Get first row of dataframe in Python Pandas based on criteria

This tutorial is a very good one for pandas slicing. Make sure you check it out. Onto some snippets… To slice a dataframe with a condition, you use this format:

>>> df[condition]

This will return a slice of your dataframe which you can index using iloc. Here are your examples:

1. Get first row where A > 3 (returns row 2)

   >>> df[df.A > 3].iloc[0]
   A    4
   B    6
   C    3
   Name: 2, dtype: int64
   

If what you actually want is the row number, rather than using iloc, it would be df[df.A > 3].index[0].

2. Get first row where A > 4 AND B > 3:

   >>> df[(df.A > 4) & (df.B > 3)].iloc[0]
   A    5
   B    4
   C    5
   Name: 4, dtype: int64
   

3. Get first row where A > 3 AND (B > 3 OR C > 2) (returns row 2)

   >>> df[(df.A > 3) & ((df.B > 3) | (df.C > 2))].iloc[0]
   A    4
   B    6
   C    3
   Name: 2, dtype: int64
   

Now, with your last case we can write a function that handles the default case of returning the descending-sorted frame:

>>> def series_or_default(X, condition, default_col, ascending=False):
...     sliced = X[condition]
...     if sliced.shape[0] == 0:
...         return X.sort_values(default_col, ascending=ascending).iloc[0]
...     return sliced.iloc[0]
>>> 
>>> series_or_default(df, df.A > 6, 'A')
A    5
B    4
C    5
Name: 4, dtype: int64

As expected, it returns row 4.