Since 2006, Paris public toilets located on sidewalks throughout the city (numbering over 400 now) are available for FREE use. I've walked past these things for years, not really noticing them...until I needed one. Those who know me well know that I know where all the good (and not so good) bathrooms are located in New York City, and over the last two years, I've come to develop a decent list of bathrooms in Paris. Necessity is the mother of invention.
Besides the McDonald's and Starbucks options for bathrooms (which are often code-locked in order to remain reserved for actual customers), most cafés give you the evil eye if you walk in and ask for a bathroom. I do it anyway, or I just breeze in like I've come in from one of the outdoor tables and march right down the stairs (usually the bathrooms are downstairs). Again, necessity...
But I'd never visited the public bathrooms located on the sidewalks, nor had I seen anyone else enter or exit one. So I assumed this would be an unworthy option - probably with a homeless person inside, or filthy and smelly à la Amtrak or NJ Transit trains (yes, I've used those, too).
These toilets are environmentally sound, use rain water, have all the proper amenities and clean themselves before each use. They are large enough for wheelchairs and have bars for the disabled. The city of Paris has a list and an interactive map of them on their website; they are called "les sanisettes." There is a video that, with great fanfare (and background music to match), announces the city-wide installation of these toilets.
So, do not be afraid. Compared to the toilets for which you will pay (such as the train stations, the big department stores and some parks/gardens), or the cafés where you have to deal with the unhappy bartender not making a sale (oh, just buy a coffee at the bar - at worst you'll pay 1 or 2 euros for the privilege and a less intimidating experience), or the Starbucks option (where you'll have to wait for someone to come out first so you can beat the code), or the Sunday problem (where many shops and bars are closed), or worse (such as "squatters," which do not only exist in China), you frequent bathroom visitors can add this to the available options. Such a list can never be too long.