REGR: pd.unique casts nan to string with other strings #16107
Description
Code Sample, a copy-pastable example if possible
In [1]: import numpy as np; import pandas as pd In [2]: pd.unique(['a', np.nan, 'c', 'c']) Out[2]: array(['a', 'nan', 'c'], dtype=object) In [3]: pd.__version__ Out[3]: '0.20.0rc1'Notice the quotes around 'nan'. This came up from the seaborn test suite
w#### Expected Output
# 0.19.2 In [1]: import numpy as np; import pandas as pd In [2]: pd.unique(['a', np.nan, 'c', 'c']) Out[2]: array(['a', nan, 'c'], dtype=object)