Why are we using Square Brackets in python to access MIMEMultipart() components like ['From'], ['To'] for sending email using python?

In a python code, which is sending an email using "smtplib" and "MIMEMultipart" libraries,

I got a doubt on, why we are using "square brackets" for [‘From’], [‘To’] and [‘Subject’] when referring to "MIMEMultipart ()". Could any anyone explain on this part ?

Below is the code snipet, observe the commenting lines :

import smtplib
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.mime.base import MIMEBase
from email import encoders
mail_content = '''Hello,This is a test mail.'''
sender_address = '[email protected]'
sender_pass = 'wesqabfucxfqcg'
receiver_address = '[email protected]'

#Setup the MIME
message = MIMEMultipart()
message['From'] = sender_address #  ** why are we using square bracket [] here ?? **
message['To'] = receiver_address # ** why are we using square bracket [] here ?? **
message['Subject'] = 'A test mail sent by Python. It has an attachment.'
# **why are we using square bracket [] here ?? **

message.attach(MIMEText(mail_content, 'plain'))
attach_file_name = 'TP_python_prev.pdf'
attach_file = open(attach_file_name, 'rb') # Open the file as binary mode
payload = MIMEBase('application', 'octate-stream')
payload.set_payload((attach_file).read())
encoders.encode_base64(payload) #encode the attachment
payload.add_header('Content-Decomposition', 'attachment', filename=attach_file_name)
message.attach(payload)
session = smtplib.SMTP('smtp.gmail.com', 587) #use gmail with port
session.starttls() #enable security
session.login(sender_address, sender_pass) #login with mail_id and password
text = message.as_string()
session.sendmail(sender_address, receiver_address, text)
session.quit()
print('Mail Sent')