Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
Comedy often uses lies and misinformation in order to get a laugh. In this way it has spread racism, sexism, ageism, homophobia, and other falsehoods that have become accepted truths, including information about snakes. Since comedies will do anything to get laughs, I can't complain that the snake scene in this movie is unnecessary to the plot, but I will complain that it re-inforces the audience's ignorance of snakes and their natural behavior so it can create fear by demonizing a snake as a vicious killer and use that for comedy. Truth never matters as long as you get a laugh. Unfortunately, this scene was like most of the movie - not funny.
Melissa McCarthy is an identity thief who steals Justin Bateman's identity. His only chance at saving his name and convincing the police that he is not a drug dealer, is to find her in Florida and bring her back to Denver. They both crash their cars so they steal the van of a skip tracer who is after McCarthy, but the van overheats 20 miles from the nearest town so they get out and walk. Half way there they take a short cut through the woods where they have to stop to spend the night. As they lay down next to a campfire to go to sleep, a snake crawls up Bateman's pant leg. He freaks out and jumps up and takes off his pants and throws the pants with the snake up into a tree. As he stands under the tree, the snake drops onto him, wrapping itself around his neck. McCarthy swats at it with a burning branch from the fire. The snake opens its mouth to show its fangs and then bites Bateman on the neck. But in the end it's all a bunch of panic for nothing as McCarthy just drops the fire branch, grabs the snake, and throws it away. Later we see two bite marks on Bateman's neck, and we learn that she carried him unconscious to a bus stop, but there is no other mention of snakebite symptoms or treatment.
The first snake we see is a real one, some kind of eastern king snake, I think, but the snake that drops around his neck and is harrassed with fire and bites him on the neck is a computer-generated effect. The snake that McCarthy grabs and tosses away is a fake snake prop, with an entirely different pattern than either of the first two.