How to use 'expected conditions' to check for an element in python-selenium?

Seems you were almost there.

The Documentation clearly says the following:

class selenium.webdriver.support.expected_conditions.visibility_of_element_located(locator)

Which is defined as :

An expectation for checking that an element is present on the DOM of a page and visible. Visibility means that the element is not only displayed but also has a height and width that is greater than 0. locator - used to find the element returns the WebElement once it is located and visible

Hence, when you mention:

return EC.visibility_of_element_located((By.XPATH, '//*[@id="kernel_indicator_icon" and @title="Kernel Idle"]'))

The found out WebElement is being returned as follows :

<selenium.webdriver.support.expected_conditions.visibility_of_element_located object at 0x110321b90>

Even the Source Code says the same as :

try:
    return _element_if_visible(_find_element(driver, self.locator))

When the search is unsuccessfull :

except StaleElementReferenceException:
    return False

To summarize in a more organized manner:

• Expected Condition is a callable (could be a function or a class with __call__() magic method defined)

• Expected Condition is supposed to be used inside the until() method of a WebDriverWait() instance:

  wait = WebDriverWait(driver, 10)
  wait.until(<Expected_condition_here>)
  

• the result of an Expected Condition does not have to be just True/False. The result would be tested on truthiness by the WebDriverWait. Note: a WebElement instance is "truthy". Read more about truthiness in Python here

• it is quite convenient that when an Expected Condition returns a WebElement instance. It allows to have a link to an element right away without the need to find it again:

  button = wait.until(EC.element_to_be_clickable((By.ID, "my_id")))
  button.click()
  

• you can write your own custom Expected Conditions