This comes up in questions and conversation all the time and frankly there isn’t much point asking your French neighbour, in fact mentioning snakes may not be a good idea unless you want to run the risk of being treated to a diatribe on how they should all be killed, (which is of course illegal).
There is a lot of confusion caused for English language speakers
due to the translations of French to English for snake “prefixes” resulting from historical errors that have never been corrected since the first dictionaries which have been copied ever since. These are the common or vernacular names in normal usage although again it’s unlikely that most French people know them.
Couleuvre is often thought to mean “Grass snake”
and the French in the vernacular tend to use this word for any snake that isn't a viper. However this isn't correct as it is used as the prefix for a group of snakes in France, 6 in all, that reproduce by laying eggs, and are all but one harmless and non venomous.
The snakes are.
Couleuvre à collier – Grass snake Couleuvre à échelons – Ladder snake Couleuvre d'Esculape – Aesculapian snake Couleuvre verte et jaune – Western whip snake Couleuvre vipérine – Viperine snake Couleuvre de Montpellier – Montpeiier snake, (venomous but with non retractable rear facing fangs at the rear of its mouth - generally harmless).
Coronelle, which won’t often be heard,
is used for 2 species of snake in France that are harmless and non venomous, one, the Southern Smooth snake lays eggs and the other, the Smooth snake is “live bearing”, that is to say although not giving live birth in the standard sense it is when the babies hatch inside the mother and are then released to the outside world and can immediately fend for themselves.