Python Dictionaries

Python Dictionaries: Hash Maps, Iteration, & Lookups

Dictionaries (dict) are key-value mappings implemented via high-performance hash tables. Since Python 3.7+, dictionaries preserve key insertion order while maintaining $O(1)$ average time complexity for lookups, insertions, and deletions.

Hash Table Mapping Architecture

flowchart LR
    subgraph Keys (Must Be Hashable)
        k1["'user_id'"]
        k2["'role'"]
    end
    subgraph Hash Engine
        h1["hash('user_id') -> 8742"]
        h2["hash('role') -> 1290"]
    end
    subgraph Value Array
        v1["10045"]
        v2["'administrator'"]
    end
    k1 --> h1 --> v1
    k2 --> h2 --> v2

Dictionary Access Methods

• dict[key]: Direct access. Raises KeyError if key is missing.
• dict.get(key, default): Safe access. Returns default (or None) if key is missing.
• dict.setdefault(key, default): Returns key value; inserts key: default if key is not present.
• dict.items(), dict.keys(), dict.values(): Dynamic view objects.

Practical Code Example

from typing import Dict, Any

def process_user_metrics(logs: list[dict]) -> Dict[str, int]:
    """Aggregate request counts per endpoint using dictionary comprehension and get()."""
    endpoint_counts: Dict[str, int] = {}
    
    for log in logs:
        endpoint = log.get("path", "/unknown")
        # Increment counter safely
        endpoint_counts[endpoint] = endpoint_counts.get(endpoint, 0) + 1
        
    return endpoint_counts

if __name__ == "__main__":
    sample_logs = [
        {"path": "/api/v1/users", "status": 200},
        {"path": "/api/v1/posts", "status": 200},
        {"path": "/api/v1/users", "status": 200},
        {"path": "/home", "status": 200},
    ]

    metrics = process_user_metrics(sample_logs)
    print("Endpoint Metrics:", metrics)

    # Dictionary Unpacking (Merging in Python 3.9+)
    default_config = {"theme": "dark", "notifications": True, "timeout": 30}
    custom_config = {"timeout": 60, "language": "en"}
    merged_config = default_config | custom_config  # Merging operator '|'
    print("Merged Configuration:", merged_config)

Best Practices & Gotchas

• Use .get() for Optional Keys: Avoid triggering unhandled KeyError exceptions when reading optional dictionary fields.
• Use Dictionary Merge Operator (|): In Python 3.9+, merge two dictionaries cleanly with dict1 | dict2.
• Key Requirements: Dict keys must be hashable (immutable objects like strings, numbers, or tuples).

Self-Check Challenge

Write a function invert_dict(d: dict) -> dict that swaps keys and values in a dictionary assuming all values are unique and hashable.