Reputation: 20896
Python OOP --action() function
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)]
Upvotes: 5
Views: 11484
Answers (4)
Reputation: 3235
in menu.py::__init__, self.choices is a dict, where the keys are integers, and the values are functions. In menu.py::run, choice is input from stdin, and is used to key into the self.choices dict. so the line
action = self.choices.get(choice)
is setting action to be one of the functions in self.choices. This function can then be called:
action()
Upvotes: 0
Reputation: 12339
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.)
Upvotes: 6
Reputation: 838836
You call the function here:
action = self.choices.get(choice) # <--- get the function based on user input
if action:
action() # <--- call the function here
If choice is equal to '1' the above code will assign self.show_notes to action and then call it.
Upvotes: 1
Reputation: 50205
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().
Upvotes: 2