5050, Mar 21 2017, 01:21:04)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin

>>> dot
[(0, 0, 0)]
>>> tests = [
...   '[0]',
...   '[1]',
...   '[-1]',
...   '[:1]',
...   '[1:]',
...   '[0:]'
... ]
>>> for test_idx in tests:
...   teststr = 'dot'+test_idx
...   print(teststr + ' ' + '-'*80)
...   try:
...     print(eval(teststr))
...   except IndexError as e:
...     print('Error:')
...     print(e)
...   print()
... 
dot[0] --------------------------------------------------------------------------------
(0, 0, 0)

dot[1] --------------------------------------------------------------------------------
Error:
list index out of range

dot[-1] --------------------------------------------------------------------------------
(0, 0, 0)

dot[:1] --------------------------------------------------------------------------------
[(0, 0, 0)]

dot[1:] --------------------------------------------------------------------------------
[]

dot[0:] --------------------------------------------------------------------------------
[(0, 0, 0)]

With this PR's changes

Auto-reload is on. Simply save files over USB to run them or enter REPL to disable.
code.py output:
dot[0] --------------------------------------------------------------------------------
(0, 0, 0)

dot[1] --------------------------------------------------------------------------------
Error:


dot[-1] --------------------------------------------------------------------------------
(0, 0, 0)

dot[:1] --------------------------------------------------------------------------------
[(0, 0, 0)]

dot[1:] --------------------------------------------------------------------------------
[]

dot[0:] --------------------------------------------------------------------------------
[(0, 0, 0)]

This matches the behavior of standard slicing, so I think this change is ✅

Note that the IndexError description doesn't seem to get bubbled up in circuitpython, but I think that should be a separate issue.

[margaret]

@tannewt Ready for review

[tannewt]

Thank you! Looks much simpler!