Overload Python 'in' to return non-bool

Question

I'm trying to overload the in operator for a class to return a non-bool object, but it seems to cast anyway. Here is my use case:

class Dataset(object):
  def __init__(self):
    self._filters = []

  def filter(self, f):
    self._filters.append(f)
    return self

class EqualFilter(object):
  def __init__(self, field, val):
    ...

class SubsetFilter(object):
  def __init__(self, field, vals):
    ...

class FilterBuilder(object):
  def __init__(self, field):
    self._field = field

  def __eq__(self, val):
    return EqualFilter(self._field, val)

  def __contains__(self, vals):
    return SubsetFilter(self._field, vals)


veggie = FilterBuilder('veggie')
fruit = FilterBuilder('fruit')
ds = Dataset().filter(veggie == 'carrot').filter(fruit in ['apple', 'orange'])

At the end of the code, ds contains an EqualFilter for veggie == 'carrot', and True for fruit in ['apple', 'orange']. Is there any way for ds to end up with a SubsetFilter?

Answer 1

There are two problems here. First, in always casts the result of __contains__ to a bool, so what you're looking for isn't possible. The second problem is that

fruit in ['apple', 'orange']

calls

['apple', 'orange'].__contains__(fruit)

There's no way for the left operand of in to override the operator, so that's also going to defeat what you're trying to do.