python - what is different between groupby.first, groupby.nth, groupby.head when as_index=False

Question

Edit: the rookie mistake I made in string np.nan having pointed out by @coldspeed, @wen-ben, @ALollz. Answers are quite good, so I don't delete this question to keep those answers.

Original:
I have read this question/answer What's the difference between groupby.first() and groupby.head(1)?

That answer explained that the differences are on handling NaN value. However, when I call groupby with as_index=False, they both pick NaN fine.

Furthermore, Pandas has groupby.nth with similar functionality to head, and first

What are difference of groupby.first(), groupby.nth(0), groupby.head(1) with as_index=False?

Example below:

In [448]: df
Out[448]:
   A       B
0  1  np.nan
1  1       4
2  1      14
3  2       8
4  2      19
5  2      12

In [449]: df.groupby('A', as_index=False).head(1)
Out[449]:
   A       B
0  1  np.nan
3  2       8

In [450]: df.groupby('A', as_index=False).first()
Out[450]:
   A       B
0  1  np.nan
1  2       8

In [451]: df.groupby('A', as_index=False).nth(0)
Out[451]:
   A       B
0  1  np.nan
3  2       8

I saw that `firs()' resets index while the other 2 doesn't. Besides that, is there any differences?

Answer 1

The major issue is that you likely have the string 'np.nan' stored and not a real null value. Here are how the three handle null values differently:

Sample Data:

import pandas as pd
df = pd.DataFrame({'A': [1,1,2,2,3,3], 'B': [None, '1', np.NaN, '2', 3, 4]})

---

first/last

This will return the first/last non-null value within each group. Oddly enough it will not skip None, though this can be made possible with the kwarg dropna=True. As a result, you may return values for columns that were part of different rows originally:

df.groupby('A', as_index=False).first()
#   A     B
#0  1  None
#1  2     2
#2  3     3

df.groupby('A', as_index=False).first(dropna=True)
#   A  B
#0  1  1
#1  2  2
#2  3  3

head(n)/tail(n)

Returns the top/bottom n rows within a group. Values remain bound within rows. If you give it an n that is more than the number of rows, it returns all rows in that group without complaining:

df.groupby('A', as_index=False).head(1)
#   A     B
#0  1  None
#2  2   NaN
#4  3     3

df.groupby('A', as_index=False).head(200)
#   A     B
#0  1  None
#1  1     1
#2  2   NaN
#3  2     2
#4  3     3
#5  3     4

nth

This takes the nth row, so again values remain bound within the row. .nth(0) is the same as .head(1), though they have different uses. For instance, if you need the 0th and 2nd row, that's difficult to do with .head(), but easy with .nth([0,2]). Also it's fair easier to write .head(10) than .nth(list(range(10)))).

df.groupby('A', as_index=False).nth(0)
#   A     B
#0  1  None
#2  2   NaN
#4  3     3

nth also supports dropping rows with any null-values, so you can use it to return the first row without any null-values, unlike .head()

df.groupby('A', as_index=False).nth(0, dropna='any')
#   A  B
#A      
#1  1  1
#2  2  2
#3  3  3