Добавление столбцов в Dataframe

Question

поведение в DF

Я вижу необъяснимое при добавлении нового столбца в DF

import pandas as pd

d = {'one' : pd.Series([1, 2, 3], index=['a', 'b', 'c']),
      'two' : pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd'])}

df = pd.DataFrame(d)
print(df)
# Adding a new column to an existing DataFrame object with column label by passing new series

print ("Adding a new column by passing as Series:")
df['three']=pd.Series([10,20,30],index=['a','b','c'])
print(df)

print ("Adding a new column using the existing columns in DataFrame:")
print("print df['one']")
print(df['one'])
print("#print(df['two']")
print(df['two'])
print("#print df['three']")
df['three']
print("df['four']=df['one']+df['three']")
df['four']=df['one']+df['three']
#print(df)
print(df)

Фактический результат:

one  two
a  1.0    1
b  2.0    2
c  3.0    3
d  NaN    4

Добавление нового столбцапередавая в виде Series:

   one  two  three
a  1.0    1   10.0
b  2.0    2   20.0
c  3.0    3   30.0
d  NaN    4    NaN

Добавление нового столбца с использованием существующих столбцов в DataFrame:

print (df ['one']):

a    1.0
b    2.0
c    3.0
d    NaN
Name: one, dtype: float64

print (df ['two']):

a    1
b    2
c    3
d    4
Name: two, dtype: int64

print (df ['three']):

nothing

df ['four'] = df ['one'] + df ['three']

  one  two  three  four
a  1.0    1   10.0  11.0
b  2.0    2   20.0  22.0
c  3.0    3   30.0  33.0
d  NaN    4    NaN   NaN

Вопрос :

Почему я не получаю данные при печати df ["three"]?