Python - 例外処理 - うまくいかないときに行うこと(密結合と抽象化、カスタム例外クラスの定義、OperationError、InterfaceError)

Head First Python 第2版
原著
楽天ブックス(Kobo) Yahoo!

開発環境

• macOS Mojave - Apple (OS)
• Emacs (Text Editor)
• Windows 10 Pro (OS)
• Visual Studio Code (Text Editor)
• Python 3.7 (プログラミング言語)
• パッケージ
  • psycopg2
  • Flask
• PostgreSQL (ORDBMS(object-relational database management system))

Head First Python 第2版 ―頭とからだで覚えるPythonの基本 (Paul Barry (著)、嶋田 健志 (監修)、木下 哲也 (翻訳)、オライリージャパン)の11章(例外処理 - うまくいかないときに行うこと)、自分で考えてみよう(p. 445)を取り組んでみる。

コード(Emacs)

DBcm.py

#!/usr/bin/env python3
import psycopg2 as sql


class ConnectionError(Exception):
    pass


class UseDatabase:
    def __init__(self, dbconfig: dict) -> None:
        self.dbconfig = dbconfig

    def __enter__(self) -> 'cursor':
        try:
            self.conn = sql.connect(**self.dbconfig)
            self.cursor = self.conn.cursor()
            return self.cursor
        except sql.OperationalError as err:
            raise ConnectionError(err)
        except sql.InterfaceError as err:
            raise ConnectionError(err)

    def __exit__(self, exc_type, exc_value, exc_trace) -> None:
        self.conn.commit()
        self.cursor.close()
        self.conn.close()


if __name__ == '__main__':
    dbconfig = {'host': '127.0.0.1',
                'database': 'vsearchlogdb'}
    with UseDatabase(dbconfig) as db:
        print(db)

vsearch4web.py

from flask import Flask, render_template, request, escape
import vsearch
import psycopg2 as sql
from DBcm import UseDatabase, ConnectionError

app = Flask(__name__)

dbconfig = {'host': '127.0.0.1',
            'database': 'vsearchlogdb'}


def log_request(req: 'flask_request', res: str) -> None:
    try:
        with UseDatabase(dbconfig) as cursor:
            _sql = '''
select phrase, letters, ip, browser_string, results
from log
'''
            cursor.execute(_sql)
            contents = cursor.fetchall()

        titles = ('phrase', 'letters', 'ユーザーエージェント', '結果')
        return render_template('viewlog.html',
                               the_title='ログの閲覧',
                               the_row_titles=titles,
                               the_data=contents,)
    except ConnectionError as err:
        print(f'データベースが動作していますか？エラー: {err}')
    except Exception as err:
        print(type(err))
        print(f'何か問題が発生しました: {err}')
    return 'Error'


@app.route('/')
@app.route('/entry')
def entry_page() -> str:
    return render_template(
        'entry.html',
        the_title='Welcom to search4letters on the web!')


@app.route('/search4', methods=['POST'])
def do_search() -> str:
    phrase = request.form['phrase']
    letters = request.form['letters']
    results = str(vsearch.search4letters(phrase, letters))
    try:
        log_request(request, results)
    except Exception as err:
        print(type(err), err)
    title = '検索結果'
    return render_template(
        'results.html',
        the_title=title,
        the_phrase=phrase,
        the_letters=letters,
        the_results=results)


@app.route('/viewlog')
def view_the_log() -> str:
    with UseDatabase(dbconfig) as cursor:
        _sql = '''
select phrase, letters, ip, browser_string, results
from log
'''
        cursor.execute(_sql)
        contents = cursor.fetchall()

    titles = ('phrase', 'letters', 'ユーザーエージェント', '結果')
    return render_template('viewlog.html',
                           the_title='ログの閲覧',
                           the_row_titles=titles,
                           the_data=contents,)


if __name__ == '__main__':
    app.run(debug=True)

入出力結果(Terminal, cmd(コマンドプロンプト), Jupyter(IPython))

$ python3 vsearch4web.py
 * Serving Flask app "vsearch4web" (lazy loading)
 * Environment: production
   WARNING: Do not use the development server in a production environment.
   Use a production WSGI server instead.
 * Debug mode: on
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 178-619-558
127.0.0.1 - - [28/Dec/2018 12:48:48] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [28/Dec/2018 12:48:48] "GET /static/hf.css HTTP/1.1" 404 -
データベースが動作していますか？エラー: could not connect to server: Connection refused
 Is the server running on host "127.0.0.1" and accepting
 TCP/IP connections on port 5432?

127.0.0.1 - - [28/Dec/2018 12:48:52] "POST /search4 HTTP/1.1" 200 -
127.0.0.1 - - [28/Dec/2018 12:48:52] "GET /static/hf.css HTTP/1.1" 404 -
127.0.0.1 - - [28/Dec/2018 12:49:16] "POST /search4 HTTP/1.1" 200 -
127.0.0.1 - - [28/Dec/2018 12:49:16] "GET /static/hf.css HTTP/1.1" 404 -
 * Detected change in '/.../vsearch4web.py', reloading
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 178-619-558
<class 'psycopg2.ProgrammingError'>
何か問題が発生しました: column "result" does not exist
LINE 2: select phrase, letters, ip, browser_string, result
                                                    ^
HINT:  Perhaps you meant to reference the column "log.results".

127.0.0.1 - - [28/Dec/2018 12:49:31] "POST /search4 HTTP/1.1" 200 -
127.0.0.1 - - [28/Dec/2018 12:49:31] "GET /static/hf.css HTTP/1.1" 404 -
 * Detected change in '/.../vsearch4web.py', reloading
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 178-619-558
127.0.0.1 - - [28/Dec/2018 12:49:38] "POST /search4 HTTP/1.1" 200 -
127.0.0.1 - - [28/Dec/2018 12:49:38] "GET /static/hf.css HTTP/1.1" 404 -
  C-c C-c$