Defaultdict
A defaultdict is a Python dictionary subclass that creates and supplies a factory-generated default value when a missing key is accessed.
Python Workout, Second Edition
defaultdict is a collections class that supplies a default value for missing dictionary keys by calling a configured factory.
- How it is used
- Solves repeated dictionary updates where missing keys need an initial value.
- How deep it goes
- Explains factory invocation and demonstrates integer accumulation, while stopping short of broader factory choices.
- Examples
- Uses a rainfall dictionary, repeated Boston entries, and defaultdict(int) to accumulate millimeters.
- What you can do after
- Apply it via collections.defaultdict to accumulate values without manually initializing missing keys.
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Python Workout: 50 ten-minute exercises
A defaultdict is a dictionary subclass that creates and returns a default value for a missing key using a configured factory function.
- How it is used
- Accumulates repeated values under dictionary keys without manually handling the first occurrence.
- How deep it goes
- Defines the missing-key factory and demonstrates integer accumulation, while contrasting it with dict.get.
- Examples
- Uses a rainfall dictionary keyed by city names, including two Boston readings totaling 60.
- What you can do after
- Apply it via collections.defaultdict to accumulate values for keys that may appear for the first time.
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Practices of the Python Pro
defaultdict is a Python mapping type that supplies a default value when a missing key is accessed.
- How it is used
- Simplifies dictionary-based counting by initializing missing keys with sensible values.
- How deep it goes
- Explains default-value behavior and demonstrates counting, but stops short of broader mapping internals.
- Examples
- Counts integers in a list and finds the number with the highest count.
- What you can do after
- Apply it via Python to build counting mappings without explicit missing-key checks.
Related concepts
- Mapkind of · A map is an associative data structure that stores key-value pairs and supports retrieving values by key.
- collectionpart of · A collection is a data structure or abstraction that groups multiple elements for storage, access, traversal, or processing.
- setdefault methodequivalent to · The setdefault method is a dictionary method that returns a key’s value, inserting the key with a specified default when absent.
- frequencyuses · Frequency is the count or rate at which an item, event, outcome, or periodic cycle occurs within a specified collection or interval.
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