Python Dataframe: Expand Single "Date" Column Dataframe by Adding Second Column From a List of Names To Each Date

Let’s say I have the following sample input dataframe below. Please note my real table contains hundreds of dates.


The code to create above table is:

import pandas as pd
df = pd.DataFrame({'date':["2023-04-02","2023-07-14"]})

And I have a list of names:

names_list = ['Matthew Perry', "Amy Winehouse", "Ted Nugent"]

I want to expand the input table by adding a second column that contains names from the names_list. The dates in "Date" column would get repeated until a new date is hit.

My desired output table looks like this:

Date Names
2023-04-02 Matthew Perry
2023-04-02 Amy Winehouse
2023-04-02 Ted Nugent
2023-07-14 Matthew Perry
2023-07-14 Amy Winehouse
2023-07-14 Ted Nugent

I searched around Stack Overflow but couldn’t find any code to do this. I’m familiar with left joins and concats but I don’t think either apply in this situation.

>Solution :

One option would be to build a Series of the names and cross-merge it with your df :

out = df.merge(pd.Series(names_list).rename("Names"), how="cross")

Alternatively, make a product and pass it to the DataFrame constructor :

from itertools import product

out = pd.DataFrame(
    product(df["date"], names_list), columns=["date", "Names"]

Output :


         date          Names
0  2023-04-02  Matthew Perry
1  2023-04-02  Amy Winehouse
2  2023-04-02     Ted Nugent
3  2023-07-14  Matthew Perry
4  2023-07-14  Amy Winehouse
5  2023-07-14     Ted Nugent

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