read_csv: if parse_dates dont appear in use_cols, we get a trace

[teto]

Code Sample, a copy-pastable example if possible

# Your code here
import pandas as pd
import io

content = io.StringIO('''
time,val
212.23, 32
''')

date_cols = ['time']

df = pd.read_csv(
    content,
    sep=',',
    usecols=['val'],
    dtype= { 'val': int },
    parse_dates=date_cols,
)

Problem description

triggers

Traceback (most recent call last):
  File "test.py", line 16, in <module>
    parse_dates=date_cols,
  File "/nix/store/k4fd48jzsyafvcifa6wi6pk4vaprnw36-python3.7-pandas-0.25.3/lib/python3.7/site-packages/pandas/io/parsers.py",
line 685, in parser_f
    return _read(filepath_or_buffer, kwds)
  File "/nix/store/k4fd48jzsyafvcifa6wi6pk4vaprnw36-python3.7-pandas-0.25.3/lib/python3.7/site-packages/pandas/io/parsers.py",
line 463, in _read
    data = parser.read(nrows)
  File "/nix/store/k4fd48jzsyafvcifa6wi6pk4vaprnw36-python3.7-pandas-0.25.3/lib/python3.7/site-packages/pandas/io/parsers.py",
line 1154, in read
    ret = self._engine.read(nrows)
  File "/nix/store/k4fd48jzsyafvcifa6wi6pk4vaprnw36-python3.7-pandas-0.25.3/lib/python3.7/site-packages/pandas/io/parsers.py",
line 2134, in read
    names, data = self._do_date_conversions(names, data)
  File "/nix/store/k4fd48jzsyafvcifa6wi6pk4vaprnw36-python3.7-pandas-0.25.3/lib/python3.7/site-packages/pandas/io/parsers.py",
line 1885, in _do_date_conversions
    keep_date_col=self.keep_date_col,
  File "/nix/store/k4fd48jzsyafvcifa6wi6pk4vaprnw36-python3.7-pandas-0.25.3/lib/python3.7/site-packages/pandas/io/parsers.py",
line 3335, in _process_date_conversion
    data_dict[colspec] = converter(data_dict[colspec])
KeyError: 'time'

i.e., if you use columns in parse_dates that dont appear in use_cols, then you are screwed.

Either parse_dates sould be added to use_cols or the documentation precise it. An assert could make the error more understandable too.
pandas 0.25.3

[murphybrendan]

I think this might have been fixed. I'm not able to reproduce that error in 1.0.0rc.

[mroeschke]

Looks fixed on master as well. Could use a regression test since it doesn't appear explicitly fixed in the whatsnew notes for 1.0.0rc

[ShaharNaveh]

I am getting the same error as well. (Python 3.8.1)

INSTALLED VERSIONS

commit : aad377d
python : 3.8.1.final.0
python-bits : 64
OS : Linux
OS-release : 5.4.13.a-1-hardened
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.0.0rc0+190.gaad377dd7
numpy : 1.18.1
pytz : 2019.3
dateutil : 2.8.1
pip : 20.0.1
setuptools : 45.1.0
Cython : 0.29.14
pytest : 5.3.4
hypothesis : 5.3.0
sphinx : 2.3.1
blosc : None
feather : None
xlsxwriter : 1.2.7
lxml.etree : 4.4.2
html5lib : 1.0.1
pymysql : None
psycopg2 : None
jinja2 : 2.10.3
IPython : 7.11.1
pandas_datareader: None
bs4 : 4.8.2
bottleneck : 1.3.1
fastparquet : 0.3.2
gcsfs : None
lxml.etree : 4.4.2
matplotlib : 3.1.2
numexpr : 2.7.1
odfpy : None
openpyxl : 3.0.1
pandas_gbq : None
pyarrow : None
pytables : None
pytest : 5.3.4
pyxlsb : None
s3fs : 0.4.0
scipy : 1.4.1
sqlalchemy : 1.3.13
tables : 3.6.1
tabulate : 0.8.6
xarray : 0.14.1
xlrd : 1.2.0
xlwt : 1.3.0
xlsxwriter : 1.2.7
numba : 0.47.0

[teto]

Thanks for looking into it.
One thing I don't get in the api is why dates are considered different than converters. Because they are multicolumn ? we could have a multicolumn converter or ignore dates altogether/have multicolumn converters (I mention this just in case 1.0 is allowed to break API).

[sathyz]

take.