Being assertive with smokers in your life means knowing what you want from them (usually to not smoke in front of you) and to tell them about it. But here's the rub. Many people, who fear they won't be heard or listened to, or who aren't used to speaking up, don't just come out with what they want directly. Instead, they may take the passive road and want someone to read their mind, which is not likely to succeed. Or they may try to control, coerce, attack or (that dreaded word) "manipulate" the other person to get their way. In this case, they are being dominant and aggressive, not genuinely self-assertive, and these tactics have a way of getting other people's backs up.
We recommend just coming out with what you want as directly and honestly as you can. Most people sincerely want to help others struggling to get free of smoking addiction (even when they smoke themselves!). So go ahead: Ask them if they can smoke outside so you won't be tempted. Ask them if you can get together at a smoke-free place so you won't be tempted. At least this way they will know what you want and have a chance to respond positively to you. They won't be failing a test they don't even know they are taking. Take a risk, be assertive. It's for a good cause...you.