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DOC: Document behaviour of head(n), tail(n) for negative values of n except on GroupBy #30556

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Dec 30, 2019
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DOC: Document behaviour of head(n), tail(n) for negative values of n except on GroupBy #30556

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DOC: Document negative values on head(n), tail(n) except for GroupBy
bharatr21 Dec 30, 2019
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Correct trailing whitespace
bharatr21 Dec 30, 2019
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bharatr21 Dec 30, 2019
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30 changes: 29 additions & 1 deletion pandas/core/generic.py
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Original file line number Diff line number Diff line change
Expand Up @@ -4652,14 +4652,17 @@ def head(self: FrameOrSeries, n: int = 5) -> FrameOrSeries:
on position. It is useful for quickly testing if your object
has the right type of data in it.

For negative values of `n`, this function returns all rows except
the last `n` rows, equivalent to ``df[:-n]``.

Parameters
----------
n : int, default 5
Number of rows to select.

Returns
-------
obj_head : same type as caller
same type as caller
The first `n` rows of the caller object.

See Also
Expand Down Expand Up @@ -4699,6 +4702,17 @@ def head(self: FrameOrSeries, n: int = 5) -> FrameOrSeries:
0 alligator
1 bee
2 falcon

For negative values of `n`

>>> df.head(-3)
animal
0 alligator
1 bee
2 falcon
3 lion
4 monkey
5 parrot
"""

return self.iloc[:n]
Expand All @@ -4711,6 +4725,9 @@ def tail(self: FrameOrSeries, n: int = 5) -> FrameOrSeries:
position. It is useful for quickly verifying data, for example,
after sorting or appending rows.

For negative values of `n`, this function returns all rows except
the first `n` rows, equivalent to ``df[n:]``.

Parameters
----------
n : int, default 5
Expand Down Expand Up @@ -4758,6 +4775,17 @@ def tail(self: FrameOrSeries, n: int = 5) -> FrameOrSeries:
6 shark
7 whale
8 zebra

For negative values of `n`

>>> df.tail(-3)
animal
3 lion
4 monkey
5 parrot
6 shark
7 whale
8 zebra
"""

if n == 0:
Expand Down
12 changes: 12 additions & 0 deletions pandas/core/groupby/groupby.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2377,6 +2377,8 @@ def head(self, n=5):
from the original DataFrame with original index and order preserved
(``as_index`` flag is ignored).

Does not work for negative values of `n`.

Returns
-------
Series or DataFrame
Expand All @@ -2390,6 +2392,10 @@ def head(self, n=5):
A B
0 1 2
2 5 6
>>> df.groupby('A').head(-1)
Empty DataFrame
Columns: [A, B]
Index: []
"""
self._reset_group_selection()
mask = self._cumcount_array() < n
Expand All @@ -2405,6 +2411,8 @@ def tail(self, n=5):
from the original DataFrame with original index and order preserved
(``as_index`` flag is ignored).

Does not work for negative values of `n`.

Returns
-------
Series or DataFrame
Expand All @@ -2418,6 +2426,10 @@ def tail(self, n=5):
A B
1 a 2
3 b 2
>>> df.groupby('A').tail(-1)
Empty DataFrame
Columns: [A, B]
Index: []
"""
self._reset_group_selection()
mask = self._cumcount_array(ascending=False) < n
Expand Down