Installation:
go get -u google.golang.org/api/gmail/v1
go get -u golang.org/x/oauth2/google

Installation via Gradle
 repositories {
       mavenCentral()
   }
   dependencies {
       compile 'com.google.api-client:google-api-client:1.30.2' 
   }

Installation:
gem install google-api-client

Installation via NuGet Package Manager Console:
Install-Package Google.Apis.Gmail.v1

Installation via npm:
npm install googleapis@39 --save

Installation via Composer:
composer require google/apiclient:"^2.0"

Installation:
pip install --upgrade google-api-python-client google-auth-httplib2 google-auth-oauthlib
or
easy_install --upgrade google-api-python-client google-auth-httplib2 google-auth-oauthlib

def create_message(sender, to, subject, message_text):
  message = MIMEText(message_text)
  message['to'] = to
  message['from'] = sender
  message['subject'] = subject
  raw_message = base64.urlsafe_b64encode(message.as_string().encode("utf-8"))
  return {
    'raw': raw_message.decode("utf-8")
  }
def create_draft(service, user_id, message_body):
  try:
    message = {'message': message_body}
    draft = service.users().drafts().create(userId=user_id, body=message).execute()
    print("Draft id: %s\nDraft message: %s" % (draft['id'], draft['message']))
    return draft
  except Exception as e:
    print('An error occurred: %s' % e)
    return None  

/**
* @param $sender string sender email address
* @param $to string recipient email address
* @param $subject string email subject
* @param $messageText string email text
* @return Google_Service_Gmail_Message
*/
function createMessage($sender, $to, $subject, $messageText) {
 $message = new Google_Service_Gmail_Message();
 $rawMessageString = "From: <{$sender}>\r\n";
 $rawMessageString .= "To: <{$to}>\r\n";
 $rawMessageString .= 'Subject: =?utf-8?B?' . base64_encode($subject) . "?=\r\n";
 $rawMessageString .= "MIME-Version: 1.0\r\n";
 $rawMessageString .= "Content-Type: text/html; charset=utf-8\r\n";
 $rawMessageString .= 'Content-Transfer-Encoding: quoted-printable' . "\r\n\r\n";
 $rawMessageString .= "{$messageText}\r\n";
 $rawMessage = strtr(base64_encode($rawMessageString), array('+' => '-', '/' => '_'));
 $message->setRaw($rawMessage);
 return $message;
}
/**
* @param $service Google_Service_Gmail an authorized Gmail API service instance.
* @param $user string User's email address or "me"
* @param $message Google_Service_Gmail_Message
* @return Google_Service_Gmail_Draft
*/
function createDraft($service, $user, $message) {
 $draft = new Google_Service_Gmail_Draft();
 $draft->setMessage($message);
 try {
   $draft = $service->users_drafts->create($user, $draft);
   print 'Draft ID: ' . $draft->getId();
 } catch (Exception $e) {
   print 'An error occurred: ' . $e->getMessage();
 }
 return $draft;
}

def send_message(service, user_id, message):
  try:
    message = service.users().messages().send(userId=user_id, body=message).execute()
    print('Message Id: %s' % message['id'])
    return message
  except Exception as e:
    print('An error occurred: %s' % e)
    return None

/**
* @param $service Google_Service_Gmail an authorized Gmail API service instance.
* @param $userId string User's email address or "me"
* @param $message Google_Service_Gmail_Message
* @return null|Google_Service_Gmail_Message
*/
function sendMessage($service, $userId, $message) {
 try {
   $message = $service->users_messages->send($userId, $message);
   print 'Message with ID: ' . $message->getId() . ' sent.';
   return $message;
 } catch (Exception $e) {
   print 'An error occurred: ' . $e->getMessage();
 }
 return null;
}

def send_message(service, user_id, message):
  try:
    message = service.users().messages().send(userId=user_id, body=message).execute()
    print('Message Id: %s' % message['id'])
    return message
  except Exception as e:
    print('An error occurred: %s' % e)
    return None
def create_message_with_attachment(sender, to, subject, message_text, file):
  message = MIMEMultipart()
  message['to'] = to
  message['from'] = sender
  message['subject'] = subject
  msg = MIMEText(message_text)
  message.attach(msg)
  content_type, encoding = mimetypes.guess_type(file)
  if content_type is None or encoding is not None:
    content_type = 'application/octet-stream'
  main_type, sub_type = content_type.split('/', 1)
  if main_type == 'text':
    fp = open(file, 'rb')
    msg = MIMEText(fp.read().decode("utf-8"), _subtype=sub_type)
    fp.close()
  elif main_type == 'image':
    fp = open(file, 'rb')
    msg = MIMEImage(fp.read(), _subtype=sub_type)
    fp.close()
  elif main_type == 'audio':
    fp = open(file, 'rb')
    msg = MIMEAudio(fp.read(), _subtype=sub_type)
    fp.close()
  else:
    fp = open(file, 'rb')
    msg = MIMEBase(main_type, sub_type)
    msg.set_payload(fp.read())
    fp.close()
  filename = os.path.basename(file)
  msg.add_header('Content-Disposition', 'attachment', filename=filename)
  message.attach(msg)
  raw_message = base64.urlsafe_b64encode(message.as_string().encode("utf-8"))
  return {'raw': raw_message.decode("utf-8")}

import base64
import email
def get_messages(service, user_id):
  try:
    return service.users().messages().list(userId=user_id).execute()
  except Exception as error:
    print('An error occurred: %s' % error)
def get_message(service, user_id, msg_id):
  try:
    return service.users().messages().get(userId=user_id, id=msg_id, format='metadata').execute()
  except Exception as error:
    print('An error occurred: %s' % error)
def get_mime_message(service, user_id, msg_id):
  try:
    message = service.users().messages().get(userId=user_id, id=msg_id,
                                             format='raw').execute()
    print('Message snippet: %s' % message['snippet'])
    msg_str = base64.urlsafe_b64decode(message['raw'].encode("utf-8")).decode("utf-8")
    mime_msg = email.message_from_string(msg_str)
    return mime_msg
  except Exception as error:
    print('An error occurred: %s' % error)

def get_attachments(service, user_id, msg_id, store_dir):
  try:
    message = service.users().messages().get(userId=user_id, id=msg_id).execute()
    for part in message['payload']['parts']:
      if(part['filename'] and part['body'] and part['body']['attachmentId']):
        attachment = service.users().messages().attachments().get(id=part['body']['attachmentId'], userId=user_id, messageId=msg_id).execute()
        file_data = base64.urlsafe_b64decode(attachment['data'].encode('utf-8'))
        path = ''.join([store_dir, part['filename']])
        f = open(path, 'wb')
        f.write(file_data)
        f.close()
  except Exception as error:
    print('An error occurred: %s' % error)