2 Dictionaries

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16 get read from a dict with a fallback

Important

  1. When there is a sensible default, set it and continue as if you got the value
  2. Use a walrus := when the control flow will change

Demo of both options

# 1) Sensible default
count = counters.get(key, 0)
counters[key] += 1

# 2) Control flow must change
for attr in data:
    if not (ts := attr.get("timestamp")): # guard clause
        continue # there's no way to handle this, skip it
    process(ts)

# Another example
if last_seen_iso:
    timestamp = ...                      # preferred source
elif attr_ts := attr.get("timestamp"):   # walrus, bind and test
    timestamp = datetime.fromtimestamp(int(attr_ts), tz=timezone.utc)
else:
    continue                             # neither, give up

Note

How .get() works:

count = counters.get(key, 0)
counters[key] += 1
# Under the hood this is what happens
try:
    count = counters[key]
except KeyError:
    count = 0
counters[key] += 1

17 setdefault/defaultdict when the lookup must create and insert a mutable default

When to use defaultdict? When you control the dict’s creation

class Visits:
    def __init__(self):
        self.data = defaultdict(set)

    def add(self, country, city):
        self.data[country].add(city)

When to use setdefault when the dict is handed to you.

# In one set reads or creates a set and places it in the dict
# and then adds to the set
visits.setdefault('France', set()).add('Arles')

Why doesn’t get work work

# this creates a new set and returns it
# but it just updates the brand new and never puts it in the dict
visits.get('France', set()).add('Arles')
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