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diff --git a/docs/pages/tutorials/repl.rst b/docs/pages/tutorials/repl.rst new file mode 100644 index 0000000..946786f --- /dev/null +++ b/docs/pages/tutorials/repl.rst @@ -0,0 +1,341 @@ +.. _tutorial_repl: + +Tutorial: Build an SQLite REPL +============================== + +The aim of this tutorial is to build an interactive command line interface for +an SQLite database using prompt_toolkit_. + +First, install the library using pip, if you haven't done this already. + +.. code:: + + pip install prompt_toolkit + + +Read User Input +--------------- + +Let's start accepting input using the +:func:`~prompt_toolkit.shortcuts.prompt()` function. This will ask the user for +input, and echo back whatever the user typed. We wrap it in a ``main()`` +function as a good practice. + +.. code:: python + + from prompt_toolkit import prompt + + def main(): + text = prompt('> ') + print('You entered:', text) + + if __name__ == '__main__': + main() + +.. image:: ../../images/repl/sqlite-1.png + + +Loop The REPL +------------- + +Now we want to call the :meth:`~prompt_toolkit.shortcuts.PromptSession.prompt` +method in a loop. In order to keep the history, the easiest way to do it is to +use a :class:`~prompt_toolkit.shortcuts.PromptSession`. This uses an +:class:`~prompt_toolkit.history.InMemoryHistory` underneath that keeps track of +the history, so that if the user presses the up-arrow, they'll see the previous +entries. + +The :meth:`~prompt_toolkit.shortcuts.PromptSession.prompt` method raises +``KeyboardInterrupt`` when ControlC has been pressed and ``EOFError`` when +ControlD has been pressed. This is what people use for cancelling commands and +exiting in a REPL. The try/except below handles these error conditions and make +sure that we go to the next iteration of the loop or quit the loop +respectively. + +.. code:: python + + from prompt_toolkit import PromptSession + + def main(): + session = PromptSession() + + while True: + try: + text = session.prompt('> ') + except KeyboardInterrupt: + continue + except EOFError: + break + else: + print('You entered:', text) + print('GoodBye!') + + if __name__ == '__main__': + main() + +.. image:: ../../images/repl/sqlite-2.png + + +Syntax Highlighting +------------------- + +This is where things get really interesting. Let's step it up a notch by adding +syntax highlighting to the user input. We know that users will be entering SQL +statements, so we can leverage the Pygments_ library for coloring the input. +The ``lexer`` parameter allows us to set the syntax lexer. We're going to use +the ``SqlLexer`` from the Pygments_ library for highlighting. + +Notice that in order to pass a Pygments lexer to prompt_toolkit, it needs to be +wrapped into a :class:`~prompt_toolkit.lexers.PygmentsLexer`. + +.. code:: python + + from prompt_toolkit import PromptSession + from prompt_toolkit.lexers import PygmentsLexer + from pygments.lexers.sql import SqlLexer + + def main(): + session = PromptSession(lexer=PygmentsLexer(SqlLexer)) + + while True: + try: + text = session.prompt('> ') + except KeyboardInterrupt: + continue + except EOFError: + break + else: + print('You entered:', text) + print('GoodBye!') + + if __name__ == '__main__': + main() + +.. image:: ../../images/repl/sqlite-3.png + + +Auto-completion +--------------- + +Now we are going to add auto completion. We'd like to display a drop down menu +of `possible keywords <https://www.sqlite.org/lang_keywords.html>`_ when the +user starts typing. + +We can do this by creating an `sql_completer` object from the +:class:`~prompt_toolkit.completion.WordCompleter` class, defining a set of +`keywords` for the auto-completion. + +Like the lexer, this ``sql_completer`` instance can be passed to either the +:class:`~prompt_toolkit.shortcuts.PromptSession` class