According to Wikipedia, a yogi is someone who lives by voluntary ethical precepts called Yamas and Niyamas. These include: non-violence, truthfulness, non-stealing, kindness, compassion, non-hypocrisy, forgiveness, fortitude, moderation, purity, cleanliness, austerity, persistence and perseverance in one's purpose, contentment, acceptance of others and of one's circumstances as they are, optimism for self, generosity, charity, sharing with others.  
Wow. This is quite a list, one that is challenging to consistently practice and embrace…. especially with what’s going on in today’s world.  
It may come as a surprise to hear these precepts being discussed, debated and ultimately embraced by students participating in Transformation Yoga Project's yoga teacher training program at the maximum security prison SCI Graterford. Understanding these concepts is one thing. Putting them into practical action within an environment lacking human compassion is quite another. 
How does one practice non-violence in a violent environment? How does one forgive others when you yourself are not forgiven? Deep stuff. Yet this is all part of doing the inward work to peel away the layers of hurt, trauma, and self-judgement preventing us from returning to our true selves. These 11 men are working hard to directly face and release their demons.
As part of this healing process, the men wrote a poem about being a yogi and becoming warriors for peace. There is an inspiring message for everyone in the deeply personal work they are doing. In my opinion, they are role models for doing the hard work to live as true yogis. 
A Yogi is a Warrior because
He faces fear and uncertainty directly
With compassion and without self judgement
He lives in union with mind and body directed by his soul.
A Yogi is a Warrior because
His focus is sharp and true
You breathe into the present moment
Rising up Rooted and Balanced.
A Yogi is a Warrior because
Surrender is not an option
Only surrender to his
True Self is worthy of consideration.
A Yogi is a Warrior because
He doesn't know fear
You do not fear because you do right
Do Right and Fear No One!
A Yogi is a Lover because
He opens to the Real
You let go of illusion
And Feel it,  Really Feel!
A Yogi is a Warrior because
He shows compassion
You show compassion because
Your heart is open.
A Yogi is a Warrior because
He protects the weak
You protect the weak because
You have great inner strength.
A Yogi is a Magician because
He sees the Trickster in us all
You work it – on and off the mat
Standing firm and walking tall
A Yogi is a King because
He integrates it all
You sink deep into the mat
Rising up and serving all.