Kelaref Kelaref - 1 month ago 13
Python Question

Unpack two variables into two columns in DataFrame

I have a function called ReturnDateRange which will return two dates.
I am trying to apply this function on a column called 'zRow' in a dataFrame, and store the result in two different columns.

The below will store both results, as a tuple in one column:

df['t1']= df['zRow'].map(ReturnDateRange)


The following returns a ValueError: Too many values to unpack (expected 2)

df['t1'], df['t2']= df['zRow'].map(ReturnDateRange)


But the function always returns two dates, or a single None.

UPDATE: I tried returning two zeros instead of None. Still get the same error.

Thanks for the help.

Answer

The "Too many values to unpack" error is because its unpacking the rows, rather than columns. Thus a transpose will help, but I feel like there's a nicer way to fix this.

In the mean time, this works, if not very elegantly...

import pandas as pd
import numpy as np

data = pd.DataFrame(np.zeros(shape=(5,2)),columns=["a","b"])

def mapper(x):
    return ('first', 'second')

data['t1'], data['t2'] = data['b'].map(mapper).apply(pd.Series).values.T

print data

Giving this result:

     a    b     t1      t2
0  0.0  0.0  first  second
1  0.0  0.0  first  second
2  0.0  0.0  first  second
3  0.0  0.0  first  second
4  0.0  0.0  first  second

This might help someone else pin-down a better solution at least.

Credit to this post here too.

Edit, found a nicer way to fix it. Use:

data[['t1', 't2']] = data['b'].map(mapper).apply(pd.Series)

So, in your case, this should work:

df[['t1', 't2']] = df['zRow'].map(ReturnDateRange).apply(pd.Series)