我决定开始使用熊猫，所以现在我正在处理MultiIndexed DataFrames。在处理索引和切片时，我注意到以下内容，我无法解释。

import pandas as pd
import numpy as np

# generate a MultiIndexed DataFrame with more or less random data
index = pd.MultiIndex.from_product([[2013,2014],[1,2]],names=['year','visit'])
columns = pd.MultiIndex.from_product([['Bob','Guido','Sue'],['HR','Temp']],names=['subject','type'])

data = np.round(np.random.randn(4,6),1)
data[:,::2] *= 10
data += 37

health_data = pd.DataFrame(data,index=index,columns=columns)
health_data
>>> 
    subject     Bob     Guido   Sue
    type    HR  Temp    HR  Temp    HR  Temp
year    visit                       
2013    1   42.0    37.4    34.0    37.2    41.0    36.2
        2   51.0    36.5    34.0    35.7    38.0    37.2
2014    1   52.0    37.2    32.0    37.9    28.0    39.3
        2   40.0    36.3    68.0    36.3    55.0    38.3

# Now why is this even working?
>>> health_data.loc[[2014,100]]
    subject     Bob     Guido   Sue
    type    HR  Temp    HR  Temp    HR  Temp
year    visit                       
2014    1   52.0    37.2    32.0    37.9    28.0    39.3
        2   40.0    36.3    68.0    36.3    55.0    38.3

我从health_data.loc[[2014]]开始，它给出了相同的结果，并且反复试验得出了奇怪的结果。那条线到底是什么？为什么起作用？