String

[Polars Documentation]

In the string module, OpenDP currently only supports parsing to temporal data types.

Python

>>> import polars as pl
>>> import opendp.prelude as dp
>>> dp.enable_features("contrib")

>>> context = dp.Context.compositor(
...     data=pl.scan_csv(dp.examples.get_france_lfs_path()),
...     privacy_unit=dp.unit_of(contributions=36),
...     privacy_loss=dp.loss_of(epsilon=1.0, delta=1e-7),
...     split_evenly_over=2,
... )

Strptime, To Date, To Datetime, To Time

Dates can be parsed from strings via .str.strptime, and its variants .str.to_date, .str.to_datetime, and .str.to_time.

Python

>>> query = (
...     context.query()
...     .with_columns(
...         pl.col.YEAR.cast(str).str.to_date(format=r"%Y")
...     )
...     .group_by("YEAR")
...     .agg(dp.len(signed=True))
... )
>>> query.release().collect().sort("YEAR")
shape: (9, 2)
┌────────────┬────────┐
│ YEAR       ┆ len    │
│ ---        ┆ ---    │
│ date       ┆ i64    │
╞════════════╪════════╡
│ 2005-01-01 ┆ ... │
│ 2006-01-01 ┆ ... │
│ 2007-01-01 ┆ ... │
│ 2008-01-01 ┆ ... │
│ 2009-01-01 ┆ ... │
│ 2010-01-01 ┆ ... │
│ 2011-01-01 ┆ ... │
│ 2012-01-01 ┆ ... │
│ 2013-01-01 ┆ ... │
└────────────┴────────┘

While Polars supports automatic inference of the datetime format from reading the data, doing so can lead to situations where the data-dependent inferred format changes or cannot be inferred upon the addition or removal of a single individual, resulting in an unstable computation. For this reason, the format argument is required.

OpenDP also does not allow parsing strings into nanosecond datetimes, because the underlying implementation throws data-dependent errors (not private) for certain inputs.

Python

>>> query = (
...     context.query()
...     .with_columns(
...         pl.col.YEAR.cast(str).str.to_datetime(
...             format=r"%Y", time_unit="ns"
...         )
...     )
...     .group_by("YEAR")
...     .agg(dp.len(signed=True))
... )
>>> try:
...     query.release()
... except dp.OpenDPException as err:
...     print(err)
...

  MakeMeasurement("Nanoseconds are not currently supported due to potential panics when parsing inputs. Please open an issue on the OpenDP repository if you would find this functionality useful. Otherwise, consider parsing into micro- or millisecond datetimes instead.")

Parsed data can then be manipulated with Temporal Expressions.