or the +:meth:`~prompt_toolkit.shortcuts.PromptSession.prompt` method. + +.. code:: python + + from prompt_toolkit import PromptSession + from prompt_toolkit.completion import WordCompleter + from prompt_toolkit.lexers import PygmentsLexer + from pygments.lexers.sql import SqlLexer + + sql_completer = WordCompleter([ + 'abort', 'action', 'add', 'after', 'all', 'alter', 'analyze', 'and', + 'as', 'asc', 'attach', 'autoincrement', 'before', 'begin', 'between', + 'by', 'cascade', 'case', 'cast', 'check', 'collate', 'column', + 'commit', 'conflict', 'constraint', 'create', 'cross', 'current_date', + 'current_time', 'current_timestamp', 'database', 'default', + 'deferrable', 'deferred', 'delete', 'desc', 'detach', 'distinct', + 'drop', 'each', 'else', 'end', 'escape', 'except', 'exclusive', + 'exists', 'explain', 'fail', 'for', 'foreign', 'from', 'full', 'glob', + 'group', 'having', 'if', 'ignore', 'immediate', 'in', 'index', + 'indexed', 'initially', 'inner', 'insert', 'instead', 'intersect', + 'into', 'is', 'isnull', 'join', 'key', 'left', 'like', 'limit', + 'match', 'natural', 'no', 'not', 'notnull', 'null', 'of', 'offset', + 'on', 'or', 'order', 'outer', 'plan', 'pragma', 'primary', 'query', + 'raise', 'recursive', 'references', 'regexp', 'reindex', 'release', + 'rename', 'replace', 'restrict', 'right', 'rollback', 'row', + 'savepoint', 'select', 'set', 'table', 'temp', 'temporary', 'then', + 'to', 'transaction', 'trigger', 'union', 'unique', 'update', 'using', + 'vacuum', 'values', 'view', 'virtual', 'when', 'where', 'with', + 'without'], ignore_case=True) + + def main(): + session = PromptSession( + lexer=PygmentsLexer(SqlLexer), completer=sql_completer) + + while True: + try: + text = session.prompt('> ') + except KeyboardInterrupt: + continue + except EOFError: + break + else: + print('You entered:', text) + print('GoodBye!') + + if __name__ == '__main__': + main() + +.. image:: ../../images/repl/sqlite-4.png + +In about 30 lines of code we got ourselves an auto completing, syntax +highlighting REPL. Let's make it even better. + + +Styling the menus +----------------- + +If we want, we can now change the colors of the completion menu. This is +possible by creating a :class:`~prompt_toolkit.styles.Style` instance and +passing it to the :meth:`~prompt_toolkit.shortcuts.PromptSession.prompt` +function. + +.. code:: python + + from prompt_toolkit import PromptSession + from prompt_toolkit.completion import WordCompleter + from prompt_toolkit.lexers import PygmentsLexer + from prompt_toolkit.styles import Style + from pygments.lexers.sql import SqlLexer + + sql_completer = WordCompleter([ + 'abort', 'action', 'add', 'after', 'all', 'alter', 'analyze', 'and', + 'as', 'asc', 'attach', 'autoincrement', 'before', 'begin', 'between', + 'by', 'cascade', 'case', 'cast', 'check', 'collate', 'column', + 'commit', 'conflict', 'constraint', 'create', 'cross', 'current_date', + 'current_time', 'current_timestamp', 'database', 'default', + 'deferrable', 'deferred', 'delete', 'desc', 'detach', 'distinct', + 'drop', 'each', 'else', 'end', 'escape', 'except', 'exclusive', + 'exists', 'explain', 'fail', 'for', 'foreign', 'from', 'full', 'glob', + 'group', 'having', 'if', 'ignore', 'immediate', 'in', 'index', + 'indexed', 'initially', 'inner', 'insert', 'instead', 'intersect', + 'into', 'is', 'isnull', 'join', 'key', 'left', 'like', 'limit', + 'match', 'natural', 'no', 'not', 'notnull', 'null', 'of', 'offset', + 'on', 'or', 'order', 'outer', 'plan', 'pragma', 'primary', 'query', + 'raise', 'recursive', 'references', 'regexp', 'reindex', 'release', + 'rename', 'replace', 'restrict', 'right', 'rollback', 'row', + 'savepoint', 'select', 'set', 'table', 'temp', 'temporary', 'then', + 'to', 'transaction', 'trigger', 'union', 'unique', 'update', 'using', + 