The documentation
gives a very clear example of the proxies usage
import requests
proxies = {
  'http': 'http://10.10.1.10:3128',
  'https': 'http://10.10.1.10:1080',
}
requests.get('http://example.org', proxies=proxies)
What isn't documented, however, is the fact that you can even configure proxies for individual urls even if the schema is the same!
This comes in handy when you want to use different proxies for different websites you wish to scrape.
proxies = {
  'http://example.org': 'http://10.10.1.10:3128',
  'http://something.test': 'http://10.10.1.10:1080',
}
requests.get('http://something.test/some/url', proxies=proxies)
Additionally, requests.get essentially uses the requests.Session under the hood, so if you need more control, use it directly
import requests
proxies = {
  'http': 'http://10.10.1.10:3128',
  'https': 'http://10.10.1.10:1080',
}
session = requests.Session()
session.proxies.update(proxies)
session.get('http://example.org')
I use it to set a fallback (a default proxy) that handles all traffic that doesn't match the schemas/urls specified in the dictionary
import requests
proxies = {
  'http': 'http://10.10.1.10:3128',
  'https': 'http://10.10.1.10:1080',
}
session = requests.Session()
session.proxies.setdefault('http', 'http://127.0.0.1:9009')
session.proxies.update(proxies)
session.get('http://example.org')