def keys(self):
    columns = set()
    for name in vars(self):
      if (not name) or name[0] == "_":
        continue
      columns.add(name)
    return columns
The left end point of 'self'.
dataset/ETHPy150Open google/transitfeed/transitfeed/gtfsobjectbase.py/GtfsObjectBase.keys--crossover with--dataset/ETHPy150Open sympy/sympy/sympy/sets/sets.py/Interval.start