source：
https://stackoverflow.com/questions/17950374/converting-a-column-within-pandas-dataframe-from-int-to-string
In [16]: df = DataFrame(np.arange(10).reshape(5,2),columns=list('AB'))
In [17]: df
Out[17]:
A B
0 0 1
1 2 3
2 4 5
3 6 7
4 8 9
In [18]: df.dtypes
Out[18]:
A int64
B int64
dtype: object
Convert a series
In [19]: df['A'].apply(str)
Out[19]:
0 0
1 2
2 4
3 6
4 8
Name: A, dtype: object
In [20]: df['A'].apply(str)[0]
Out[20]: '0'
Convert the whole frame
In [21]: df.applymap(str)
Out[21]:
A B
0 0 1
1 2 3
2 4 5
3 6 7
4 8 9
In [22]: df.applymap(str).iloc[0,0]
Out[22]: '0'
