typeline

Write dataclasses to delimited text formats and read them back again.

Features type-safe parsing, optional field support, and an intuitive API for working with structured data.

Installation

The package can be installed with pip:

pip install typeline

Quickstart

Building a Test Dataclass

>>> from dataclasses import dataclass
>>>
>>> @dataclass
... class MyData:
...     field1: int
...     field2: str
...     field3: float | None

Writing

>>> from tempfile import NamedTemporaryFile
>>> from typeline import TsvWriter
>>> 
>>> temp_file = NamedTemporaryFile(mode="w+t", suffix=".tsv")
>>>
>>> with TsvWriter.from_path[MyData](temp_file.name) as writer:
...     writer.write_header()
...     writer.write(MyData(10, "test1", 0.2))
...     writer.write(MyData(20, "test2", None))

Reading

>>> from typeline import TsvReader
>>> 
>>> with TsvReader.from_path[MyData](temp_file.name) as reader:
...     for record in reader:
...         print(record)
MyData(field1=10, field2='test1', field3=0.2)
MyData(field1=20, field2='test2', field3=None)

Any Text Stream

To use an open text stream instead of a path, subscript the reader or writer class itself.

>>> import gzip
>>>
>>> with TsvWriter[MyData](gzip.open(f"{temp_file.name}.gz", "wt")) as writer:
...     writer.write(MyData(10, "test1", 0.2))
>>>
>>> with TsvReader[MyData](gzip.open(f"{temp_file.name}.gz", "rt"), header=False) as reader:
...     print(list(reader))
[MyData(field1=10, field2='test1', field3=0.2)]

Missing Values

None is written as an empty field. When read, an empty field is None if the field allows None, and "" if it is a str. Set none_field, e.g. to "NA", when an optional text field must tell "" and None apart.

Comments

A reader skips lines that start with any of its comment_prefixes, and hands each one to on_comment as a Comment with its line number. A writer writes comments with write_comment, so comments can be passed straight from a reader to a writer and keep their places.

>>> _ = open(temp_file.name, "w").write("# made by a tool\nfield1\tfield2\tfield3\n10\ttest1\t0.2\n")
>>>
>>> with (
...     TsvWriter.from_path[MyData](f"{temp_file.name}.copy") as writer,
...     TsvReader.from_path[MyData](temp_file.name, comment_prefixes={"#"}, on_comment=writer.write_comment) as reader,
... ):
...     writer.write_header()
...     for record in reader:
...         writer.write(record)
>>>
>>> print(open(f"{temp_file.name}.copy").read(), end="")
# made by a tool
field1	field2	field3
10	test1	0.2

Custom Field Formats

Lists, dicts, sets, enums, and nested dataclasses are written as JSON by default. For any other text format, a FieldCodec reads a field from its text and writes it back, chosen by the field's type. Helpers in typeline.codecs cover common formats.

>>> from datetime import date
>>> from typeline import Codecs, FieldCodec
>>> from typeline.codecs import boolean, delimited
>>>
>>> @dataclass
... class Visit:
...     patient: str
...     seen: date
...     consented: bool
...     blocks: list[int]
>>>
>>> codecs: Codecs = {
...     date: FieldCodec(from_text=date.fromisoformat, into_text=date.isoformat),
...     bool: boolean(true="Y", false="N"),
...     list[int]: delimited(int, sep=";"),
... }
>>>
>>> with TsvWriter.from_path[Visit](temp_file.name, codecs=codecs) as writer:
...     writer.write(Visit("P-001", date(2026, 9, 29), True, [3, 1, 4]))
>>>
>>> print(open(temp_file.name).read(), end="")
P-001	2026-09-29	Y	3;1;4
>>>
>>> with TsvReader.from_path[Visit](temp_file.name, header=False, codecs=codecs) as reader:
...     print(list(reader))
[Visit(patient='P-001', seen=datetime.date(2026, 9, 29), consented=True, blocks=[3, 1, 4])]

Custom types nested anywhere inside a field, like a list[Interval], are handled by enc_hook, which turns such an object into builtin values, and dec_hook, which builds it back from them. Each hook raises NotImplementedError for types it does not handle.

>>> class Interval:
...     def __init__(self, start: int, end: int) -> None:
...         self.start = start
...         self.end = end
...
...     def __repr__(self) -> str:
...         return f"Interval({self.start}, {self.end})"
>>>
>>> def enc_hook(obj: object) -> object:
...     if isinstance(obj, Interval):
...         return [obj.start, obj.end]
...     raise NotImplementedError
>>>
>>> def dec_hook(kind: type, obj: object) -> object:
...     if kind is Interval:
...         return Interval(*obj)
...     raise NotImplementedError
>>>
>>> @dataclass
... class Target:
...     gene: str
...     intervals: list[Interval]
>>>
>>> with TsvWriter.from_path[Target](temp_file.name, enc_hook=enc_hook) as writer:
...     writer.write(Target("BRCA1", [Interval(1, 9), Interval(20, 25)]))
>>>
>>> print(open(temp_file.name).read(), end="")
BRCA1	[[1,9],[20,25]]
>>>
>>> with TsvReader.from_path[Target](temp_file.name, header=False, dec_hook=dec_hook) as reader:
...     print(list(reader))
[Target(gene='BRCA1', intervals=[Interval(1, 9), Interval(20, 25)])]

Your Own Format

Subclass a reader to give a format its own defaults.

>>> from typing import TextIO
>>> from typing_extensions import Unpack
>>> from typeline import ReaderOptions, RecordType
>>> from typeline.codecs import key_value
>>>
>>> class VcfLikeReader(TsvReader[RecordType]):
...     def __init__(self, handle: TextIO, /, **options: Unpack[ReaderOptions]) -> None:
...         _ = options.setdefault("header", False)
...         _ = options.setdefault("comment_prefixes", {"#"})
...         _ = options.setdefault("none_field", ".")
...         _ = options.setdefault("codecs", {dict[str, str]: key_value()})
...         super().__init__(handle, **options)
>>>
>>> @dataclass
... class Site:
...     chrom: str
...     pos: int
...     ident: str | None
...     info: dict[str, str]
>>>
>>> _ = open(temp_file.name, "w").write("#CHROM\tPOS\tID\tINFO\nchr1\t100\t.\tDP=10;AF=0.5\n")
>>>
>>> with VcfLikeReader.from_path[Site](temp_file.name) as reader:
...     print(list(reader))
[Site(chrom='chr1', pos=100, ident=None, info={'DP': '10', 'AF': '0.5'})]

Type checkers see VcfLikeReader.from_path[Site](...) as a TsvReader[Site], its closest built-in reader.

A reader fixed to one record type can add FixedRecordType to its bases, and is then built without a subscript.

>>> from typeline import FixedRecordType
>>>
>>> class SiteReader(VcfLikeReader[Site], FixedRecordType):
...     pass
>>>
>>> with SiteReader.from_path(temp_file.name) as reader:
...     print(list(reader))
[Site(chrom='chr1', pos=100, ident=None, info={'DP': '10', 'AF': '0.5'})]

More examples, from sample sheets to GFF3 and BED-like data, are in tests/test_real_world_examples.py.

Development and Testing

See the contributing guide for more information.