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This beautiful flower is definitely from a cactus, but not Hylocereus [in which dragon fruit belong to]. This spotting is that of Epiphyllum oxypetalum http://en.wikipedia.org/wiki/Epiphyllum_...
"Epiphyllum oxypetalum (Dutchman's Pipe, kardable, Night Queen or निशागंधी Nishagandhi or Gul-e-Bakawali) is a species of cactus and one of the most cultivated species in the genus. It is a variety of nightblooming Cereus and often confused with species of Selenicereus."
I think its a type of Nightblooming Cactus - Hylocereus
http://en.wikipedia.org/wiki/Hylocereus