__init__() missing 1 required positional argument

Your constructor is expecting one parameter (data). You're not passing it in the call. I guess you wanted to initialise a field in the object. That would look like this:

class DHT:
    def __init__(self):
        self.data = {}
        self.data['one'] = '1'
        self.data['two'] = '2'
        self.data['three'] = '3'
    def showData(self):
        print(self.data)

if __name__ == '__main__':
    DHT().showData()

Or even just:

class DHT:
    def __init__(self):
        self.data = {'one': '1', 'two': '2', 'three': '3'}
    def showData(self):
        print(self.data)

You're receiving this error because you did not pass a data variable to the DHT constructor.

aIKid and Alexander's answers are nice but it wont work because you still have to initialize self.data in the class constructor like this:

class DHT:
   def __init__(self, data=None):
      if data is None:
         data = {}
      else:
         self.data = data
      self.data['one'] = '1'
      self.data['two'] = '2'
      self.data['three'] = '3'
   def showData(self):
      print(self.data)

And then calling the method showData like this:

DHT().showData()

Or like this:

DHT({'six':6,'seven':'7'}).showData()

or like this:

# Build the class first
dht = DHT({'six':6,'seven':'7'})
# The call whatever method you want (In our case only 1 method available)
dht.showData()