to_numeric fails with empty data and downcast="unsigned"

[pspeter]

Code Sample, a copy-pastable example if possible

pd.to_numeric([], downcast="unsigned")

Problem description

The code currently calls np.min() on the data which fails when the data is empty. This does not happen for any other downcast.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/.../lib/python3.7/site-packages/pandas/core/tools/numeric.py", line 163, in to_numeric
    elif downcast == "unsigned" and np.min(values) >= 0:
  File "<__array_function__ internals>", line 6, in amin
  File "/.../lib/python3.7/site-packages/numpy/core/fromnumeric.py", line 2793, in amin
    keepdims=keepdims, initial=initial, where=where)
  File "/.../lib/python3.7/site-packages/numpy/core/fromnumeric.py", line 90, in _wrapreduction
    return ufunc.reduce(obj, axis, dtype, out, **passkwargs)
ValueError: zero-size array to reduction operation minimum which has no identity

Expected Output

array([], dtype=uint8)

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.6.final.0
python-bits : 64
OS : Darwin
OS-release : 19.3.0
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 1.0.1
numpy : 1.18.1
pytz : 2019.3
dateutil : 2.8.1
pip : 20.0.2
setuptools : 41.2.0
Cython : None
pytest : 5.3.5
hypothesis : None
sphinx : None
blosc : None
feather : 0.4.0
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.1
IPython : 7.13.0
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : 0.6.0
lxml.etree : None
matplotlib : 3.1.3
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 0.16.0
pytables : None
pytest : 5.3.5
pyxlsb : None
s3fs : None
scipy : 1.4.1
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None
numba : 0.48.0

[TomAugspurger]

Thanks for the report. I think it's safe to include an initial=0 in that call to np.min in

pandas/pandas/core/tools/numeric.py

Lines 165 to 166 in 970499d

	elif downcast == "unsigned" and np.min(values) >= 0:
	typecodes = np.typecodes["UnsignedInteger"]

.

[MillanSharma]

take

[pspeter]

There's already a PR for this bug!

[pspeter]

Thanks @mikekutzma! :)