Initialize parameter of method with default value

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The common idiom here is to set the default to some sentinel value (None is typical, although some have suggested Ellipsis for this purpose) which you can then check.

class Example(object): #inherit from object.  It's just a good idea.
   def __init__(self, data = None):
      self.data = self.default_data() if data is None else data

   def default_data(self):  #probably need `self` here, unless this is a @staticmethod ...
      # ....
      return something

You might also see an instance of object() used for the sentinel.

SENTINEL = object()
class Example(object):
   def __init__(self, data = SENTINEL):
      self.data = self.default_data() if data is SENTINEL else data

This latter version has the benefit that you can pass None to your function but has a few downsides (see comments by @larsmans below). If you don’t forsee the need to pass None as a meaningful argument to your methods, I would advocate using that.

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