How can I save all cookies in Python’s Selenium WebDriver to a txt-file, then load them later? The documentation doesn’t say much of anything about the getCookies function.

You can save the current cookies as a python object using pickle. For example:

import pickle
import selenium.webdriver 

driver = selenium.webdriver.Firefox()
driver.get("http://www.google.com")
pickle.dump( driver.get_cookies() , open("cookies.pkl","wb"))

and later to add them back:

import pickle
import selenium.webdriver 

driver = selenium.webdriver.Firefox()
driver.get("http://www.google.com")
cookies = pickle.load(open("cookies.pkl", "rb"))
for cookie in cookies:
    driver.add_cookie(cookie)

When you need cookies from session to session there is another way to do it, use the Chrome options user-data-dir in order to use folders as profiles, I run:

#you need to: from selenium.webdriver.chrome.options import Options
chrome_options = Options()
chrome_options.add_argument("user-data-dir=selenium") 
driver = webdriver.Chrome(chrome_options=chrome_options)
driver.get("www.google.com")

You can do here the logins that check for human interaction, I do this and then the cookies I need now every-time I start the Webdriver with that folder everything is in there. You can also manually install the Extensions and have them in every session. Secon time I run, all the cookies are there:

#you need to: from selenium.webdriver.chrome.options import Options    
chrome_options = Options()
chrome_options.add_argument("user-data-dir=selenium") 
driver = webdriver.Chrome(chrome_options=chrome_options)
driver.get("www.google.com") #Now you can see  the cookies, the settings, extensions, etc, and the logins done in the previous session are present here. 

The advantage is you can use multiple folders with different settings and cookies, Extensions without the need to load, unload cookies, install and uninstall Extensions, change settings, change logins via code, and thus no way to have the logic of the program break, etc Also this is faster than havin to do it all by code.

Remember, you can only a add cookie for the current domain. If you wanna add a cookie for your Google account, do

browser.get('http://google.com')
for cookie in cookies:
    browser.add_cookie(cookie)

Based on answer by @Eduard Florinescu but with newer code and missing import added:

$ cat work-auth.py 
#!/usr/bin/python3

# Setup:
# sudo apt-get install chromium-chromedriver
# sudo -H python3 -m pip install selenium

import time
from selenium import webdriver
from selenium.webdriver.chrome.options import Options

chrome_options = Options()
chrome_options.add_argument("--user-data-dir=chrome-data")
driver = webdriver.Chrome('/usr/bin/chromedriver',options=chrome_options)
chrome_options.add_argument("user-data-dir=chrome-data") 
driver.get('https://www.somedomainthatrequireslogin.com')
time.sleep(30)  # Time to enter credentials
driver.quit()

$ cat work.py 
#!/usr/bin/python3

import time
from selenium import webdriver
from selenium.webdriver.chrome.options import Options

chrome_options = Options()
chrome_options.add_argument("--user-data-dir=chrome-data")
driver = webdriver.Chrome('/usr/bin/chromedriver',options=chrome_options)
driver.get('https://www.somedomainthatrequireslogin.com')  # Already authenticated
time.sleep(10)
driver.quit()

Just a slight modification for the code written by @Roel Van de Paar, as all credit goes to him. I am using this in Windows and it is working perfectly, both for setting and adding cookies:

from selenium import webdriver
from selenium.webdriver.chrome.options import Options

chrome_options = Options()
chrome_options.add_argument("--user-data-dir=chrome-data")
driver = webdriver.Chrome('chromedriver.exe',options=chrome_options)
driver.get('https://web.whatsapp.com')  # Already authenticated
time.sleep(30)

this is code I used in windows, It works.

 for item in COOKIES.split(';'):
            name,value = item.split('=',1)
            name=name.replace(' ','').replace('\r','').replace('\n','')
            value = value.replace(' ','').replace('\r','').replace('\n','')
            cookie_dict={  
                    'name':name,
                    'value':value,
                    "domain": "",  # google chrome
                    "expires": "",
                    'path': '/',
                    'httpOnly': False,
                    'HostOnly': False,
                    'Secure': False
                    }
            self.driver_.add_cookie(cookie_dict)

my os is Windows 10, and the chrome version is 75.0.3770.100. I have tried the ‘user-data-dir’ solution, didn’t work. try the solution of @ Eric Klien fails too. finally, I make the chrome setting like the picture, it works!but it didn’t work on windows server 2012.

setting