In the french judicial system there is no difference between classes of drugs ie. A, B ,C.... Allthough a judge will hand out a lower sentance for cannabis dealing than cocaine, there technically is no difference between hard and soft drugs.
France is the biggest consumer per person of weed/hash in western europe yet has the most harsh repressive laws.
Cops here can stop you for no reason and search your person, but once you into your twenties you stop getting harrassed unless you look like a hoodlum or are arab (cops are very racist here)
You might get away with a verbal warning for possession of weed or hash lets say under 10 grams, depending on where you are (in a nice or shitty neighbourhood) and depending on your cooperation, but you can also spend the night in jail for one spliff, theres no fixed rules there, anything other than weed or hash, and you'll go to jail for the day/night up to 48 hours, but a simple user is extremely unlikely to get sent to prison for coke, heroin etc.
France has needle exchanges, as well as free methadone/subutex centers, there are people with progressive HR views towards drugs, from doctors to judges, but most are harsher on drugs than other western european countries.
What you can and cant find:
Hash is everywhere, go to any "project" and they're selling,but don't blindly trust a rasta with your money coz he has dreads, and speaks a bit of english... it's cheap and potent esp. in THC (no CBD) Weed is more expensive, past a certain age, most people have friends who are connects. People smoke in the evenings by the river Seine, or in parks etc, just gotta be a bit smart and aware.
Cocaine is sold in the ghetto's but is 90% cut or entire rip off, but there are very high grade relatively cheap serious dealers, but you're likely to get mediocre stuff, unless you've been around for a while.
There are a couple open air drug markets for crack, in and around the capital, there is also a lot of rip-offs
If you like electronic music, dnb etc. and make friends you will have no problem getting good MDMA (xtc pills are shit though) those "sort of people" will likely have access to decent weed and LSD, maybe shrooms.
Subutex is everywwhere on the black marcket, and morphine (skenan) is relatively easy to find, allthough the government has clamped down recently on crooked doctors, and have secure systems to prevent doc shopping, and sellers if they feel a hint you're not french will try to rip you off, and definately double the price (i don't know if i'm allowed to PM you the average prices of opiates, and open air opiate drug markets...).
Oxycodone and hydromorphone and other opiate pharms exept tramadol are very hard to come by.
GHB is piss easy to get if you usse that thing between your ears.
At underground raves, as well as organised "technivals" you can get pretty much anything, the K comes from england , opium from italy, the prices are slightly bumped up, and french ravers especially like their LSD.
RC's like the rest of the world, but usually banned in france a few months after the UK (hard to keep track of what is being made illegal and when)
Heroin is rather low grade apparantly, but all over the north of the city, and northern subburbs (not my thing though)... like with everything, it's best to get introduced, if you don't sound french, or seem unsure you WILL be ripped off.
Codeine and promethazine are otc
Benzos ar ridiculously easy to get Rx'ed by any doctor.
Never seen PCP.
Methamphetamine is still very rare, can be found at raves.
Booze is cheap in the shops, but very expensive in bars and clubs.
The best way is to make friends and connects through electronic music nights, and there are lots of international people in Paris nightclubs and you can get most things, raves even more so...., but like most places DON'T give your money up front to a stranger who will "do the run for you"....
On the other hand if you're only looking for opiates, they are mostly frowned upon by the clubbing, and to a certain extent raving community....
Hope this helps a bit, feel free to PM