pandas Gotchas of pandas Automatic Data Alignment (index-awared behaviour)
Example
If you want to append a series of values [1,2] to the column of dataframe df, you will get NaNs:
import pandas as pd
series=pd.Series([1,2])
df=pd.DataFrame(index=[3,4])
df['col']=series
df
col
3 NaN
4 NaN
because setting a new column automatically aligns the data by the indexe, and your values 1 and 2 would get the indexes 0 and 1, and not 3 and 4 as in your data frame:
df=pd.DataFrame(index=[1,2])
df['col']=series
df
col
1 2.0
2 NaN
If you want to ignore index, you should set the .values at the end:
df['col']=series.values
col
3 1
4 2
