COMPAT: pandas 0.21.0

[TomAugspurger]

In pandas-dev/pandas#17710 pandas delays the import of various libraries. An upshot of this is that pandas.core.computation/__init__.py doesn't import pandas.core.computation.expression, so the code in

dask/dask/dataframe/core.py

Lines 38 to 43 in 0f29e7b

	if PANDAS_VERSION >= '0.20.0':
	from pandas.util import cache_readonly
	pd.core.computation.expressions.set_use_numexpr(False)
	else:
	from pandas.util.decorators import cache_readonly
	pd.computation.expressions.set_use_numexpr(False)

fails.

The recommended way to set that value is pd.set_option("compute.use_numexpr", True).

Does anyone know what that's being set in the first place? I'll look around the history.

[mrocklin]

My guess is that we wanted to avoid oversubscribing threads.

[TomAugspurger]

Seems to go back to #502 (cachey)