import pandas as pd
df = pd.DataFrame([
   ['green', 'M', 10.1, 'class1'], 
   ['red', 'L', 13.5, 'class2'], 
   ['blue', 'XL', 15.3, 'class1']])

df.columns = ['color', 'size', 'prize', 'class label']

size_mapping = {
   'XL': 3,
   'L': 2,
   'M': 1}
df['size'] = df['size'].map(size_mapping)

class_mapping = {label:idx for idx,label in enumerate(set(df['class label']))}
df['class label'] = df['class label'].map(class_mapping)

Using the get_dummies will create a new column for every unique string in a certain column:使用get_dummies进行one-hot 

pd.get_dummies(df)