'vacuum', 'values', 'view', 'virtual', 'when', 'where', 'with', + 'without'], ignore_case=True) + + style = Style.from_dict({ + 'completion-menu.completion': 'bg:#008888 #ffffff', + 'completion-menu.completion.current': 'bg:#00aaaa #000000', + 'scrollbar.background': 'bg:#88aaaa', + 'scrollbar.button': 'bg:#222222', + }) + + def main(): + session = PromptSession( + lexer=PygmentsLexer(SqlLexer), completer=sql_completer, style=style) + + while True: + try: + text = session.prompt('> ') + except KeyboardInterrupt: + continue + except EOFError: + break + else: + print('You entered:', text) + print('GoodBye!') + + if __name__ == '__main__': + main() + +.. image:: ../../images/repl/sqlite-5.png + +All that's left is hooking up the sqlite backend, which is left as an exercise +for the reader. Just kidding... Keep reading. + + +Hook up Sqlite +-------------- + +This step is the final step to make the SQLite REPL actually work. It's time +to relay the input to SQLite. + +Obviously I haven't done the due diligence to deal with the errors. But it +gives a good idea of how to get started. + +.. code:: python + + #!/usr/bin/env python + import sys + import sqlite3 + + from prompt_toolkit import PromptSession + from prompt_toolkit.completion import WordCompleter + from prompt_toolkit.lexers import PygmentsLexer + from prompt_toolkit.styles import Style + from pygments.lexers.sql import SqlLexer + + sql_completer = WordCompleter([ + 'abort', 'action', 'add', 'after', 'all', 'alter', 'analyze', 'and', + 'as', 'asc', 'attach', 'autoincrement', 'before', 'begin', 'between', + 'by', 'cascade', 'case', 'cast', 'check', 'collate', 'column', + 'commit', 'conflict', 'constraint', 'create', 'cross', 'current_date', + 'current_time', 'current_timestamp', 'database', 'default', + 'deferrable', 'deferred', 'delete', 'desc', 'detach', 'distinct', + 'drop', 'each', 'else', 'end', 'escape', 'except', 'exclusive', + 'exists', 'explain', 'fail', 'for', 'foreign', 'from', 'full', 'glob', + 'group', 'having', 'if', 'ignore', 'immediate', 'in', 'index', + 'indexed', 'initially', 'inner', 'insert', 'instead', 'intersect', + 'into', 'is', 'isnull', 'join', 'key', 'left', 'like', 'limit', + 'match', 'natural', 'no', 'not', 'notnull', 'null', 'of', 'offset', + 'on', 'or', 'order', 'outer', 'plan', 'pragma', 'primary', 'query', + 'raise', 'recursive', 'references', 'regexp', 'reindex', 'release', + 'rename', 'replace', 'restrict', 'right', 'rollback', 'row', + 'savepoint', 'select', 'set', 'table', 'temp', 'temporary', 'then', + 'to', 'transaction', 'trigger', 'union', 'unique', 'update', 'using', + 'vacuum', 'values', 'view', 'virtual', 'when', 'where', 'with', + 'without'], ignore_case=True) + + style = Style.from_dict({ + 'completion-menu.completion': 'bg:#008888 #ffffff', + 'completion-menu.completion.current': 'bg:#00aaaa #000000', + 'scrollbar.background': 'bg:#88aaaa', + 'scrollbar.button': 'bg:#222222', + }) + + def main(database): + connection = sqlite3.connect(database) + session = PromptSession( + lexer=PygmentsLexer(SqlLexer), completer=sql_completer, style=style) + + while True: + try: + text = session.prompt('> ') + except KeyboardInterrupt: + continue # Control-C pressed. Try again. + except EOFError: + break # Control-D pressed. + + with connection: + try: + messages = connection.execute(text) + except Exception as e: + print(repr(e)) + else: + for message in messages: + print(message) + + print('GoodBye!') + + if __name__ == '__main__': + if len(sys.argv) < 2: + db = ':memory:' + else: + db = sys.argv[1] + + main(db) + +.. image:: ../../images/repl/sqlite-6.png + +I hope that gives an idea of how to get started on building command line +interfaces. + +The End. + +.. _prompt_toolkit: https://github.com/prompt-toolkit/python-prompt-toolkit +.. _Pygments: http://pygments.org/ |