error: self.name = "NPC{}".format(num_of_NPCs + 1)

[Exsul]

Code:

class NPC:

    num_of_NPCs = 0

    def __init__(self):
        self.name = "NPC{}".format(num_of_NPCs + 1)
        NPC.num_of_NPCs += 1

Error:

NameError: name 'num_of_NPCs' is not defined

I'm trying to name class instances "NPC" plus an increasing number (1, 2, 3) each time a new class instance is created. The above code gives me a name error. What am I doing wrong?