to_timedelta does not work properly with nanoseconds

[Sup3rGeo]

Code Sample, a copy-pastable example if possible

>>> index = pandas.to_timedelta([15e-7], unit="s")
>>> index[0].nanoseconds
0
>>> index[0].microseconds
2

Problem description

nanoseconds range input for to_timedelta function are rounded to microseconds.

Expected Output

>>> index = pandas.to_timedelta([15e-7], unit="s")
>>> index[0].nanoseconds
500
>>> index[0].microseconds
1

However this works fine:

>>> index = pandas.to_timedelta([1.5], unit="us")
>>> index[0].nanoseconds
500
>>> index[0].microseconds
1

So it seems to be something when se choose unit="s"

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.21.1
pytest: 3.4.0
pip: 9.0.1
setuptools: 38.4.0
Cython: 0.27.3
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.6
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.1
openpyxl: 1.7.0
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[jorisvandenbossche]

@Sup3rGeo thanks for the report!

It seems this happens in this function:

In [126]: pd._libs.tslibs.timedeltas.cast_from_unit(1.5e-6, 's')
Out[126]: 2000

because we round at 6 decimals:

pandas/pandas/_libs/tslibs/timedeltas.pyx

Line 234 in 405ed25

frac = round(frac, p)

Not fully sure why we choose 6 decimals to be allowed, and not 9

[jorisvandenbossche]

Ah, this is a duplicate of #14156. Continuing discussion there further.