Variables, strings, lists, dicts, conditions, loops

02A list position is not a dict key

Use square brackets to look up an item, but inspect what is inside them. labels[0] uses a list position; scores["lab"] uses a dictionary key. Python dictionaries preserve insertion order, but their lookup is still by key, not by numeric position. A missing key causes an error rather than supplying an automatic zero.

A condition is a test that chooses whether an action happens. The operator == compares values; it differs from =, which assigns a value. An if line ends in a colon, and the indented statements beneath it run only when its condition is true. A for line also ends in a colon and repeats its indented body for each item. The extra indentation in the activity puts the addition inside both the loop and the condition.

JSON, short for JavaScript Object Notation, is a text format for data. A JSON object contains named pairs; an array contains an ordered list. Python’s json module is part of its standard library, the tools supplied with Python. import json makes that module available, and json.load reads JSON from a file. The activity’s object becomes a dict and its array becomes a list. The file contains data, not Python instructions.

print(scores["lab"])

scores = {"lab": 1}
print(scores["lab"])

labels = ["red", "blue"]
for label in labels:
    print(label)

scores["lab"] = 1 (number)

1

03Count the ready records

The activity files contain three records and a script that counts records whose status is ready. A record here is a small dictionary describing one item. Follow the source against all three records before reading the predicted trace.

{
  "records": [
    {"id": "n1", "status": "ready"},
    {"id": "n2", "status": "hold"},
    {"id": "n3", "status": "ready"}
  ]
}

import json

with open("records.json", encoding="utf-8") as handle:
    payload = json.load(handle)

chosen = 0
for record in payload["records"]:
    if record["status"] == "ready":
        chosen = chosen + 1

print(chosen)

Predicted intermediate trace. The script prints only the final count:
n1 status ready -> chosen becomes 1
n2 status hold  -> chosen stays 1
n3 status ready -> chosen becomes 2
print(chosen) -> 2

1. In records.json, find the outer object, its records key, and the array inside square brackets. Read the three status values: ready, hold, ready.
2. Read python-basics.py. The with block opens records.json as UTF-8 text, names the open file handle, and lets json.load read it. Leaving with closes the file. payload is the resulting outer dictionary.
3. Start chosen at 0. For each record in payload["records"], compare record["status"] with ready. Add one only when they match. Your running counts should be 1, 1, and 2. The addition reads the old chosen value before assigning the new one.
4. Compare with the predicted trace: the script prints only the final 2. A prior Python 3.12.3 run recorded that final line. Stop after tracing, or later place both unchanged files in a fresh lab directory and enter python3 python-basics.py there. No extra JSON package is needed.

04Optional challenge

Spend about ten extra minutes imagining one added record with id n9 and status ready. Trace all four records without editing the download. The predicted total becomes 3 because three statuses match. Explain which condition caused each addition.

05Fresh cases for the checks

Use these fictional cases for the checks below. They have different inputs from the worked examples. Read them here or make notes; no installation or external service is needed.

records = [
    {"id": "a7", "status": "wait"},
    {"id": "b4", "status": "send"},
    {"id": "c8", "status": "wait"},
]
chosen = 0
for record in records:
    if record["status"] == "send":
        chosen = chosen + 1
print(chosen)
 
Source-reading case only; this code was not run.

The loop visits these four dictionaries in the printed order:
{"id": "k2", "status": "ready"}
{"id": "m9", "status": "ready"}
{"id": "p5", "status": "paused"}
{"id": "v3", "status": "ready"}
 
The addition is inside this condition:
if record["status"] == "ready":
    chosen = chosen + 1
Assume chosen starts at 0. No other statement changes it.

{
  "records": [
    {"id": "r6", "status": "held"}
  ],
  "limit": 8
}
 
Assume json.load reads this valid JSON into a name called payload.

records = [
    {"id": "t8", "status": "open"},
    {"id": "u2", "status": "wait"},
    {"id": "w6", "status": "open"},
]
chosen = 0
for record in records:
    if record["status"] == "open":
        chosen = chosen + 1
print(chosen)
 
For the changed case, replace only the comparison string "open" with "closed".
Leave every dictionary, the starting count, and the indentation unchanged.
Predict from the source; no run is recorded.

records = [
    {"id": "q4", "name": "memo", "stage": "send"},
    {"id": "q9", "name": "list", "stage": "wait"},
]
 
Intended rule: count a record when its processing state is send.
Current condition: if record["name"] == "send":
Change only the dictionary key inside the square brackets.

06Explain a conditional count

Optional self-check: choose one card and explain why a loop can visit a record without increasing the count. Explain how a dictionary key differs from a list position.

Check your explanation: name the value retrieved by the key, compare it with the condition's string, connect the result to the indented addition, and distinguish the final print from intermediate counts. A JSON array becomes a list; a JSON object becomes a dict. These are predictions from supplied source, not observations of an execution.