Data manipulation

Update or insert a key-value pair in a dictionary.

When you need to store a computed result under a specific key; when building a lookup table; when accumulating counts.

Used in scripts, data processing pipelines, configuration builders, and any code that works with mapping objects.

dictionary[key] = value

...[...] = ...

dictionary: dict, key: hashable, value: any object

• dict.get
• dict.update
• defaultdict
• setdefault

• Using dict.setdefault(key
• value) if you only want to set when missing
• using dict.update({key: value})

Using immutable key types like lists; forgetting that assignment overwrites existing value; confusing with equality check (==).

dict[key] += value (for numeric accumulation) vs direct assignment; dict.pop(key) removes.

Coming from languages where arrays use bracket assignment but require pre-sizing (e.g., C, Java): learners may expect dictionary to need pre-allocation → dictionaries do not require pre-allocation.

• Dictionary key access
• dictionary update
• dictionary comprehension

Keys must be hashable; assignment does not raise KeyError for missing keys; performance is O(1) average.

Provides a concise way to build or modify mappings, making intent explicit.

Think of putting a label on a folder: dictionary[key] = value.

Note that dictionary assignment modifies the dictionary in place and returns None; attempting to use a mutable (unhashable) key raises TypeError.

Using collections.defaultdict or collections.Counter for automatic default values; using dict comprehension for bulk creation.