>>> df = session.create_dataframe([1, -1, 1, -1, -1], schema=["a"])
>>> df.select(var_pop(col("a"))).collect()
[Row(VAR_POP("A")=Decimal('0.960000'))]
>>> df = session.create_dataframe([1, None, 2, 3, None, 5, 6], schema=["a"])
>>> df.select(var_pop(col("a"))).collect()
[Row(VAR_POP("A")=Decimal('3.440000'))]
>>> df = session.create_dataframe([None, None, None], schema=["a"])
>>> df.select(var_pop(col("a"))).collect()
[Row(VAR_POP("A")=None)]
>>> df = session.create_dataframe([42], schema=["a"])
>>> df.select(var_pop(col("a"))).collect()
[Row(VAR_POP("A")=Decimal('0.000000'))]