Based on my observations that I posted about here The pictures and analysis and here RE: What is this? Opinions please, I would venture that yours is most likely an Anaconda, but without seeing the dorsal pattern/coloration, it's hard to make any hard determination when only seeing half of the animal, low-expression or not. If you have some dorsal pictures to share, that might be helpful as some guessed that the female I shared in the above posts was a low-expression Anaconda before even seeing the belly.
I love seeing the belly button in the first pic. 
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Troy Rexroth
Rextiles
