feat: allow lineterminator specification in MetricWriter (#220)

I stumbled upon the fact that `csv.DictWriter` uses windows new line
endings by default. This PR lets you change that when using
`MetricWriter`.

I'm arguing for the default to be `"\n"` instead of `os.linesep`
because:
- [Python has universal new line
support](https://peps.python.org/pep-0278/) (so it doesn't care what we
do)
- If you're working on a shared analysis and checking in files to
version control, you don't want to worry about if you're running on the
same OS as your collaborator
- In my experience, more bioinformatics tools have problems with windows
line endings than with unix line endings

fgpyo/util/metric.py

@@ -450,6 +450,7 @@ def __init__(
         delimiter: str = "\t",
         include_fields: Optional[List[str]] = None,
         exclude_fields: Optional[List[str]] = None,
+        lineterminator: str = "\n",
     ) -> None:
         """
         Args:
@@ -464,6 +465,8 @@ def __init__(
             exclude_fields: If specified, any listed fieldnames will be excluded when writing
                 records to file.
                 May not be used together with `include_fields`.
+            lineterminator: The string used to terminate lines produced by the MetricWriter.
+                Default = "\n".
 
         Raises:
             TypeError: If the provided metric class is not a dataclass- or attr-decorated
@@ -506,6 +509,7 @@ def __init__(
             f=self._fout,
             fieldnames=self._fieldnames,
             delimiter=delimiter,
+            lineterminator=lineterminator,
         )
 
         # If we aren't appending to an existing file, write the header before any rows

tests/fgpyo/util/test_metric.py

@@ -618,6 +618,23 @@ def test_writer_from_str(tmp_path: Path) -> None:
             next(f)
 
 
+@pytest.mark.parametrize("lineterminator", ["\n", "\r", "\r\n"])
+def test_writer_lineterminator(tmp_path: Path, lineterminator: str) -> None:
+    fpath = tmp_path / "test.txt"
+
+    with MetricWriter(
+        filename=fpath, append=False, metric_class=FakeMetric, lineterminator=lineterminator
+    ) as writer:
+        writer.write(FakeMetric(foo="abc", bar=1))
+
+    with fpath.open("r") as f:
+        assert next(f) == "foo\tbar\n"
+        assert repr(f.newlines) == repr(lineterminator)
+        assert next(f) == "abc\t1\n"
+        with pytest.raises(StopIteration):
+            next(f)
+
+
 def test_writer_writeall(tmp_path: Path) -> None:
     fpath = tmp_path / "test.txt"