Sample code for: How to Use Claude Code to Write and Refactor Python

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: RPBot

claude-code/README.md

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+# How to Use Claude Code to Write and Refactor Python
+
+This folder contains code associated with the Real Python tutorial [How to Use Claude Code to Write and Refactor Python](https://realpython.com/how-to-use-claude-code/).
+
+The `mini-contacts/` project is the finished state of the command-line contact manager that you build with Claude Code in the tutorial. The prompts that produced it are collected in [`prompts.md`](prompts.md), in the order they appear.
+
+Because Claude Code is nondeterministic, your own run won't match this code line for line. Expect the same structure, a storage module for CSV operations and a CLI module for argument parsing, with different naming and implementation details.
+
+## Run the Project
+
+```sh
+$ cd mini-contacts/
+$ python -m mini_contacts add --name "Alice" --email "alice@example.com" --phone "555-1234"
+$ python -m mini_contacts list
+```
+
+Contacts are stored at `~/.mini-contacts.csv` by default. Pass `--path` to use a different file.
+
+## Run the Tests
+
+```sh
+$ cd mini-contacts/
+$ python -m unittest
+```
+
+The suite uses `unittest` from the standard library, so there's nothing to install.
+
+## About the Author
+
+Real Python - Email: office@realpython.com
+
+## License
+
+Distributed under the MIT license. See `LICENSE` for more information.

claude-code/mini-contacts/CLAUDE.md

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+# Project Conventions
+
+- Python 3.14+, four-space indentation, PEP 8 compliant
+- Prefer the standard library; use third-party packages only when needed
+- Use type hints on all public functions

claude-code/mini-contacts/mini_contacts/__init__.py

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+from mini_contacts.cli import main
+
+main()

claude-code/mini-contacts/mini_contacts/__main__.py

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+import argparse
+import sys
+
+from mini_contacts import storage
+
+
+def _print_table(contacts: list[dict[str, str]]) -> None:
+    if not contacts:
+        print("No contacts found.")
+        return
+    headers = {"name": "Name", "email": "Email", "phone": "Phone"}
+    widths = {
+        field: max(len(headers[field]), max(len(row[field]) for row in contacts))
+        for field in storage.FIELDNAMES
+    }
+    header_line = " | ".join(headers[f].ljust(widths[f]) for f in storage.FIELDNAMES)
+    separator = "-+-".join("-" * widths[f] for f in storage.FIELDNAMES)
+    print(header_line)
+    print(separator)
+    for row in contacts:
+        print(" | ".join(row[f].ljust(widths[f]) for f in storage.FIELDNAMES))
+
+
+def main(argv: list[str] | None = None) -> None:
+    parser = argparse.ArgumentParser(description="Minimal contact manager")
+    parser.add_argument(
+        "--path", default="~/.mini-contacts.csv", help="Path to the CSV file"
+    )
+    subparsers = parser.add_subparsers(dest="command")
+    subparsers.required = True
+    add_parser = subparsers.add_parser("add", help="Add a new contact")
+    add_parser.add_argument("--name", required=True)
+    add_parser.add_argument("--email", required=True)
+    add_parser.add_argument("--phone", required=True)
+    subparsers.add_parser("list", help="List all contacts")
+    args = parser.parse_args(argv)
+
+    try:
+        if args.command == "add":
+            storage.add_contact(args.path, args.name, args.email, args.phone)
+            print(f"Added contact: {args.name}")
+        elif args.command == "list":
+            contacts = storage.read_contacts(args.path)
+            _print_table(contacts)
+    except (ValueError, OSError) as e:
+        print(f"Error: {e}", file=sys.stderr)
+        sys.exit(1)

claude-code/mini-contacts/mini_contacts/cli.py

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+import csv
+import os
+
+FIELDNAMES = ("name", "email", "phone")
+
+
+def read_contacts(path: str) -> list[dict[str, str]]:
+    path = os.path.expanduser(path)
+    try:
+        with open(path, newline="") as f:
+            reader = csv.DictReader(f, restval="")
+            if reader.fieldnames is None:
+                return []
+            if tuple(reader.fieldnames) != FIELDNAMES:
+                raise ValueError(
+                    f"Expected CSV headers {FIELDNAMES}, got {tuple(reader.fieldnames)}"
+                )
+            return list(reader)
+    except FileNotFoundError:
+        return []
+
+
+def add_contact(path: str, name: str, email: str, phone: str) -> None:
+    if not all([name, email, phone]):
+        raise ValueError("name, email, and phone must not be empty")
+    path = os.path.expanduser(path)
+    write_header = not os.path.exists(path) or os.path.getsize(path) == 0
+    with open(path, "a", newline="") as f:
+        writer = csv.DictWriter(f, fieldnames=FIELDNAMES)
+        if write_header:
+            writer.writeheader()
+        writer.writerow({"name": name, "email": email, "phone": phone})

