pandas - Selection with .loc in python

Question

I saw this code in someone's iPython notebook, and I'm very confused as to how this code works. As far as I understood, pd.loc[] is used as a location based indexer where the format is:

df.loc[index,column_name]

However, in this case, the first index seems to be a series of boolean values. Could someone please explain to me how this selection works. I tried to read through the documentation but I couldn't figure out an explanation. Thanks!

iris_data.loc[iris_data['class'] == 'versicolor', 'class'] = 'Iris-versicolor'

Answer 1

pd.DataFrame.loc can take one or two indexers. For the rest of the post, I'll represent the first indexer as i and the second indexer as j.

If only one indexer is provided, it applies to the index of the dataframe and the missing indexer is assumed to represent all columns. So the following two examples are equivalent.

1. df.loc[i]
2. df.loc[i, :]

Where : is used to represent all columns.

If both indexers are present, i references index values and j references column values.

---

Now we can focus on what types of values i and j can assume. Let's use the following dataframe df as our example:

    df = pd.DataFrame([[1, 2], [3, 4]], index=['A', 'B'], columns=['X', 'Y'])

loc has been written such that i and j can be

1. scalars that should be values in the respective index objects

   df.loc['A', 'Y']
   
   2
   

2. arrays whose elements are also members of the respective index object (notice that the order of the array I pass to loc is respected

   df.loc[['B', 'A'], 'X']
   
   B    3
   A    1
   Name: X, dtype: int64
   

   • Notice the dimensionality of the return object when passing arrays. i is an array as it was above, loc returns an object in which an index with those values is returned. In this case, because j was a scalar, loc returned a pd.Series object. We could've manipulated this to return a dataframe if we passed an array for i and j, and the array could've have just been a single value'd array.

     df.loc[['B', 'A'], ['X']]
     
        X
     B  3
     A  1
     

3. boolean arrays whose elements are True or False and whose length matches the length of the respective index. In this case, loc simply grabs the rows (or columns) in which the boolean array is True.

   df.loc[[True, False], ['X']]
   
      X
   A  1
   

---

In addition to what indexers you can pass to loc, it also enables you to make assignments. Now we can break down the line of code you provided.

iris_data.loc[iris_data['class'] == 'versicolor', 'class'] = 'Iris-versicolor'

1. iris_data['class'] == 'versicolor' returns a boolean array.
2. class is a scalar that represents a value in the columns object.
3. iris_data.loc[iris_data['class'] == 'versicolor', 'class'] returns a pd.Series object consisting of the 'class' column for all rows where 'class' is 'versicolor'

4. When used with an assignment operator:

   iris_data.loc[iris_data['class'] == 'versicolor', 'class'] = 'Iris-versicolor'
   

   We assign 'Iris-versicolor' for all elements in column 'class' where 'class' was 'versicolor'