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Parsing

Polars has native support for parsing time series data and doing more sophisticated operations such as temporal grouping and resampling.

Datatypes

Polars has the following datetime datatypes:

  • Date: Date representation e.g. 2014-07-08. It is internally represented as days since UNIX epoch encoded by a 32-bit signed integer.
  • Datetime: Datetime representation e.g. 2014-07-08 07:00:00. It is internally represented as a 64 bit integer since the Unix epoch and can have different units such as ns, us, ms.
  • Duration: A time delta type that is created when subtracting Date/Datetime. Similar to timedelta in Python.
  • Time: Time representation, internally represented as nanoseconds since midnight.

Parsing dates from a file

When loading from a CSV file Polars attempts to parse dates and times if the try_parse_dates flag is set to True:

read_csv 

df = pl.read_csv("docs/assets/data/apple_stock.csv", try_parse_dates=True)
print(df)

CsvReader · Available on feature csv 

let df = CsvReadOptions::default()
    .map_parse_options(|parse_options| parse_options.with_try_parse_dates(true))
    .try_into_reader_with_file_path(Some("docs/assets/data/apple_stock.csv".into()))
    .unwrap()
    .finish()
    .unwrap();
println!("{}", &df);


shape: (100, 2)
┌────────────┬────────┐
│ Date       ┆ Close  │
│ ---        ┆ ---    │
│ date       ┆ f64    │
╞════════════╪════════╡
│ 1981-02-23 ┆ 24.62  │
│ 1981-05-06 ┆ 27.38  │
│ 1981-05-18 ┆ 28.0   │
│ 1981-09-25 ┆ 14.25  │
│ 1982-07-08 ┆ 11.0   │
│ …          ┆ …      │
│ 2012-05-16 ┆ 546.08 │
│ 2012-12-04 ┆ 575.85 │
│ 2013-07-05 ┆ 417.42 │
│ 2013-11-07 ┆ 512.49 │
│ 2014-02-25 ┆ 522.06 │
└────────────┴────────┘

On the other hand binary formats such as parquet have a schema that is respected by Polars.

Casting strings to dates

You can also cast a column of datetimes encoded as strings to a datetime type. You do this by calling the string str.to_date method and passing the format of the date string:

read_csv · str.to_date 

df = pl.read_csv("docs/assets/data/apple_stock.csv", try_parse_dates=False)

df = df.with_columns(pl.col("Date").str.to_date("%Y-%m-%d"))
print(df)

CsvReader · str.replace_all · Available on feature csv · Available on feature dtype-date 

let df = CsvReadOptions::default()
    .map_parse_options(|parse_options| parse_options.with_try_parse_dates(false))
    .try_into_reader_with_file_path(Some("docs/assets/data/apple_stock.csv".into()))
    .unwrap()
    .finish()
    .unwrap();
let df = df
    .clone()
    .lazy()
    .with_columns([col("Date").str().to_date(StrptimeOptions::default())])
    .collect()?;
println!("{}", &df);


shape: (100, 2)
┌────────────┬────────┐
│ Date       ┆ Close  │
│ ---        ┆ ---    │
│ date       ┆ f64    │
╞════════════╪════════╡
│ 1981-02-23 ┆ 24.62  │
│ 1981-05-06 ┆ 27.38  │
│ 1981-05-18 ┆ 28.0   │
│ 1981-09-25 ┆ 14.25  │
│ 1982-07-08 ┆ 11.0   │
│ …          ┆ …      │
│ 2012-05-16 ┆ 546.08 │
│ 2012-12-04 ┆ 575.85 │
│ 2013-07-05 ┆ 417.42 │
│ 2013-11-07 ┆ 512.49 │
│ 2014-02-25 ┆ 522.06 │
└────────────┴────────┘

The format string specification can be found here..

Extracting date features from a date column

You can extract data features such as the year or day from a date column using the .dt namespace on a date column:

dt.year 

df_with_year = df.with_columns(pl.col("Date").dt.year().alias("year"))
print(df_with_year)

dt.year 

let df_with_year = df
    .clone()
    .lazy()
    .with_columns([col("Date").dt().year().alias("year")])
    .collect()?;
println!("{}", &df_with_year);


shape: (100, 3)
┌────────────┬────────┬──────┐
│ Date       ┆ Close  ┆ year │
│ ---        ┆ ---    ┆ ---  │
│ date       ┆ f64    ┆ i32  │
╞════════════╪════════╪══════╡
│ 1981-02-23 ┆ 24.62  ┆ 1981 │
│ 1981-05-06 ┆ 27.38  ┆ 1981 │
│ 1981-05-18 ┆ 28.0   ┆ 1981 │
│ 1981-09-25 ┆ 14.25  ┆ 1981 │
│ 1982-07-08 ┆ 11.0   ┆ 1982 │
│ …          ┆ …      ┆ …    │
│ 2012-05-16 ┆ 546.08 ┆ 2012 │
│ 2012-12-04 ┆ 575.85 ┆ 2012 │
│ 2013-07-05 ┆ 417.42 ┆ 2013 │
│ 2013-11-07 ┆ 512.49 ┆ 2013 │
│ 2014-02-25 ┆ 522.06 ┆ 2014 │
└────────────┴────────┴──────┘

Mixed offsets

If you have mixed offsets (say, due to crossing daylight saving time), then you can use utc=True and then convert to your time zone:

str.to_datetime · dt.convert_time_zone · Available on feature timezone 

data = [
    "2021-03-27T00:00:00+0100",
    "2021-03-28T00:00:00+0100",
    "2021-03-29T00:00:00+0200",
    "2021-03-30T00:00:00+0200",
]
mixed_parsed = (
    pl.Series(data)
    .str.to_datetime("%Y-%m-%dT%H:%M:%S%z")
    .dt.convert_time_zone("Europe/Brussels")
)
print(mixed_parsed)

str.replace_all · dt.convert_time_zone · Available on feature dtype-datetime · Available on feature timezones 

let data = [
    "2021-03-27T00:00:00+0100",
    "2021-03-28T00:00:00+0100",
    "2021-03-29T00:00:00+0200",
    "2021-03-30T00:00:00+0200",
];
let q = col("date")
    .str()
    .to_datetime(
        Some(TimeUnit::Microseconds),
        None,
        StrptimeOptions {
            format: Some("%Y-%m-%dT%H:%M:%S%z".into()),
            ..Default::default()
        },
        lit("raise"),
    )
    .dt()
    .convert_time_zone("Europe/Brussels".into());
let mixed_parsed = df!("date" => &data)?.lazy().select([q]).collect()?;

println!("{}", &mixed_parsed);


shape: (4,)
Series: '' [datetime[μs, Europe/Brussels]]
[
    2021-03-27 00:00:00 CET
    2021-03-28 00:00:00 CET
    2021-03-29 00:00:00 CEST
    2021-03-30 00:00:00 CEST
]