<p>Building a custom notes application in Python can be achieved using various frameworks and libraries. For simplicity and quick development, let’s use the <code>tkinter</code> library for the GUI and <code>sqlite3</code> for the database to store the notes.</p>
<p>Here’s a basic implementation with <code>tkinter</code> for the GUI and <code>sqlite3</code> for storing and retrieving notes:</p>
<p><strong>Step 1: Set up SQLite Database</strong></p>
<p>First, create a SQLite database to store the notes.</p>
<pre class="hljs"><code><a class="copy-code" onclick="copyCode(this)" href="#">Copy</a><span class="hljs-keyword">import</span> sqlite3

<span class="hljs-keyword">def</span> <span class="hljs-title function_">init_db</span>():
    conn = sqlite3.connect(<span class="hljs-string">'notes.db'</span>)
    cursor = conn.cursor()
    cursor.execute(<span class="hljs-string">'''
        CREATE TABLE IF NOT EXISTS notes (
            id INTEGER PRIMARY KEY,
            title TEXT NOT NULL,
            content TEXT NOT NULL
        )
    '''</span>)
    conn.commit()
    conn.close()

init_db()
</code></pre>
<p><strong>Step 2: Build the GUI Application</strong></p>
<p>Next, use <code>tkinter</code> to create the GUI for the notes application.</p>
<pre class="hljs"><code><a class="copy-code" onclick="copyCode(this)" href="#">Copy</a>import tkinter as tk
from tkinter import messagebox
import sqlite3

class <span class="hljs-built_in">NotesApp</span>(tk.Tk):
    def <span class="hljs-built_in">__init__</span>(self):
        <span class="hljs-built_in">super</span>().<span class="hljs-built_in">__init__</span>()
        self.<span class="hljs-built_in">title</span>(<span class="hljs-string">"Notes Application"</span>)
        self.<span class="hljs-built_in">geometry</span>(<span class="hljs-string">"600x400"</span>)
        
        self.title_label = tk.<span class="hljs-built_in">Label</span>(self, text=<span class="hljs-string">"Title:"</span>)
        self.title_label.<span class="hljs-built_in">pack</span>(pady=<span class="hljs-number">5</span>)
        
        self.title_entry = tk.<span class="hljs-built_in">Entry</span>(self, width=<span class="hljs-number">50</span>)
        self.title_entry.<span class="hljs-built_in">pack</span>(pady=<span class="hljs-number">5</span>)
        
        self.content_label = tk.<span class="hljs-built_in">Label</span>(self, text=<span class="hljs-string">"Content:"</span>)
        self.content_label.<span class="hljs-built_in">pack</span>(pady=<span class="hljs-number">5</span>)
        
        self.content_text = tk.<span class="hljs-built_in">Text</span>(self, width=<span class="hljs-number">50</span>, height=<span class="hljs-number">10</span>)
        self.content_text.<span class="hljs-built_in">pack</span>(pady=<span class="hljs-number">5</span>)
        
        self.save_button = tk.<span class="hljs-built_in">Button</span>(self, text=<span class="hljs-string">"Save Note"</span>, command=self.save_note)
        self.save_button.<span class="hljs-built_in">pack</span>(pady=<span class="hljs-number">5</span>)
        
        self.notes_list_label = tk.<span class="hljs-built_in">Label</span>(self, text=<span class="hljs-string">"Notes List:"</span>)
        self.notes_list_label.<span class="hljs-built_in">pack</span>(pady=<span class="hljs-number">5</span>)
        
        self.notes_listbox = tk.<span class="hljs-built_in">Listbox</span>(self, width=<span class="hljs-number">50</span>, height=<span class="hljs-number">10</span>)
        self.notes_listbox.<span class="hljs-built_in">pack</span>(pady=<span class="hljs-number">5</span>)
        self.notes_listbox.<span class="hljs-built_in">bind</span>(<span class="hljs-string">'&lt;&lt;ListboxSelect&gt;&gt;'</span>, self.show_note)
        
        self.delete_button = tk.<span class="hljs-built_in">Button</span>(self, text=<span class="hljs-string">"Delete Note"</span>, command=self.delete_note)
        self.delete_button.<span class="hljs-built_in">pack</span>(pady=<span class="hljs-number">5</span>)
        
        self.<span class="hljs-built_in">load_notes</span>()
    
    def <span class="hljs-built_in">save_note</span>(self):
        title = self.title_entry.<span class="hljs-built_in">get</span>()
        content = self.content_text.<span class="hljs-built_in">get</span>(<span class="hljs-string">"1.0"</span>, tk.END).<span class="hljs-built_in">strip</span>()
        
