Supply an automatic value for a missing dictionary key.
What You Will Learn
defaultdict(factory) calls the zero-argument factory upon __getitem__ for an absent key, inserting the result. get() does not call the factory and can still return None.
Step-by-Step Example
The example below makes the behavior visible with small input values that you can change safely.
from collections import defaultdict
tasks_by_owner = defaultdict(list)
tasks_by_owner['Ava'].append('review')
print(tasks_by_owner['Ava'])
print(tasks_by_owner.get('Leo'))How the Example Works
- The imports provide the standard-library tools used in the demonstration: collections.
- The variable tasks_by_owner holds the result of defaultdict(list). Later lines use that value or inspect how it changes.
- The print() calls make the return values or state visible so you can compare them with the explanation and check your prediction.
Expected output from the example:
['review']
NoneWhen You Would Use This
Decide which input types are valid, what output should be produced, and which cases should fail clearly. A small self-contained demonstration is a good starting point before adding file access, network calls, or application state.
Practice: Modify the Code
Group tasks by owner without explicit existence checks.
Verify the changed output against the behavior described above, including the relevant invalid or boundary case.
For complete parameters and version details, consult the official Python documentation.
Common Beginner Mistakes
- Passing list() instead of list as the factory, or expecting get() to initialize a key.
- Assuming the operation has the same behavior for missing values, empty inputs and regular inputs.
Key Takeaways
- Supply an automatic value for a missing dictionary key.
- defaultdict(factory) calls the zero-argument factory upon __getitem__ for an absent key, inserting the result. get() does not call the factory and can still return None.
- Apply this operation only after checking the assumptions and input conditions shown in the example.