Both positivity and optimism are about "looking on the bright side” of the things.
Optimism is an outward view to see the bright side of the things; while positive is an internal mental attitude to achieve a better result. Being optimistic is the view look for the best, come with “never give up" attitude. Your behavior, the way you showcase your attitude and mood others to put on their thinking cap. Being positive is within yourself, reflects your personality, thought process put into action. They both can be defined as "looking on the bright side." Optimism is looking on the bright side of a situation (external), whereas, being positive is looking on the bright side of all (internal) and reflecting that positivity outwards for others to benefit from and add to.
A good positive chain of thoughts followed by positive actions brought good results. Incidentally, a single unexpected success suddenly triggers a chain of positive thought process. A person comes inbuilt with all the attributes, attitudes in his/her personal package. What matters is his/her ability to harness and leverage them and gain from it for the greater good. Ultimately, it depends on the person who faces a situation, how he/she understands it, how he/she understands him/herself, harnesses/leverages the right attributes within him/herself and moves forward. Also, he/she recognizes that failure, pessimism, negative thoughts are also very valuable, as long as he/she deals, learns and improves in the process by it.
Optimism is good but not at extreme levels. Everything should be at the right dose. Optimism and pessimism are outlooks or states of mind/heart and they may affect the approach one takes to accomplish a task, but these are not exclusive. A practical approach can be taken independent of a person's positivity or negativity towards the situation or life, in general, likewise, a purely impractical approach can be taken, just the same. Additionally, practical or impractical is a viewpoint that is retained by the individual and while there may be things that are thought of as 'generally practical,' such as taking the shortest distance to get somewhere when you are in a hurry, practicality may be a matter of preference.