def sign_in(self, email=None, password=None):
"""Signs in a user, either with the specified email address and password, or a returning user."""
from pages.sign_in import SignIn
sign_in = SignIn(self.selenium, timeout=self.timeout)
if sign_in.is_initial_sign_in:
sign_in.sign_in(email, password)
else:
sign_in.sign_in_returning_user()
if len(self.selenium.window_handles) == 2:
sign_in.click_this_is_not_my_computer()
sign_in.switch_to_main_window()
