Extension dtypes do not work with dataframe query method 
In the latest pandas version (0.24.1), extension dtypes do not work with numexpr (e.g. the “query” method on dataframes fails when the engine parameter is set to the default ):

https://stackoverflow.com/questions/54759936/extension-dtypes-in-pandas-appear-to-have-a-bug-with-query

Code to reproduce:
df_test = pd.DataFrame(data=[4,5,6], columns=["col_test"])
df_test = df_test.astype(dtype={"col_test": pd.Int32Dtype()})
df_test.query("col_test != 6")
Last lines of the long error message are:

File "...\site_packages\numexpr\necompiler.py", line 822, in evaluate zip(names, arguments)] File "...\site_packages\numexpr\necompiler.py", line 821, in signature = [(name, getType(arg)) for (name, arg) in File "...\site_packages\numexpr\necompiler.py", line 703, in getType raise ValueError("unknown type %s" % a.dtype.name) ValueError: unknown type object