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I can't pass in a variable my search for xpath with selenium. What is my mistake?

btn_login = "'.//a[contains(text(), 'Login to')]'"
btn = WebDriverWait(driver, 20).until(EC.presence_of_element_located((By.XPATH, f'{btn_login}')))​
btn.click()

like that it works, I don't know if it's possible to use an f string in this case.

btn = WebDriverWait(driver, 20).until(EC.presence_of_element_located((By.XPATH, .//a[contains(text(), 'Login to')]')))
btn.click()