Architecting the Python Dictionary Mappings, Hashability, and 10
10 – Dictionaries ● Creating Dictionaries ● Accessing Values ● Dictionary Methods ● Nested Dictionary
Dictionaries in Python
1. What is a Dictionary?
In Python, a dictionary (or dict) is a built-in mutable mapping type that represents a finite, associative array of key-value pairs.
The Associative Paradigm: Dictionaries map arbitrary, unique keys to associated values (similar to "hashes" in Perl). Unlike sequence types (like lists and tuples) which are indexed by sequential integers, dictionaries are indexed by their keys.
Keys Mutability and Hashability Requirements: To maintain an extremely efficient internal lookup structure, keys must be immutable and hashable. An object is hashable if it has a hash value that never changes during its lifetime (implementing __hash__() and __eq__() methods).
Standard mutable types (such as lists, dictionaries, or other containers containing mutable elements) compare by value rather than object identity, meaning their hash values are not constant; consequently, they cannot be used as dictionary keys.
The Numeric Comparison Rule: Numeric keys obey standard mathematical comparison rules. If two numbers compare as mathematically equal (such as the integer 1 and the floating-point 1.0), they are treated as interchangeable keys that index the exact same dictionary entry.
Insertion Order Preservation: Starting in Python 3.7, dictionaries are officially guaranteed to preserve the insertion order of their elements (this was previously an implementation detail in CPython 3.6).
Replacing the value of an existing key does not alter its position in the insertion sequence; however, removing a key and then re-inserting it appends it directly to the end of the dictionary.
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