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✦ Python Foundations Collections module

Concept 13 · Nested Collections

Reach inside collections.
One level at a time.

Nested collections store one collection inside another. Read the data from the outside in, one level at a time.

01 Learn02 Try03 Predict

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01 Learn

Read nested data from the outside in, one level at a time.

Lists select values by position and dictionaries select values by meaningful keys. A nested collection stores another collection as one of its values, which helps programs organize students and scores, courses and lessons, or projects and tasks.

student = {"name": "Ana", "scores": [8, 9]}
print(student["scores"][1])

Do not read a long expression all at once. students[0] selects the first dictionary; students[0]["scores"] selects its scores list; students[0]["scores"][1] selects one value inside that list.

Follow the path: students → list; students[0] → first dictionary; students[0]["scores"] → [8, 9]; then index 1 → 9. If the current value is a dictionary, use a key; if it is a list, use an index.

↳

Nested values can change too: students[0]["scores"][1] = 10 changes one value deep in the structure. The outer list, dictionary, and scores list remain the same collections.

A dictionary can contain another dictionary
course = {"name": "Python Foundations", "student": {"name": "Ana", "scores": [8, 10, 9]}}

print(course["student"]["scores"][1])

First select "student", then the inner "scores" list, then index 1. The same outside-in strategy works at every level.

A loop can select one dictionary at a time
for student in students:
    print(student["name"])

The loop gives one student dictionary per iteration, so student["name"] uses a key in that current dictionary.

Trace a project path
projects[1]["tasks"][2]

projects[1] selects the second dictionary; ["tasks"] selects its list; [2] selects "Deploy".

Follow the data one level at a time

  1. Identify the current valueAsk what value and collection type you have now.
  2. Choose a key or an indexUse a key for a dictionary and an index for a list.
  3. Use the returned value for the next stepEach access returns the next value in the path.

Common beginner mistakes

  • Using a key on a liststudents["name"] fails because students is a list; select a student with an index first.
  • Using an index on a dictionarystudent[0] fails because a dictionary needs a key.
  • Skipping a levelFollow each returned collection before trying to select its inner value.
  • Using a wrong index or keyAn index outside a list can raise IndexError; a missing dictionary key can raise KeyError.
02 Try

Python Playground

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Predict the result, then follow each access level by level. Change one index, key, or nested value and run the code again.

nested_collections.pyPython
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03 Predict

What will this program print?

Read each operation before choosing an answer.

courses = {
    "python": ["Ana", "Marko"],
    "django": ["Jelena"]
}

print(courses["python"][1])
print(len(courses["django"]))
Choose the output
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