Snakes are ectotherms and normally take their body temperature from the environment. Brooding pythons are an exception. A female wraps her body around the clutch and, when the ambient temperature falls, begins rhythmic contractions of her trunk muscles, a shivering thermogenesis that can hold the eggs several degrees above the air around them.
The cost is substantial. Shivering raises the female's metabolic rate many times over resting level, and she does not feed through an incubation that lasts roughly two to three months. Females typically emerge from brooding in poor condition and may not breed again the following year. The rate of contraction is adjusted to the temperature deficit, so the effort scales with need.
This is not warm-bloodedness. The heat is generated only during brooding, only by reproductive females, and only when it is needed; outside that window the snake is as ectothermic as any other. The behaviour is known in several python species and has been studied most closely in Python molurus and the Burmese python.