Python Dictionary Methods and Tricks: The Complete Guide

user = {'name':'Alice','age':30}
user.get('missing','default')   # 'default' — no KeyError
user.pop('age', None)           # Remove, return None if missing
user.setdefault('tags',[]).append('admin')  # Init once, append safely

# Merge (Python 3.9+)
merged = base | overrides        # New dict
base |= overrides                # In-place

# Iteration
for k,v in user.items(): print(k,v)
keys = list(user.keys())
values = list(user.values())

from collections import defaultdict

# Grouping
grouped = defaultdict(list)
for item in items:
    grouped[item.category].append(item)
# No 'if key not in grouped' needed!

# Counting
counts = defaultdict(int)
for word in text.split():
    counts[word] += 1

# Nested
nested = defaultdict(lambda: defaultdict(int))
nested['users']['alice'] += 1

from collections import Counter

words = ['apple','banana','apple','cherry','apple']
c = Counter(words)
c.most_common(2)  # [('apple',3),('banana',1)]
c['missing']      # 0, not KeyError

# Count characters
Counter('hello')  # Counter({'l':2,'h':1,'e':1,'o':1})

# Arithmetic
Counter({'a':3,'b':2}) - Counter({'a':1}) # Counter({'a':2,'b':2})

{x:x**2 for x in range(5)}           # {0:0,1:1,2:4,3:9,4:16}
{v:k for k,v in d.items()}            # Invert dict
{k.lower():v for k,v in data.items()} # Lowercase keys
{k:v for k,v in scores.items() if v>=90} # Filter

from collections import ChainMap

defaults = {'debug':False,'timeout':30}
project  = {'timeout':60}
user     = {'timeout':5}

config = ChainMap(user,project,defaults)
config['timeout']  # 5 (user wins)
config['debug']    # False (falls back)