Python OOP --action() function

Question

I'm new to Python OOP and trying to create a OOP program to manage a library. This code is from a book.

This code is working as expected but I need to understand how the action() calls the corresponding function when I select a particular option, e.g.: when I select 1 the show_notes function is called even though we don't call it.

Menu.py

import sys
from notebook import Notebook, Note

class Menu:
    '''Display a menu and respond to choices when run.'''
    def __init__(self):
        self.notebook = Notebook()
        self.choices = {
                "1": self.show_notes,
                "2": self.search_notes,
                "3": self.add_note,
                "4": self.modify_note,
                "5": self.quit
                }

    def display_menu(self):
        print("""
Notebook Menu

1. Show all Notes
2. Search Notes
3. Add Note
4. Modify Note
5. Quit
""")

    def run(self):
        '''Display the menu and respond to choices.'''
        while True:
            self.display_menu()
            choice = input("Enter an option: ")
            action = self.choices.get(choice)
            if action:
                action()
            else:
                print("{0} is not a valid choice".format(choice))

    def show_notes(self, notes=None):
        if not notes:
            notes = self.notebook.notes
        for note in notes:
            print("{0}: {1}\n{2}".format(
                note.id, note.tags, note.memo))

    def search_notes(self):
        filter = input("Search for: ")
        notes = self.notebook.search(filter)
        self.show_notes(notes)

    def add_note(self):
        memo = input("Enter a memo: ")
        self.notebook.new_note(memo)
        print("Your note has been added.")

    def modify_note(self):
        id = input("Enter a note id: ")
        memo = input("Enter a memo: ")
        tags = input("Enter tags: ")
        if memo:
            self.notebook.modify_memo(id, memo)
        if tags:
            self.notebook.modify_tags(id, tags)

    def quit(self):
        print("Thank you for using your notebook today.")
        sys.exit(0)

if __name__ == "__main__":
    Menu().run()

notebook.py

import datetime

# Store the next available id for all new notes
last_id = 0

class Note:
    '''Represent a note in the notebook. Match against a
    string in searches and store tags for each note.'''


    def __init__(self, memo, tags=''):
        '''initialize a note with memo and optional
        space-separated tags. Automatically set the note's
        creation date and a unique id'''
        self.memo = memo
        self.tags = tags
        self.creation_date = datetime.date.today()
        global last_id
        last_id += 1
        self.id = last_id

    def match(self, filter):
        '''Determine if this note matches the filter
        text. Return True if it matches, False otherwise.

        Search is case sensitive and matches both text and
        tags.'''
        return filter in self.memo or filter in self.tags

class Notebook:

    '''Represent a collection of notes that can be tagged,
    modified, and searched.'''

    def __init__(self):
        '''Initialize a notebook with an empty list.'''
        self.notes = []

    def new_note(self, memo, tags=''):
        '''Create a new note and add it to the list.'''
        self.notes.append(Note(memo, tags))

    def _find_note(self, note_id):
        '''Locate the note with the given id.'''
        for note in self.notes:
            if str(note.id) == str(note_id):
                return note
        return None

    def modify_memo(self, note_id, memo):
        '''Find the note with the given id and change its
        memo to the given value.'''
        note = self._find_note(note_id)
        if note:
            note.memo = memo
            return True
        return False

    def modify_tags(self, note_id, tags):
        '''Find the note with the given id and change its
        tags to the given value.'''
        note = self._find_note(note_id)
        if note:
            note.tags = tags
            return True
        return False

    def search(self, filter):
        '''Find all notes that match the given filter
        string.'''
        return [note for note in self.notes if
                note.match(filter)]

Answer 1

Functions and methods are themselves objects. So the dict contains objects which we then call.

So self.choices is a dictionary with "1", "2", etc as keys. The values are method objects. When you get the value from the dictionary, you get that object (called a "callable") and assign it to action. Then you call that object with action().

The key bit of code is this:

 self.choices = {
            "1": self.show_notes,
            "2": self.search_notes,
            "3": self.add_note,
            "4": self.modify_note,
            "5": self.quit
            }

self.choices["1"] evaluates to the value of self.show_notes, which is then assigned to action, which you then call with action().

(As an aside: The menu in the example code is hardcoded, but in fact could be autogenerated, if the methods had docstrings. "\n".join("%s: %s" % (key, action.__doc__) for key, action in sorted(self.choices.iteritems())) To properly handle a menu with >=10 items, you'd want to make the keys into integers, or expand the one-liner.)

Answer 2

self.choices is a dictionary of methods. When you select "1" this is happening:

action = self.choices.get("1")  # action = self.show_notes

Therefore when you call action() you're actually calling self.show_notes().