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Comment by qwertox | Hacker News Reader

  days = {}

  day = '2021-05-05'
  if day not in days:
    days[day] = []
  days[day].append(event)

vs. just

  days = defaultdict(list)

  day = '2021-05-05'
  days[day].append(event)

what a blessing! Thanks for posting this. I do this so often.

Now maybe add an `OrderedDefaultDict`.

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  This is the TXR Lisp interactive listener of TXR 257.
  Quit with :quit or Ctrl-D on an empty line. Ctrl-X ? for cheatsheet.
  Caution: objects in heap are farther from reality than they appear.
  1> (defun pend (list item) (append list (list item)))
  pend
  2> (define-modify-macro pendto (item) pend)
  pendto
  3> (defvar x)
  x
  4> (pendto x 3)
  (3)
  5> (pendto x 4)
  (3 4)
  6> (defvar days (hash))
  days
  7> (pendto [days "2021-05-01"] 'event0)
  (event0)
  8> (pendto [days "2021-05-01"] 'event1)
  (event0 event1)
  9> (pendto [days "2021-05-01"] 'event2)
  (event0 event1 event2)
  10> days
  #H(() ("2021-05-01" (event0 event1 event2)))
  11> [days "2021-05-01"]
  (event0 event1 event2)

Dictionaries are ordered since, I think, 3.6. I believe this is an implementation detail of CPython, rather than it being “standard” (e.g., I don’t know if you can rely on, say, PyPy respecting that, although I suspect it does.)

In fact it looks like PyPy implemented it first [0], and it was an implementation detail in 3.6, but declared part of the standard in 3.7 [1].

[0]: https://docs.python.org/3.6/whatsnew/3.6.html#new-dict-imple...

[1]: https://docs.python.org/3.7/whatsnew/3.7.html

There's also the `setdefault` method you could use on ordinary dicts:

  -if day not in days:
  -  days[day] = []
  +days.setdefault(day, [])

Interestingly, in Common Lisp,

  (push event (gethash day days))

does this with zero extra effort, since the default value for fetching from a hashtable is NIL. The magic of reasonable defaults... I feel like it's underappreciated sometimes.

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