Respect existing line endings (#122)

Fixes #104

src/tox_ini_fmt/__main__.py

@@ -27,13 +27,19 @@ def run(args: Sequence[str] | None = None) -> int:
     opts = cli_args(sys.argv[1:] if args is None else args)
     changed = False
     for tox_ini in opts.tox_ini:
-        formatted = format_tox_ini(tox_ini, opts)
-        before = tox_ini.read_text()
+        with tox_ini.open("rt") as file:
+            before = file.read()
+            original_newlines = file.newlines
+        if isinstance(original_newlines, tuple):
+            original_newlines = original_newlines[0]
+        formatted = format_tox_ini(before, opts)
         changed |= before != formatted
         if opts.stdout:  # stdout just prints new format to stdout
             print(formatted, end="")
         else:
-            tox_ini.write_text(formatted)
+            if before != formatted:
+                with tox_ini.open("wt", newline=original_newlines) as file:
+                    file.write(formatted)
             try:
                 name = str(tox_ini.relative_to(Path.cwd()))
             except ValueError:

src/tox_ini_fmt/formatter/__init__.py

@@ -13,12 +13,15 @@
 INDENTATION = "    "
 
 
-def format_tox_ini(tox_ini: Path, opts: ToxIniFmtNamespace | None = None) -> str:
+def format_tox_ini(tox_ini: str | Path, opts: ToxIniFmtNamespace | None = None) -> str:
     if opts is None:
         opts = ToxIniFmtNamespace(pin_toxenvs=[])
     parser = ConfigParser(interpolation=None)
-    with tox_ini.open("rt"):
-        parser.read([tox_ini])
+    if isinstance(tox_ini, Path):
+        text = tox_ini.read_text()
+    else:
+        text = tox_ini
+    parser.read_string(text)
 
     order_sections(parser, opts.pin_toxenvs)
     format_tox_section(parser, opts.pin_toxenvs)

tests/formatter/test_line_endings.py

@@ -0,0 +1,46 @@
+from __future__ import annotations
+
+import os
+
+import pytest
+
+from tox_ini_fmt.__main__ import run
+
+
+def test_platform_default(tox_ini):
+    """If the ini file has no newlines, the platform default may be inserted."""
+
+    tox_ini.write_bytes(b"[tox]")
+    run([str(tox_ini)])
+    assert tox_ini.read_bytes() == f"[tox]{os.linesep}".encode()
+
+
+@pytest.mark.parametrize("newline", ["\r\n", "\n", "\r"])
+def test_line_endings(tox_ini, newline):
+    """The ini file's existing newlines must be respected when reformatting."""
+
+    original_text = f"[tox]{newline}envlist=py39"
+    expected_text = f"[tox]{newline}envlist ={newline}    py39{newline}"
+    tox_ini.write_bytes(original_text.encode("utf8"))
+    run([str(tox_ini)])
+    assert tox_ini.read_bytes() == expected_text.encode("utf8")
+
+
+def test_mixed_line_endings(tox_ini):
+    """If mixed line endings are found, the first one in the tuple should be used.
+
+    Note that this does not mean the first newline in the file will be used!
+    Python does not report the newlines in the order they're encountered.
+    """
+
+    original_text = "[tox]\r\n \r \nenvlist=py39"
+    expected_text = "[tox]!!envlist =!!    py39!!"
+    tox_ini.write_bytes(original_text.encode("utf8"))
+    with tox_ini.open("rt") as file:
+        file.read()
+        assert set(file.newlines) == {"\r", "\n", "\r\n"}
+        first_newline = file.newlines[0]
+
+    expected_text = expected_text.replace("!!", first_newline)
+    run([str(tox_ini)])
+    assert tox_ini.read_bytes() == expected_text.encode("utf8")