After this lesson, you will be able to:

• Access values inside dictionaries that contain lists or other dictionaries.
• Update a nested value without replacing unrelated data.
• Iterate through nested records and summarize their fields.
• Handle a missing nested dictionary key while keeping valid zero values.

Real responses and configurations often combine collections: a dictionary may hold a list of services, and each service may itself be a dictionary. Read a nested path one level at a time. config["services"] selects the list; [0] selects a record; ["name"] selects a field.

Nested collections are mutable. Changing a nested field through its path changes the original collection. The outer collection's type does not remove the mutability of objects inside it.

When a nested mapping is optional, use .get(key, {}) to obtain an empty dictionary as a fallback. Read an optional numeric value with .get(key, default) rather than a truthiness fallback: the value 0 can be intentional.

1. Read a value one level at a time

config = {"targets": [{"name": "staging", "port": 8081}, {"name": "prod", "port": 443}]}
print(config["targets"][1]["port"])

Output:

443

The first lookup returns a list; the second selects a dictionary within it.

2. Change one nested setting

limits = {"search": {"retries": 2, "timeout": 30}}
limits["search"]["retries"] += 1
print(limits["search"]["retries"])
print(limits["search"]["timeout"])

Output:

3
30

Only the retry setting changes.

3. Process a list of records

jobs = [{"name": "index", "done": True}, {"name": "backup", "done": False}]
for job in jobs:
    if job["done"]:
        print(f"Finished: {job['name']}")

Output:

Finished: index

The loop variable is already one dictionary; no additional numeric index is needed.

4. Use a safe nested fallback

settings = {"thresholds": {"api": 0}}
thresholds = settings.get("thresholds", {})
print(thresholds.get("api", 10))
print(thresholds.get("worker", 10))

Output:

0
10

A stored zero is preserved, but a missing key uses the fallback.

1. A dictionary record from for record in records is not a numeric index.
2. data["missing"] raises KeyError when a key is absent.
3. dict.get("key") or default wrongly replaces valid falsy values such as 0.
4. Updating a nested dictionary changes the existing object; it is not automatically copied.