date_range fails when I try to generate ones with 1 periods and freq equal WOM-1MON

[asdf8601]

Code Sample

import pandas as pd
pd.date_range('20100104', periods=2, freq='WOM-1MON')  # works
pd.date_range('20100104', periods=1, freq='WOM-1MON')  # fails

Traceback (most recent call last):
  File "/Users/mmngreco/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 2910, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-31-ec0b4bad59c9>", line 1, in <module>
    pd.date_range('20100104', periods=1, freq='WOM-1MON')
  File "/Users/mmngreco/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pandas/core/indexes/datetimes.py", line 2057, in date_range
    closed=closed, **kwargs)
  File "/Users/mmngreco/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pandas/util/_decorators.py", line 118, in wrapper
    return func(*args, **kwargs)
  File "/Users/mmngreco/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pandas/core/indexes/datetimes.py", line 324, in __new__
    ambiguous=ambiguous)
  File "/Users/mmngreco/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pandas/core/indexes/datetimes.py", line 531, in _generate
    index = _generate_regular_range(start, end, periods, offset)
  File "/Users/mmngreco/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pandas/core/indexes/datetimes.py", line 2009, in _generate_regular_range
    dates = list(xdr)
  File "/Users/mmngreco/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pandas/tseries/offsets.py", line 2960, in generate_range
    end = start + (periods - 1) * offset
  File "/Users/mmngreco/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pandas/tseries/offsets.py", line 425, in __rmul__
    return self.__mul__(someInt)
  File "/Users/mmngreco/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pandas/tseries/offsets.py", line 422, in __mul__
    **self.kwds)
  File "/Users/mmngreco/miniconda3/envs/pandas-dev/lib/python3.6/site-packages/pandas/tseries/offsets.py", line 1671, in __init__
    raise ValueError('N cannot be 0')
ValueError: N cannot be 0

Problem description

If N is equal to periods then is not 0 as we can see, that make me think that probably there is something wrong in the code.

Expected Output

Out[33]: DatetimeIndex(['2010-01-04'], dtype='datetime64[ns]', freq='WOM-1MON')

Output of pd.show_versions()

pd.show_versions()
INSTALLED VERSIONS

commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: None
LOCALE: es_ES.UTF-8
pandas: 0.22.0
pytest: 3.4.2
pip: 9.0.1
setuptools: 38.5.1
Cython: 0.27.3
numpy: 1.14.2
scipy: 1.0.0
pyarrow: 0.9.0
xarray: 0.10.2
IPython: 6.2.1
sphinx: 1.7.1
patsy: 0.5.0
dateutil: 2.7.0
pytz: 2018.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: 0.4.0
matplotlib: 2.2.2
openpyxl: 2.5.1
xlrd: 1.1.0
xlwt: 1.2.0
xlsxwriter: 1.0.2
lxml: 4.2.0
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.5
pymysql: 0.8.0
psycopg2: None
jinja2: 2.10
s3fs: 0.1.3
fastparquet: 0.1.4
pandas_gbq: None
pandas_datareader: None

[WillAyd]

Well the traceback here points exactly to the offending line of code - within the constructor there is an explicit check that you have more than one period which I've linked below for reference (generate_periods up one level in the stack subtracts one from periods)

pandas/pandas/tseries/offsets.py

Line 1464 in c4b4a81

if self.n == 0:

If you remove that assertion you get the value you are expecting. I'm not overly familiar with offsets though - @jbrockmendel any chance you know whether it makes sense to relax that assertion or not?

[jbrockmendel]

This is one of those "it was like that when I got here" things. I'd guess that n==0 would cause trouble with incrementing, but not really sure.

[WillAyd]

@mmngreco do you want to try a PR for this? Would need test cases to cover this and any other edge case you can think of

[asdf8601]

Ok, I would like to try.

[tv3141]

The same check can be found for three other offsets:

import pandas as pd
from pandas.tseries.offsets import *

# all three lines below return ValueError: N cannot be 0
pd.date_range('20100104', periods=1, freq=LastWeekOfMonth(weekday=2))
pd.date_range('20100104', periods=1, freq=FY5253(startingMonth=4, weekday=2, variation='nearest'))
pd.date_range('20100104', periods=1, freq=FY5253Quarter(qtr_with_extra_week=1, startingMonth=4, weekday=2, variation='nearest'))

LastWeekOfMonth

pandas/pandas/tseries/offsets.py

Lines 1538 to 1539 in 601b8c9

	if self.n == 0:
	raise ValueError('N cannot be 0')

FY5253

pandas/pandas/tseries/offsets.py

Lines 1824 to 1825 in 601b8c9

	if self.n == 0:
	raise ValueError('N cannot be 0')

FY5253Quarter

pandas/pandas/tseries/offsets.py

Lines 2025 to 2026 in 601b8c9

	if self.n == 0:
	raise ValueError('N cannot be 0')