shift does not always preserve freqstr if series contains only one or two elements #21275
Description
Problem description
The freqstr information is lost if series contains only one or two elements AND the freqstr by which is shifted is not the same as the freqstr of the series. In all other cases it works fine.
import pandas as pd
s1 = pd.Series([1], index=pd.date_range('2016-1-1', periods=1, freq='H'))
s2 = pd.Series([1, 2], index=pd.date_range('2016-1-1', periods=2, freq='H'))
s3 = pd.Series([1, 2, 3], index=pd.date_range('2016-1-1', periods=3, freq='H'))
# following DOES NOT preserve freqstr
print(s1.shift(1, '2H'))
print(s2.shift(1, '2H'))
print(s1.shift(1, 'min'))
print(s2.shift(1, 'min'))
# for comparison, following is all OK (preserves freqstr)
print(s1.shift(1, 'H'))
print(s2.shift(1, 'H'))
print(s3.shift(1, 'H'))
print(s3.shift(1, '2H'))
print(s3.shift(1, 'min'))
print(s3.shift(1, 'min'))Expected Output
Preservation of the freqstr in all above cases.