Default-dicts

dict_defaultdict.py

from collections import defaultdict

# by default a dict
a = defaultdict(dict)
assert a[5] == {}
a[5]["a"] = 5
assert a[5] == { "a": 5 }

# by default a list
a = defaultdict(list)
assert a[5] == []
a[5].append(3)
assert a[5] == [3]

# you can also specify a lambda-expression
# to create the default value
a = defaultdict(lambda: 10)
assert a[5] == 10
assert a[6] + 1 == 11

# in essense, this way we can avoid doing
# if key not in a:
#    a[key] = ...

# and it can get especially useful if things are nested
a = defaultdict(lambda: defaultdict(dict))
assert a[5][5] == {}