claude-code/mini-contacts/mini_contacts/storage.py

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+import contextlib
+import io
+import tempfile
+import unittest
+from pathlib import Path
+
+from mini_contacts.cli import main
+
+
+class CLITests(unittest.TestCase):
+    def setUp(self):
+        tmp = tempfile.TemporaryDirectory()
+        self.addCleanup(tmp.cleanup)
+        self.tmp_path = Path(tmp.name)
+
+    def run_cli(self, *argv):
+        stdout = io.StringIO()
+        with contextlib.redirect_stdout(stdout):
+            main(list(argv))
+        return stdout.getvalue()
+
+    def test_add_then_list(self):
+        path = str(self.tmp_path / "contacts.csv")
+        self.run_cli(
+            "--path",
+            path,
+            "add",
+            "--name",
+            "Alice",
+            "--email",
+            "alice@example.com",
+            "--phone",
+            "555-1234",
+        )
+        out = self.run_cli("--path", path, "list")
+        self.assertIn("Alice", out)
+        self.assertIn("alice@example.com", out)
+
+    def test_list_empty(self):
+        out = self.run_cli("--path", str(self.tmp_path / "empty.csv"), "list")
+        self.assertIn("No contacts found.", out)
+
+    def test_list_short_row_exits_cleanly(self):
+        path = self.tmp_path / "contacts.csv"
+        path.write_text("name,email,phone\nAlice,alice@example.com\n")
+        out = self.run_cli("--path", str(path), "list")
+        self.assertIn("Alice", out)

claude-code/mini-contacts/tests/__init__.py

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+import tempfile
+import unittest
+from pathlib import Path
+
+from mini_contacts import storage
+
+
+class StorageTests(unittest.TestCase):
+    def setUp(self):
+        tmp = tempfile.TemporaryDirectory()
+        self.addCleanup(tmp.cleanup)
+        self.tmp_path = Path(tmp.name)
+
+    def test_round_trip(self):
+        path = self.tmp_path / "contacts.csv"
+        storage.add_contact(str(path), "Alice", "alice@example.com", "555-1234")
+        expected = {
+            "name": "Alice",
+            "email": "alice@example.com",
+            "phone": "555-1234",
+        }
+        self.assertEqual(storage.read_contacts(str(path)), [expected])
+
+    def test_header_written_once(self):
+        path = self.tmp_path / "contacts.csv"
+        storage.add_contact(str(path), "Alice", "alice@example.com", "555-1234")
+        storage.add_contact(str(path), "Bob", "bob@example.com", "555-5678")
+        self.assertEqual(path.read_text().count("name,email,phone"), 1)
+
+    def test_read_missing_file_returns_empty(self):
+        missing = self.tmp_path / "missing.csv"
+        self.assertEqual(storage.read_contacts(str(missing)), [])
+
+    def test_short_row_filled_with_blanks(self):
+        path = self.tmp_path / "contacts.csv"
+        path.write_text("name,email,phone\nAlice,alice@example.com\n")
+        contacts = storage.read_contacts(str(path))
+        self.assertEqual(contacts[0]["phone"], "")
+
+    def test_add_blank_field_raises(self):
+        with self.assertRaises(ValueError):
+            storage.add_contact(str(self.tmp_path / "c.csv"), "Alice", "", "555-1234")

claude-code/mini-contacts/tests/test_cli.py

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+# Prompts
+
+These are the prompts used in the Real Python tutorial
+[How to Use Claude Code to Write and Refactor Python](https://realpython.com/how-to-use-claude-code/),
+in the order they appear.
+
+Claude Code is nondeterministic, so your results will differ in wording and
+detail. Expect the same general shape, not identical output.
+
+## 1. Plan the project structure (Plan Mode)
+
+```text
+I want to build a minimal command-line contact manager in Python.
+
+It should have two commands:
+
+- `add` to append a new contact with name, email, and phone to a CSV file
+- `list` to print all contacts in a readable table format
+
+Use `argparse` for the CLI. Store the CSV at ~/.mini-contacts.csv
+by default with a --path flag to override it.
+
+Split the project into a mini_contacts/ package with storage.py for CSV
+read/write and cli.py for the `argparse` interface.
+```
+
+## 2. Implement the plan
+
+```text
+Implement the plan.
+```
+
+## 3. Exercise the happy path
+
+```text
+Add a contact named "John Doe" with email john@example.com and
+phone 555-0100, then list all contacts to verify it was saved.
+```
+
+## 4. Write the tests
+
+```text
+Write tests for the storage and CLI modules. Cover the happy path for
+add and list, and test what happens when the CSV file doesn't exist yet.
+```
+
+## 5. Run the tests with Shell mode
+
+```text
+! python -m unittest
+```
+
+## 6. Commit the initial implementation
+
+```text
+Create a commit with the message "Add initial mini-contacts implementation".
+```
+
+## 7. Explore the codebase in a fresh session
+
+```text
+Review this codebase. Tell me what the application does, describe
+each module and how they connect, and summarize the test coverage.
+```
+
+## 8. Hunt for bugs and edge cases
+
+```text
+Now look for bugs, security issues, and edge cases that could cause
+crashes or data loss. Be specific about each finding.
+```
+
+## 9. Plan the fixes (Plan Mode)
+
+```text
+Fix the crash bugs and the edge case for empty-string validation.
+For the crash on short CSV rows, handle rows with missing fields gracefully.
+For the empty-string fields, reject blank values for name, email, and phone.
+```
+
+## 10. Implement the fixes
+
+```text
+Implement the fixes.
+```
+
+## 11. Re-run the tests
+
+```text
+Run the tests and show me the results.
+```
+
+## 12. Commit the fixes
+
+```text
+Create a commit with the message "Add input validation and fix crash bugs".
+```