typing.List and Dict
- Python course
- Advanced
- Lesson 9/83
- 5 min
Learn generic types with typing.List and Dict.
typing.List and Dict
To declare the inner type of a list, use typing.List: List[int] means a list of integers. For dicts use Dict: Dict[str, int] means a dict with string keys and integer values.
from typing import List, Dict
def total(nums: List[int]) -> int:
return sum(nums)
def counts(words: List[str]) -> Dict[str, int]:
return {w: len(w) for w in words}The type inside brackets is the element type of the list or dict. Python 3.9+ allows lowercase list[int] and dict[str, int]; this lesson uses the typing module.
Nested types and reading them
Types can nest. List[Dict[str, int]] means a list whose items are dicts. This is useful for representing database rows or config settings.
When reading, parse right to left: find the innermost type, then add the outer container. Example: Dict[str, List[int]] is a dict with string keys and integer-list values.
Nested hints look complex, but IDEs and mypy follow them; you catch errors early.
Practice total function with List[int]
Write total(nums: List[int]) -> int. The body contains return sum(nums). Import List from typing. Call print(total([1, 2, 3])).
Starter code
# import List from typing
# total(nums: List[int]) -> int: sum the nums list
# print the result of total([1, 2, 3])Sample solution
from typing import List
def total(nums: List[int]) -> int:
return sum(nums)
print(total([1, 2, 3]))Expected output
6
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the container types
| Term | Meaning |
|---|---|
List[int] |
List of integers |
Dict[str, int] |
String→int dict |
List[str] |
List of strings |
Dict[str, str] |
String→string dict |
2 Complete the Dict type
from typing import Dict
def lengths(words):
result: Dict[str, ____] = {}
for w in words:
result[w] = len(w)
return resultfrom typing import Dict
def lengths(words):
result: Dict[str, int] = {}
for w in words:
result[w] = len(w)
return resultExplanation: Dict[str, int] is a dict with string keys and integer values; len(w) returns an integer.
3 What does Dict[str, int] describe?
- A dict with string keys and integer values Correct answer
- A list of integers
- A dict containing only None
- A list of strings
Explanation: Dict[str, int] is a dict with string keys and integer values.