There are about a dozen species of snake in France but only two (or in the south, three) species in France are really venomous - these are the Vipers or Adders. (To be absolutely accurate, one species of grass snake is venomous too, but because of the way its fangs are positioned, could not physically get enough poison into a person to do any lasting damage.) For comparison, there are 3 species of snake in the British Isles (one poisonous), and Australia has about 140 species of terrestrial snakes and 32 marine species. About 100 Australian species are venomous.
Vipers and Adders can be identified most reliably by their slit shaped pupils, but are also usually less than 50cm long, grey or reddish brown with a black zigzag pattern down the back. They are quite venomous, but almost never fatal. If bitten by any snake, the most important thing is to stay calm and get to a hospital. You will probably know if the bite is poisonous fairly quickly, as the area around the bite will hurt like hell and swell alarmingly.
If you or a companion are feeling particularly cool, calm and collected (as if!) photograph the snake, to help ID it at the hospital. I don't suggest trying to catch it or kill it - you risk being bitten again, and anyway, all snake species in France are protected.
Having said all that, snakes are not normally agressive, except in the breeding season (early spring).
Non-venomous snakes have round pupils. Any snake longer than 50cm is unlikely to be venomous and any snake longer than 1m is certainly not going to be a venomous species (in France, that is). Any snake that exudes a ghastly smelling substance is not venomous - it is a grass snake of some sort. (The term 'grass snake', used in the general sense, is the same as the French term 'couleuvre'. Grass Snake, in the specific sense, is the species Natrix natrix)
A Western Whip Snake Coluber viridiflavus (Couleuvre verte et jaune), taken in the Charente at our friend Bas's place. It had come to drink at a roadside ditch in a hamlet.
Many visitors to France will never see a snake, but if you do want to see them, water bodies such as ponds and ditches are a real draw, as are buildings infested with mice. Snakes like cool quiet dark places as much as sunny spots. That's why finding them in laundries is not uncommon - a room with a door to the outside, with a cool tiled floor, not in constant use, and with a lovely dark quiet retreat under the tub or washing machine.
My personal experience in France is that you would only ever meet a snake in a rural area. I've only seen a few, either on the road (usually road kill) or at ditches and puddles coming to drink. All of these have been grass snakes (couleuvre) of some sort, so not really venomous. You can pick up grass snakes and handle them quite safely generally so long as you are calm with them, but it is not considered good practice to do so unless you are studying them in a serious way, and anyway, you may not find the experience very pleasant. They exude a rather disgusting musk gunk which you will get all over your hands - the smell lasts for hours no matter how much you wash.
A Grass Snake Natrix natrix (Couleuvre à collier).(Note: This particular specimen is not French, but was caught in Shropshire when I was doing my Reptiles and Amphibians module at university. It was a rather cool day, and the snake realised quite quickly that being held in nice warmblooded hands was not all bad.)
There are several species of grass snake in France, and they tend to be long and thin. Many snakes will swim quite readily. Many grass snakes will take frogs (indeed frogs often form the major part of their diet), which is why you do quite often see them in the water. They will also slip into the water as a preferred escape route if you approach. Ironically, frogs will return the favour and take young grass snakes. (Some species of grass snake, inevitably, buck this trend, and avoid water and only eat small mammals.)
Snakes are extraordinarily cryptic (which is the techie term for well camouflaged). They choose their basking places with great care, and unless you are really tuned in or very lucky, you just won't see them. They will also just disappear under the leaf litter or a log in less than the blink of an eye if they hear anything approaching. They will hear you long before you see them. If you are surveying for reptiles you need to be already familiar with the site so you know where the likely basking spots are, and you need to check them out with binoculars from a fair distance if you are going to get any real idea of the snake population on a site.
There is now an updated post on Snakes in the Loire Valley, with more pictures and the benefit of 10 years of personal observations to inform it.
For details of our private guided tours of chateaux, wineries, markets and more please visit the Loire Valley Time Travel website. We would be delighted to design a tour for you.