CSV file format

csv is the most basic file format to store tabular data, where all the values are strings and are separated by a delimiter (typically comma). dlt uses it for specific use cases - mostly for the performance and compatibility reasons.

Internally we use two implementations:

• pyarrow csv writer - very fast, multithreaded writer for the arrow tables
• python stdlib writer - a csv writer included in the Python standard library for Python objects

Supported Destinations

Supported by: Postgres, Filesystem

By setting the loader_file_format argument to csv in the run command, the pipeline will store your data in the csv format at the destination:

info = pipeline.run(some_source(), loader_file_format="csv")

Default Settings

dlt attempts to make both writers to generate similarly looking files

• separators are commas
• quotes are " and are escaped as ""
• NULL values are empty strings
• UNIX new lines are used
• dates are represented as ISO 8601
• quoting style is "when needed"

Change settings

You can change basic csv settings, this may be handy when working with filesystem destination. Other destinations are tested with standard settings:

• delimiter: change the delimiting character (default: ',')
• include_header: include the header row (default: True)
• quoting: quote_all - all values are quoted, quote_needed - quote only values that need quoting (default: quote_needed)

When quote_needed is selected: in case of Python csv writer all non-numeric values are quoted. In case of pyarrow csv writer, the exact behavior is not described in the documentation. We observed that in some cases, strings are not quoted as well.

[normalize.data_writer]
delimiter="|"
include_header=false
quoting="quote_all"

Or using environment variables:

NORMALIZE__DATA_WRITER__DELIMITER=|
NORMALIZE__DATA_WRITER__INCLUDE_HEADER=False
NORMALIZE__DATA_WRITER__QUOTING=quote_all

Limitations

arrow writer

• binary columns are supported only if they contain valid UTF-8 characters
• complex (nested, struct) types are not supported

csv writer

• binary columns are supported only if they contain valid UTF-8 characters (easy to add more encodings)
• complex columns dumped with json.dumps
• None values are always quoted