when should I use query vs eval on a pandas dataframe?

Question

I used to perform operations such as df.query("a > 10") and they work fine.

However, I noticed that when I tried df.query("(a + b) / 2 > 10"), it failed.

Fortunately, when I tried df.eval("(a + b) / 2 > 10"), it works fine.

This leads me to the question what makes eval different from query and when should we use one vs the other?

Answer 1

The df.query is to get the selected rows (where condition is fulfilled) and df.eval is to get boolean output for each row. e.g.

df123 = pd.DataFrame({'A': range(1, 6),
                      'B': range(10, 0, -2),
                      'C C': range(10, 5, -1)})
print(df123)

   A   B  C C
0  1  10   10
1  2   8    9
2  3   6    8
3  4   4    7
4  5   2    6

df123.query('A > B')

  A   B   C C
4 5   2   6

df123.eval('A > B')

0    False
1    False
2    False
3    False
4     True
dtype: bool

Hope this helps!