`read_gbq` fails with ValueError when a column is of type `TIME`

I believe that the smallest example to reproduce this is by running:

from pandas_gbq import read_gbq

read_gbq('select current_time()')

the traceback is:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/german/Documents/c4v/infra-tools/probe-analyzer/env/lib/python3.6/site-packages/pandas_gbq/gbq.py", line 967, in read_gbq
    progress_bar_type=progress_bar_type,
  File "/home/german/Documents/c4v/infra-tools/probe-analyzer/env/lib/python3.6/site-packages/pandas_gbq/gbq.py", line 532, in run_query
    progress_bar_type=progress_bar_type,
  File "/home/german/Documents/c4v/infra-tools/probe-analyzer/env/lib/python3.6/site-packages/pandas_gbq/gbq.py", line 562, in _download_results
    progress_bar_type=progress_bar_type,
  File "/home/german/Documents/c4v/infra-tools/probe-analyzer/env/lib/python3.6/site-packages/google/cloud/bigquery/table.py", line 1776, in to_dataframe
    for frame in self.to_dataframe_iterable(dtypes=dtypes):
  File "/home/german/Documents/c4v/infra-tools/probe-analyzer/env/lib/python3.6/site-packages/google/cloud/bigquery/table.py", line 1439, in _to_page_iterable
    for item in tabledata_list_download():
  File "/home/german/Documents/c4v/infra-tools/probe-analyzer/env/lib/python3.6/site-packages/google/cloud/bigquery/_pandas_helpers.py", line 565, in download_dataframe_tabledata_list
    yield _tabledata_list_page_to_dataframe(page, column_names, dtypes)
  File "/home/german/Documents/c4v/infra-tools/probe-analyzer/env/lib/python3.6/site-packages/google/cloud/bigquery/_pandas_helpers.py", line 539, in _tabledata_list_page_to_dataframe
    columns[column_name] = pandas.Series(page._columns[column_index], dtype=dtype)
  File "/home/german/Documents/c4v/infra-tools/probe-analyzer/env/lib/python3.6/site-packages/pandas/core/series.py", line 327, in __init__
    data = sanitize_array(data, index, dtype, copy, raise_cast_failure=True)
  File "/home/german/Documents/c4v/infra-tools/probe-analyzer/env/lib/python3.6/site-packages/pandas/core/construction.py", line 447, in sanitize_array
    subarr = _try_cast(data, dtype, copy, raise_cast_failure)
  File "/home/german/Documents/c4v/infra-tools/probe-analyzer/env/lib/python3.6/site-packages/pandas/core/construction.py", line 568, in _try_cast
    subarr = construct_1d_ndarray_preserving_na(subarr, dtype, copy=copy)
  File "/home/german/Documents/c4v/infra-tools/probe-analyzer/env/lib/python3.6/site-packages/pandas/core/dtypes/cast.py", line 1618, in construct_1d_ndarray_preserving_na
    subarr = np.array(values, dtype=dtype, copy=copy)

I believe that changing the mapping between the TIME data-type and its dtype to object can fix the issue:

https://github.com/pydata/pandas-gbq/blob/d251db03b159447331ac9ae63e13d295d75bad70/pandas_gbq/gbq.py#L698-L710

… but haven’t tried yet and I have never collaborated to this repository before.

Any other idea to solve this issue?

[tswast]

Oh, and thanks for the reproducible test case!

[tswast]

I just stepped through the code, and https://github.com/googleapis/python-bigquery/blob/0c387dadd57fba9cdbfd39abe530de209943db9a/google/cloud/bigquery/table.py#L1721 isn’t failing on my machine, but it’s also not trying to convert from object to datetime64[ns] despite being passed that as a dtype.