to_datetime parses dates incorrectly when the format includes '%W' and does not include day of week plus calendar year

[wes-turner]

Code Sample, a copy-pastable example if possible

import pandas as pd

# I'm not sure how a bare week should be interpreted.  But the result should
# probably still be of the same week number
print(pd.to_datetime('20', format='%W').strftime('%W'))  # 01

# Parsing is broken even when the week is fully specified
print(pd.to_datetime('2017-20', format='%Y-%W').strftime('%Y-%W'))  # 2017-00

Problem description

to_datetime parses a week XX as 0 for any valid XX. Interestingly, an impossible XX (ex: '70') throws a ValueError as expected.

Expected Output

20
2017-20

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.4.3.final.0 python-bits: 64 OS: Linux OS-release: 4.4.0-79-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8

pandas: 0.20.2
pytest: None
pip: 9.0.1
setuptools: 36.0.1
Cython: 0.24.1
numpy: 1.13.0
scipy: 0.18.1
xarray: None
IPython: 6.1.0
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.0
openpyxl: 2.4.0
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 3.3.3
bs4: 4.2.1
html5lib: 0.999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
pandas_gbq: None
pandas_datareader: None

[wes-turner]

In the docs for datetime.datetime.strptime:

When used with the strptime() method, %U and %W are only used in calculations when the day of the week and the calendar year (%Y) are specified.

Turns out this is true with to_datetime, too. It'd also be a totally-valid resolution to this issue to include this note in the to_datetime docstring.

[jorisvandenbossche]

When specifying the day of the week, it indeed seems to work (my locale is set to nl_BE here):

In [83]: pd.to_datetime('2017-20 Zondag', format='%Y-%W %A')
Out[83]: Timestamp('2017-05-21 00:00:00')

[wes-turner]

I should have phrased the bug more clearly: "to_datetime /only/ parses %W when both day and year are also specified."

Ideal behavior would be for a timestamp to be generated for, say, the beginning of the week (as a timestamp is analagously generated for the beginning of a day when no time is specified). Alternate behavior would be for to_datetime to refuse (like datetime.datetime.strptime) to parse such dates but to document this behavior.

[jreback]

I would raise on this if %W is passed but not also day and year.

note #16661/ #16607 as well which does this type of checking (for ISO week)