By the end of this lesson
Distinguish valid JSON syntax from a valid record list.
Before you start
- Python 3.12 or a compatible newer Python 3 interpreter; no external packages.
- Complete lesson 15 in this route, or be comfortable with its examples.
Three words to know
- JSON
- A text format for values such as objects, arrays, strings and numbers.
- Decode
- Turn a JSON document into Python values.
- Schema rule
- A requirement about the structure or meaning of data.
1. Decode the document
json.loads reads JSON text; it does not execute Python source. JSON objects become dictionaries and arrays become lists. JSON uses double-quoted strings and lowercase true, false and null; Python uses True, False and None. A syntax parser does not decide whether fields make sense for your application.
import json
text = '[{"status": "open"}, {"status": "closed"}]'
records = json.loads(text)
print(type(records).__name__)
print(records[0]["status"])
What you should see · example output
list open
2. Validate before counting
This function checks the outer list and each required field. A missing field is different from a present but unsupported value. Extra fields are deliberately allowed here. This short example is for already-decoded small data; it is not a bounded file reader.
def check_records(records):
if not isinstance(records, list):
raise ValueError("expected a list")
for record in records:
if not isinstance(record, dict):
raise ValueError("expected objects")
if record.get("status") not in ("open", "closed"):
raise ValueError("unsupported status")
return len(records)
print(check_records([{"status": "open"}]))
What you should see · example output
1
3. Serialise a result
json.dumps converts supported Python values into JSON text. indent makes it readable; sort_keys gives a predictable key order for review. Python’s default JSON decoder accepts some extensions such as NaN, so strict interoperability may need extra rules. The final project rejects those constants explicitly.
import json
result = {"records": 2, "open": 1}
print(json.dumps(result, sort_keys=True))
What you should see · example output
{"open": 1, "records": 2}
YOUR TURN
A small check before you continue
Is {"status": "open"} valid JSON? Does it meet the record-list rule above?
Show a suggested answer
It is valid JSON syntax but it is an object, not the required outer list. The corresponding valid input is [{"status": "open"}].
Sources and scope
Documentation and examples reviewed 2026-10-18. Provider screens, package versions and output wording can differ. Examples show expected outcomes; they are not a claim that OpenViST connected to your server or verified your setup.