Make Series.shift always a copy?

[TomAugspurger]

Right now, Series.shift(0) will just return the series. Shifting for all other periods induces a copy:

In [1]: import pandas as pd

In [2]: a = pd.Series([1, 2])

In [3]: a.shift(1) is a
Out[3]: False

In [4]: a.shift(0) is a
Out[4]: True

Should we defensively copy on 0 as well, for a consistent user experience?

pandas/pandas/core/generic.py

Lines 8084 to 8086 in e669fae

	def shift(self, periods=1, freq=None, axis=0):
	if periods == 0:
	return self

[WillAyd]

What if we disallowed shifting by zero? Seems like a useless argument for this function anyway

[TomAugspurger]

Special casing edge cases seems to always end up biting us in the end. Since a length-0 shift is well-defined, I'd prefer to just allow it.

[WillAyd]

Fair enough - in that case I'm +1 on always copy