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Define your BigQuery tables as dataclasses.

Project description


Python package PyPI version Codecov Code style: black Imports: isort


At limehome we are heavy users of python and bigquery. This library was created to mainly solve the following issues:

  • Define table schemas in code and have a migration script to apply changes.
  • On deploy make sure that all schemas were applied, otherwise abort.
  • Guarantee that when we try to write data to a table, the data matches the schema of the table (required / optional, datatypes)
  • Version our tables and enable migrations to a new schema

Additionally this library aims to help the users through the usage of python typing.

  • Specify your schema as a python dataclass
  • Our migration script converts the data class into a bigquery schema definition
  • Deserialize rows into a dataclass instance, while reading from a table
  • Serialize a dataclass instance into a dictionary and write it to the table.

The main benefit of combining all these features is, that we can guarantee that our code will run, before we deploy to production.


Since this library makes use of newer features of python, you need at least python3.7.

  1. Install the package
pip install bq_schema
  1. Create a schema and a table definition in
class Schema:
    string_field: str = field(metadata={"description": "This is a STRING field."})
    int_field: Optional[int]
    some_floats: List[float]
    bool_field: bool

class MyTable(BigqueryTable):
    name = "my_table_name"
    schema = Schema

If you have already tables created in your account, you can use the convert-table script to create a schema.

Note: The script produces a file which is meant to be a starting point. You will most likely have to add some imports yourself!

  1. Create your table

Hint: Make sure to have you credentials set:


Alternativly you can set the service_file as a environment variable:

export GOOGLE_SERVICE_FILE={"type": "service_account", ...}

Now create your table

migrate-tables --project my_project --dataset my_dataset --module-path my_table --apply
  1. Write a row
from import bigquery
row = Schema(string_field="foo", int_field=1, some_floats=[1.0, 2.0], bool_field=True)
row_transformer = RowTransformer[Schema](Schema)
serialized_row = RowTransformer.dataclass_instance_to_bq_row(row)

bigquery_client = bigquery.Client()
table = bigquery_client.get_table("project.dataset.my_table_name")
bigquery_client.insert_rows(table, [serialized_row])
  1. Validate you code with a type checker like mypy
  1. Read a row
query = "SELECT * FROM project.dataset.my_table_name"
for row in bigquery_client.query(query=query):
    deserialized_row = row_transformer.bq_row_to_dataclass_instance(row)
    assert isinstance(deserialized_row, Schema)


Schema definitions

For a full list of supported types check the following schema:

from typing import Optional
from dataclasses import dataclass
from bq_schema.types.type_mapping import Timestamp, Geography

class RequiredNestedField:
    int_field: int = field(metadata={"description": "This field is an INT field."})

class RequiredSchema:
    string_field: str = field(metadata={"description": "This field is a STRING field."})
    string_field_optional = Optional[str]
    bytes_field: bytes
    int_field: int
    float_field: float
    numeric_field: Decimal
    bool_field: bool
    timestamp_field: Timestamp
    date_field: date
    time_field: time
    datetime_field: datetime
    geography_field: Geography
    required_nested_field: RequiredNestedField = field(metadata={"description": "This field is a STRUCT field."})
    optional_nested_field: Optional[RequiredNestedField] 
    repeated_nested_field: List[RequiredNestedField]

Abstract tables

If you want to have an class that inherits from BigqueryTable but does not actually map to a table in BigQuery (is abstract, common interface etc.), you can have it inherit from ABC and it will not be discovered if you pass the flag "--ignore-abstract"

migrate-tables ... --ignore-abstract

class SomeInterface(BigqueryTable, ABC):
class ConcreteImplementation(SomeInterface):
    name="Some value here"
    schema = SomeSchema

Without the flag, this would fail due to name and schema being required in "SomeInterface"


Timestamps are deserialized into datetime objects, due to the nature of the underlying bq library. To distinguish between datetime and timestamp use bq_schema.types.type_mapping. Usage:

from bq_schema.types.type_mapping import Timestamp
from datetime import datetime

the_timestamp = Timestamp(datetime.utcnow())


This library treats the geography data type as a string. BigQuery accepts geography values either in the WKT or GeoJson format. To actually parse and work with geodata in python, one could use the shapely library. Here is an example how to load a point from the WKT format:

from shapely.wkt import loads
loads('POINT (0 0)')

Table definitions

The bigquery class is used for:

  • Recursive table discovery by our migrate-tables script
  • Define table properties like name and schema

Required properties

  • name: The name of the table
  • schema: table schema either as dataclass or a list of schema fields

Optional properties

  • project: name of the project, can be overwritten by the migrate-tables script
  • dataset: name of the dataset, can be overwritten by the migrate-tables script

Versioning tables

Since bigquery does not allow backwards incompatible schema changes, you might want to version your schemas.project

class MyTable(BigqueryTable):
    name = "my_table_name"
    schema = Schema
    version = "1"

By default the version will be appended to the table name, like so: my_table_name_v1. If you want to overwrite this behaviour, you can implement the full_table_name method.

Time partitioning

Define time partitioning for your table:

from bq_schema.types.type_mapping import Timestamp
from import TimePartitioning, TimePartitioningType

class MyTable:
    time_partitioning = TimePartitioning(
        type_=TimePartitioningType.DAY, field="some_column"



This script has two uses:

  • Check if locally defined schemas are in sync with the schemas in bigquery
  • If a difference is detected, we try to apply the changes

The script will find all defined tables recursivly for a given python module.

Note: If you have not defined your project and / or dataset in code, you will have to pass it as a parameter to the script. Show the help:

migrate-tables --help

Check if tables are in sync. List all changes.

migrate-tables --module-path module/

If you want the script to fail on a change, add the validate flag. Useful for running inside your CI:

migrate-tables --module-path module/ --validate

Apply changes

migrate-tables --module-path src/jobs/ --apply


If you already have tables created in bigquery, this script print the corresponding dataclass for you.

Note: The script produces a file which is meant to be a starting point. You will most likely have to add some imports yourself!

Show the help:

convert-table --help

Print a table:

convert-table --project project --dataset scraper --table-name table_name >>


Setting up your dev environment

  1. Clone the project.

  2. Navigate into the cloned project.

  3. Create a virtual environment with python version >=3.7

    pipenv --python PYTHON_VERSION

    $ pipenv --python 3.7



    $ virtualenv -p /usr/bin/python3.7 placeholder
  4. Install flit via pip

    $ pip install flit
  5. Install packages

    $ flit install --symlink

Code quality

Run all code quality checks:

inv check-all
inv test
inv lint
inv type-check
Code format
inv format-code

Validate code is correctly formatted:

inv check-code-format

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