Preprocessing with Parsers

new in 0.19.0

Parsers allow you to do some custom preprocessing on dataframes, columns, and series objects before running the validation checks. This is useful when you want to normalize, clip, or otherwise clean data values before applying validation checks.

Parsing versus validation

Pandera distinguishes between data validation and parsing. Validation is the act of verifying whether data follows some set of contraints, whereas parsing transforms raw data into some desired set of constraints.

Pandera ships with a few core parsers that you may already be familiar with:

• coerce=True will convert the datatypes of the incoming data to validate. This option is available in both DataFrameSchema and Column objects. See here for more details.

• strict="filter" will remove columns in the data that are not specified in the DataFrameSchema. See here for more details.

• add_missing_columns=True will add missing columns to the data if the Column is nullable or specifies a default value. See here.

The Parser abstraction allows you to specify any arbitrary transform that occurs before validation so that you can codify and standardize the preprocessing steps needed to get your raw data into a valid state.

Important

This feature is currently only supported with the pandas validation backend.

With parsers, you can codify and reuse preprocessing logic as part of the schema. Note that this feature is optional, meaning that you can always do preprocessing before calling schema.validate with the native dataframe API:

import pandas as pd
import pandera as pa

schema = pa.DataFrameSchema({"a": pa.Column(int, pa.Check.ge(0))})
data = pd.DataFrame({"a": [1, 2, -1]})

# clip negative values
data["a"] = data["a"].clip(lower=0)
schema.validate(data)

	a
0	1
1	2
2	0

Let’s encode the preprocessing step as a parser:

schema = pa.DataFrameSchema({
    "a": pa.Column(
        int,
        parsers=pa.Parser(lambda s: s.clip(lower=0)),
        checks=pa.Check.ge(0),
    )
})

data = pd.DataFrame({"a": [1, 2, -1]})
schema.validate(data)

	a
0	1
1	2
2	0

You can specify both dataframe- and column-level parsers, where dataframe-level parsers are performed before column-level parsers. Assuming that a schema contains parsers and checks, the validation process consists of the following steps:

1. dataframe-level parsing

2. column-level parsing

3. dataframe-level checks

4. column-level and index-level checks

Parsing columns

Parser objects accept a function as a required argument, which is expected to take a Series input and output a parsed Series, for example:

import numpy as np


schema = pa.DataFrameSchema({
    "sqrt_values": pa.Column(parsers=pa.Parser(lambda s: np.sqrt(s)))
})
schema.validate(pd.DataFrame({"sqrt_values": [1., 2., 3.]}))

	sqrt_values
0	1.000000
1	1.414214
2	1.732051

Multiple parsers can be applied to a column:

Important

The order of parsers is preserved at validation time.

schema = pa.DataFrameSchema({
    "string_numbers": pa.Column(
        str,
        parsers=[
            pa.Parser(lambda s: s.str.zfill(10)),
            pa.Parser(lambda s: s.str[2:]),
        ]
    ),
})

schema.validate(pd.DataFrame({"string_numbers": ["12345", "67890"]}))

	string_numbers
0	00012345
1	00067890

Parsing the dataframe

For any dataframe-wide preprocessing logic, you can specify the parsers kwarg in the DataFrameSchema object.

schema = pa.DataFrameSchema(
    parsers=pa.Parser(lambda df: df.transform("sqrt")),
    columns={
        "a": pa.Column(float),
        "b": pa.Column(float, parsers=pa.Parser(lambda s: s * -1)),
        "c": pa.Column(float, parsers=pa.Parser(lambda s: s + 1)),
    }
)

data = pd.DataFrame({
    "a": [2.0, 4.0, 9.0],
    "b": [2.0, 4.0, 9.0],
    "c": [2.0, 4.0, 9.0],
})

schema.validate(data)

	a	b	c
0	1.414214	-1.414214	2.414214
1	2.000000	-2.000000	3.000000
2	3.000000	-3.000000	4.000000

Note

Similar to the column-level parsers, you can also provide a list of Parsers at the dataframe level.

Parsers in DataFrameModel

We can write a DataFrameModel that’s equivalent to the schema above with the parse() and dataframe_parse() decorators:

class DFModel(pa.DataFrameModel):
    a: float
    b: float
    c: float

    @pa.dataframe_parser
    def sqrt(cls, df):
        return df.transform("sqrt")

    @pa.parser("b")
    def negate(cls, series):
        return series * -1

    @pa.parser("c")
    def plus_one(cls, series):
        return series + 1