defaultdict
- Python course
- Intermediate
- Lesson 89/116
- 5 min
Learn default-value dicts with defaultdict.
defaultdict
defaultdict produces a default value instead of raising KeyError when you access a missing key. A factory function is given to the constructor; types like int, list, dict can be used.
Example: d = defaultdict(int). Writing d["a"] += 1 first creates d["a"] as 0, then it becomes 1. So for counting or grouping tasks you do not need to check whether the key exists first.
Which factory gives what?
defaultdict(int) defaults to 0; ideal for counting. defaultdict(list) produces an empty list; ideal for grouping. defaultdict(dict) gives an empty dict; useful for nested structures.
Grouping example: groups = defaultdict(list)\nfor word in words:\n groups[len(word)].append(word). This code collects words of the same length into a list. Each length triggers an automatic empty list when first seen; no if len(word) in groups check is needed.
Practice Count with defaultdict
Start with from collections import defaultdict. Create a defaultdict with d = defaultdict(int). Call d["a"] += 1 twice. Then print the count of a with print(d["a"]).
Starter code
# Count with defaultdict(int)Sample solution
# Count with defaultdict(int)
from collections import defaultdict
d = defaultdict(int)
d["a"] += 1
d["a"] += 1
print(d["a"])Expected output
2
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 defaultdict factory argument
from collections import defaultdict
d = defaultdict(____)
d["a"] += 1
print(d["a"])from collections import defaultdict
d = defaultdict(int)
d["a"] += 1
print(d["a"])Explanation: The int factory produces 0 as default.
2 What does defaultdict(int) return for a missing key?
- KeyError
- None
- 0 Correct answer
- Empty string
Explanation: defaultdict(int) produces 0 for missing keys.
3 Match factory and default
| Term | Meaning |
|---|---|
int |
0 |
list |
[] |
dict |
{} |
str |
"" |