Boolean Series key will be reindexed to match DataFrame index

Question

Here is how I encountered the error:

df.loc[a_list][df.a_col.isnull()]

The type of a_list is Int64Index， it contains a list of row indexes. All of these row indexes belong to df.

The df.a_col.isnull() part is a condition I need for filtering.

If I execute the following commands individually, I do not get any warnings:

df.loc[a_list]

df[df.a_col.isnull()]

But if I put them together df.loc[a_list][df.a_col.isnull()], I get the warning message (but I can see the result):

Boolean Series key will be reindexed to match DataFrame index

What is the meaning of this error message? Does it affect the result that is returned?

Answer 1

Your approach will work but notwithstanding the warning, but it's best not to rely on implicit, unclear action.

Solution for this is to make the selection of indices in a_list a boolean mask:

df[df.index.isin(a_list) & df.a_col.isnull()]

Answer 2

First of all lets Understand the Warning 
Boolean Indexing: When you apply any condition to a DataFrame it returns a Boolean Series For example: 
df = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6] #code to create a dataframe 
condition = df['A'] > 1 # This will create a boolean series 
Index Mismatch: If the Boolean Series is generated from a DataFrame or Series with an index other than the one you are filtering, pandas will issue a warning and reindex the Boolean Series by index to the DataFrame.