How can I check that a Python file-writing task produced the expected output?

Treat writing and verification as two separate steps. Finish writing and close the file, then reopen it in read mode and compare what you read with specific expectations. A successful write call does not establish that the file contains the intended values or structure. [S1][S2]

For example, suppose your task is to write a JSON object containing a status and a list of names. Decide what the expected object should be before checking the file. Then read it back and compare:

import json
from pathlib import Path

path = Path("[output-file.json]")
expected = {"status": "ready", "names": ["Ari", "Bo"]}

with path.open("w", encoding="utf-8") as f:
    json.dump(expected, f)

with path.open("r", encoding="utf-8") as f:
    actual = json.load(f)

assert isinstance(actual, dict)
assert actual == expected

Replace the placeholder path and sample values with those your task requires. The equality check verifies the decoded content, not merely that the file exists. Invalid JSON raises JSONDecodeError. [S2]

Check the header and the expected rows, rather than relying only on a row count. Reopen the file with csv.reader; CSV files should be opened with newline='', and you can specify UTF-8 explicitly. [S1]

For instance, if the intended CSV has a name,score header and two known records, the check could look like this:

import csv
from pathlib import Path

path = Path("[output-file.csv]")
expected_rows = [
    ["name", "score"],
    ["Ari", "8"],
    ["Bo", "10"],
]

with path.open("r", encoding="utf-8", newline="") as f:
    actual_rows = list(csv.reader(f))

assert actual_rows == expected_rows

If the output differs, compare the actual rows with the expected header and records to find what is missing or unexpected.

Reopening confirms that you can read a file at the path you checked, but correctness depends on the expectations you compare against. A well-formed JSON document can still have the wrong value, and a readable CSV can still have a missing or incorrect row. The Python documentation describes ways to read and write these formats; it cannot determine what your particular task intended. [S1][S2]

For plain text, choose checks that fit the requirement: compare the whole string when exact content matters, or check specific lines or required phrases when other differences are acceptable. Keep verification read-only by opening the completed file in mode "r", not a mode that truncates or rewrites it.

Use this sequence:

1. Finish writing to the intended path and close the file before checking it.
2. Define the expected content or structure: exact text, CSV header and rows, or JSON type, keys, and values.
3. Reopen the completed file in read mode, with an appropriate encoding. For CSV, include newline=''; for JSON, parse it with json.load(). [S1][S2]
4. Compare the actual result with the expectation, including meaningful details such as an empty list or a missing row.
5. If a check fails, inspect the read-back content and the path you opened; do not treat a successful write as proof of correctness.

This checks the output against your stated requirements. It does not establish that those requirements themselves are complete or correct.