Counting in a python nested dictionary | fromkeys() function How it works?

Question

Given a list of runs up to a maximum reached stage, I would like to transfer the maximum values to a nested dictionary holding the number of tries (t) for each stage.

This means runs of 3, 4 and 7 should yield:

{0: {'t': 3}, 1: {'t': 3}, 2: {'t': 3}, 3: {'t': 2}, 4: {'t': 1}, 5: {'t': 1}, 6: {'t': 1}}

Stages 0,1,2 has been played for 3 times each, stage 3 for 2 times and stages 4, 5 and 6 only once.

I Get instead the following result:

{0: {'t': 14}, 1: {'t': 14}, 2: {'t': 14}, 3: {'t': 14}, 4: {'t': 14}, 5: {'t': 14}, 6: {'t': 14}}

Source Code

p = {}
p = dict.fromkeys(range(7), {})

runs = (3, 4, 7)

for r in runs:
    for l in range(r):
        if "t" in p[l]:
          p[l]["t"] += 1
        else:
          p[l]["t"] = 1

Why are the values the same in all the dictionary's key?

The documentation for [dict.fromkeys](https://docs.python.org/3/library/stdtypes.html#dict.fromkeys) says they share the same value. So the empty dict you pass in, `{}`, is shared between them all. — Peter Wood, Dec 21 '20 at 11:28
See this answer: https://stackoverflow.com/a/43772300/1084416 — Peter Wood, Dec 21 '20 at 11:31

score 1 · Answer 1 · answered Dec 21 '20 at 11:37

That is because dict.fromkeys() sets the same object for each key in p

Try doing something like this instead

p = { i : {} for i in range(7) }

runs = (3, 4, 7)

for r in runs:
    for l in range(r):
        if "t" in p[l]:
          p[l]["t"] += 1
        else:
          p[l]["t"] = 1

Federico Baù · Answer 2 · 2020-12-21T11:56:28.170

Use a Dict Comprehension

 p = {n:{} for n in range(7)}
  runs = (3, 4, 7)

  for r in runs:
      for l in range(r):
          if "t" in p[l]:
              p[l]["t"] += 1
          else:
              p[l]["t"] = 1
  print(p)

fromkeys() is a class method that returns a new dictionary. value defaults to None.

All of the values refer to just a single instance, so it generally doesn’t make sense for value to be a mutable object such as an empty list.

To get distinct values, use a dict comprehension instead.

Edit

Worth to mention:

Use setdefault Method:

 for r in runs:
          for l in range(r):
              if "t" in p[l]:
                  p[l]['t'] += 1
              else:
                  p[l].setdefault('t', 1)

Pre-Build the Dict with the Key 't' in it.

  p = {n: {'t': 0} for n in range(7)}
  runs = (3, 4, 7)

  for l in range(len(runs)):
      for run in range(runs[l]):
          p[run]['t'] += 1

Counting in a python nested dictionary | fromkeys() function How it works?

Source Code

2 Answers2

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