Webpage contained within one dynamic page, unable to use driver interactions with Selenium (Python)
As suggested by saurabh , use 
1 self.wait.until(EC.visibility_of_element_located((By.CSS_SELECTOR, OR.Sub_categories)))
Else put a sleep and see however it is not advisable to use that, may be the xpath you have changes at the time of page load
The homepage for the web application I'm testing has a loading screen when you first load it, then a username/password box appears.  It is a dynamically generated UI element and the cursor defaults to being inside the username field.
I looked around and someone suggested using action chains.  When I use action chains, I can immediately input text into the username and password fields and then press enter and the next page loads fine.  Unfortunately, action chains are not a viable long-term answer for me due to my particular setup.
When I use the webdriver's find_element_by_id I am able to locate it and I am not able to send_keys to the element though because it is somehow not visible.  I receive 

selenium.common.exceptions.ElementNotVisibleException: Message: element not visible.  

I'm also not able to click the field or otherwise interact with it without getting this error.
I have also tried identifying and interacting with the elements via other means, such as "xpaths" and css, to no avail.  They are always not visible.
Strangely, it works with dynamic page titles.  When the page first loads it is Loading... and when finished it is Login.  The driver will return the current title when driver.title is called.
Does anyone have a suggestion?