pandas attribute error : no attribute 'Factor' found

报错：

Traceback (most recent call last):
File "rf.py", line 10, in <module>
    df['species'] = pd.Factor(iris.target, iris.target_names)
AttributeError: module 'pandas' has no attribute 'Factor'

代码：

import numpy as np
import pandas as pd
from sklearn.datasets import load_iris
from sklearn.ensemble import RandomForestClassifier

iris = load_iris()
df = pd.DataFrame(iris.data, columns=iris.feature_names)
df['is_train'] = np.random.uniform(0, 1, len(df)) <= .75
df['species'] = pd.Factor(iris.target, iris.target_names)

请问这个是什么问题？