python assign value to pandas df if falls between range of dates in another df

Question

What is the best way to create a new column and assign a value if date falls between two dates in another dataframe ?

e.g.

dataframe A    
date             


        

Answer 1

Using an IntervalIndex, which is new in Pandas 0.20.0. This looks to still be in the experimental phase though, so other solutions may be more reliable.

# Get the 'id' column indexed by the 'start'/'end' intervals.
s = pd.Series(df_b['id'].values, pd.IntervalIndex.from_arrays(df_b['start'], df_b['end']))

# Map based on the date of df_a.
df_a['id'] = df_a['date'].map(s)

The resulting output:

        date values  id
0 2017-05-16      x  34
1 2017-04-12      Y  32

Alternatively, if you don't mind altering the index of df_b, you could just directly convert to an IntervalIndex on it:

# Create an IntervalIndex on df_b.
df_b = df_b.set_index(['start', 'end'])
df_b.index = pd.IntervalIndex.from_tuples(df_b.index)

# Map based on the date of df_a.
df_a['id'] = df_a['date'].map(df_b['id'])

Answer 2

This is an example of something that's really straightforward to do in one step in sql, but not so much in Pandas. So with the proviso that I don't love this approach, here it is.

1. do a full cartesian join
2. filter down to desired rows & columns.

_

# First Full Outer Join Dataframes 
# (Requires a Common Column in Pandas Unlike SQL)
df_A['fake key'] = 1
df_B['fake key'] = 1
outer_join = pd.merge(df_A, df_B, how='outer', on='fake key')

# Now Filter Back down to Desired Rows/Columns
desired_rows    = outer_join.query('date < end and date > start')
desired_columns = ['date', 'values', 'id']

final = desired_rows[desired_columns]
final

output:

        date values  id
0 2017-05-16      x  34
5 2017-04-12      y  32

The things that make this answer somewhat unsatisfying to me are:

1. The first step of a full cartesian join doesn't scale well at all to large data
2. The cartesian join requires a common column, so in this case first creating that fake key column (see this github issue