        if title and content:
            conn = sqlite3.<span class="hljs-built_in">connect</span>(<span class="hljs-string">'notes.db'</span>)
            cursor = conn.<span class="hljs-built_in">cursor</span>()
            cursor.<span class="hljs-built_in">execute</span>(<span class="hljs-string">'INSERT INTO notes (title, content) VALUES (?, ?)'</span>, (title, content))
            conn.<span class="hljs-built_in">commit</span>()
            conn.<span class="hljs-built_in">close</span>()
            self.<span class="hljs-built_in">load_notes</span>()
            self.<span class="hljs-built_in">clear_entries</span>()
        else:
            messagebox.<span class="hljs-built_in">showwarning</span>(<span class="hljs-string">"Input Error"</span>, <span class="hljs-string">"Both title and content are required!"</span>)
    
    def <span class="hljs-built_in">load_notes</span>(self):
        self.notes_listbox.<span class="hljs-built_in">delete</span>(<span class="hljs-number">0</span>, tk.END)
        
        conn = sqlite3.<span class="hljs-built_in">connect</span>(<span class="hljs-string">'notes.db'</span>)
        cursor = conn.<span class="hljs-built_in">cursor</span>()
        cursor.<span class="hljs-built_in">execute</span>(<span class="hljs-string">'SELECT id, title FROM notes'</span>)
        notes = cursor.<span class="hljs-built_in">fetchall</span>()
        conn.<span class="hljs-built_in">close</span>()
        
        for note in notes:
            self.notes_listbox.<span class="hljs-built_in">insert</span>(tk.END, (note[<span class="hljs-number">0</span>], note[<span class="hljs-number">1</span>]))
    
    def <span class="hljs-built_in">show_note</span>(self, event):
        selection = self.notes_listbox.<span class="hljs-built_in">curselection</span>()
        if selection:
            note_id = self.notes_listbox.<span class="hljs-built_in">get</span>(selection[<span class="hljs-number">0</span>])[<span class="hljs-number">0</span>]
            
            conn = sqlite3.<span class="hljs-built_in">connect</span>(<span class="hljs-string">'notes.db'</span>)
            cursor = conn.<span class="hljs-built_in">cursor</span>()
            cursor.<span class="hljs-built_in">execute</span>(<span class="hljs-string">'SELECT title, content FROM notes WHERE id = ?'</span>, (note_id,))
            note = cursor.<span class="hljs-built_in">fetchone</span>()
            conn.<span class="hljs-built_in">close</span>()
            
            self.title_entry.<span class="hljs-built_in">delete</span>(<span class="hljs-number">0</span>, tk.END)
            self.title_entry.<span class="hljs-built_in">insert</span>(<span class="hljs-number">0</span>, note[<span class="hljs-number">0</span>])
            self.content_text.<span class="hljs-built_in">delete</span>(<span class="hljs-string">"1.0"</span>, tk.END)
            self.content_text.<span class="hljs-built_in">insert</span>(tk.END, note[<span class="hljs-number">1</span>])
    
    def <span class="hljs-built_in">delete_note</span>(self):
        selection = self.notes_listbox.<span class="hljs-built_in">curselection</span>()
        if selection:
            note_id = self.notes_listbox.<span class="hljs-built_in">get</span>(selection[<span class="hljs-number">0</span>])[<span class="hljs-number">0</span>]
            
            conn = sqlite3.<span class="hljs-built_in">connect</span>(<span class="hljs-string">'notes.db'</span>)
            cursor = conn.<span class="hljs-built_in">cursor</span>()
            cursor.<span class="hljs-built_in">execute</span>(<span class="hljs-string">'DELETE FROM notes WHERE id = ?'</span>, (note_id,))
            conn.<span class="hljs-built_in">commit</span>()
            conn.<span class="hljs-built_in">close</span>()
            
            self.<span class="hljs-built_in">load_notes</span>()
            self.<span class="hljs-built_in">clear_entries</span>()
    
    def <span class="hljs-built_in">clear_entries</span>(self):
        self.title_entry.<span class="hljs-built_in">delete</span>(<span class="hljs-number">0</span>, tk.END)
        self.content_text.<span class="hljs-built_in">delete</span>(<span class="hljs-string">"1.0"</span>, tk.END)

if __name__ == <span class="hljs-string">"__main__"</span>:
    app = <span class="hljs-built_in">NotesApp</span>()
    app.<span class="hljs-built_in">mainloop</span>()
</code></pre>
<p><strong>Explanation:</strong></p>
<ol>
<li><strong>Database Initialization:</strong> Creates a SQLite database (<code>notes.db</code>) and a table (<code>notes</code>) for storing notes.</li>
<li><strong>GUI Application:</strong> Uses <code>tkinter</code> to create a basic GUI. It includes:
<ul>
<li><code>Title</code> entry field.</li>
<li><code>Content</code> text area.</li>
<li><code>Save Note</code> button to save a note.</li>
<li>A listbox to display the list of saved notes.</li>
<li><code>Delete Note</code> button to remove a selected note.</li>
</ul>
</li>
</ol>
<p>When you run the code, you will see a window where you can type the title and content of your note, save it, view it in the list, and delete it if necessary.</p>
<p>This should give you a basic idea of how to create a custom notes application using Python. You can extend this by adding features like searching, editing notes, etc.</p>