ValueError: Series lengths must match to compare when matching dates in Pandas

Question

I apologize in advance for asking such a basic question but I am stumped.

This is a very simple, dummy example. I\'m having some issue matching dates in Pandas and I

Answer 1

You say:

some_date = df.iloc[1:2]['Date']  # gives 2016-01-01

but that's not what it gives. It gives a Series with one element, not simply a value -- when you use [1:2] as your slice, you don't get a single element, but a container with one element:

>>> some_date
1   2016-01-01
Name: Date, dtype: datetime64[ns]

Instead, do

>>> some_date = df.iloc[1]['Date']
>>> some_date
Timestamp('2016-01-01 00:00:00')

after which

>>> df[(df['ID']==some_id) & (df['Date'] == some_date)] 
   ID       Date
0   1 2016-01-01

(Note that there are more efficient patterns if you have a lot of some_id and some_date values to look up, but that's a separate issue.)