Ignore UTF-8 BOM sequences in various scenarios (#69)

- Adapt the .open() helper to use encoding=utf-8-sig (for reading only)
- Adapt the Reader() to ignore a (single) UTF-8 BOM sequences at the
  start of a line; this handles concatenated files, non-file input, etc.
- Add exhaustive tests

Note that this deliberately does not strip multiple concatenated BOM
sequences since that's indicative of malformed input.

Fixes #68.

jsonlines/jsonlines.py

@@ -3,6 +3,7 @@
 """
 
 import builtins
+import codecs
 import enum
 import io
 import json
@@ -42,6 +43,13 @@
     str,
 }
 
+# Characters to skip at the beginning of a line. Note: at most one such
+# character is skipped per line.
+SKIPPABLE_SINGLE_INITIAL_CHARS = (
+    "\x1e",  # RFC7464 text sequence
+    codecs.BOM_UTF8.decode(),
+)
+
 
 class DumpsResultConversion(enum.Enum):
     LeaveAsIs = enum.auto()
@@ -293,7 +301,7 @@ def read(
                 )
                 raise exc from orig_exc
 
-        if line.startswith("\x1e"):  # RFC7464 text sequence
+        if line.startswith(SKIPPABLE_SINGLE_INITIAL_CHARS):
             line = line[1:]
 
         try:
@@ -611,7 +619,8 @@ def open(
         raise ValueError("'mode' must be either 'r', 'w', or 'a'")
 
     cls = Reader if mode == "r" else Writer
-    fp = builtins.open(file, mode=mode + "t", encoding="utf-8")
+    encoding = "utf-8-sig" if mode == "r" else "utf-8"
+    fp = builtins.open(file, mode=mode + "t", encoding=encoding)
     kwargs = dict(
         loads=loads,
         dumps=dumps,

tests/test_jsonlines.py

@@ -2,8 +2,10 @@
 Tests for the jsonlines library.
 """
 
+import codecs
 import collections
 import io
+import json
 import tempfile
 
 import jsonlines
@@ -38,6 +40,48 @@ def test_reader_rfc7464_text_sequences() -> None:
         assert list(reader) == ["a", "b"]
 
 
+def test_reader_utf8_bom_bytes() -> None:
+    """
+    UTF-8 BOM is ignored, even if it occurs in the middle of a stream.
+    """
+    chunks = [
+        codecs.BOM_UTF8,
+        b"1\n",
+        codecs.BOM_UTF8,
+        b"2\n",
+    ]
+    fp = io.BytesIO(b"".join(chunks))
+    with jsonlines.Reader(fp) as reader:
+        assert list(reader) == [1, 2]
+
+
+def test_reader_utf8_bom_text() -> None:
+    """
+    Text version of ``test_reader_utf8_bom_bytes()``.
+    """
+    chunks = [
+        "1\n",
+        codecs.BOM_UTF8.decode(),
+        "2\n",
+    ]
+    fp = io.StringIO("".join(chunks))
+    with jsonlines.Reader(fp) as reader:
+        assert list(reader) == [1, 2]
+
+
+def test_reader_utf8_bom_bom_bom() -> None:
+    """
+    Too many UTF-8 BOM BOM BOM chars cause BOOM 💥 BOOM.
+    """
+    reader = jsonlines.Reader([codecs.BOM_UTF8.decode() * 3 + "1\n"])
+    with pytest.raises(jsonlines.InvalidLineError) as excinfo:
+        reader.read()
+
+    exc = excinfo.value
+    assert "invalid json" in str(exc)
+    assert isinstance(exc.__cause__, json.JSONDecodeError)
+
+
 def test_writer_text() -> None:
     fp = io.StringIO()
     with jsonlines.Writer(fp) as writer:
@@ -78,6 +122,7 @@ def test_invalid_lines() -> None:
         exc = excinfo.value
         assert "invalid json" in str(exc)
         assert exc.line == data
+        assert isinstance(exc.__cause__, json.JSONDecodeError)
 
 
 def test_skip_invalid() -> None:
@@ -203,6 +248,18 @@ def test_open_reading() -> None:
             assert list(reader) == [123]
 
 
+def test_open_reading_with_utf8_bom() -> None:
+    """
+    The ``.open()`` helper ignores a UTF-8 BOM.
+    """
+    with tempfile.NamedTemporaryFile("wb") as fp:
+        fp.write(codecs.BOM_UTF8)
+        fp.write(b"123\n")
+        fp.flush()
+        with jsonlines.open(fp.name) as reader:
+            assert list(reader) == [123]
+
+
 def test_open_writing() -> None:
     with tempfile.NamedTemporaryFile("w+b") as fp:
         with jsonlines.open(fp.name, mode="w") as writer:
@@ -224,3 +281,10 @@ def test_open_invalid_mode() -> None:
     with pytest.raises(ValueError) as excinfo:
         jsonlines.open("foo", mode="foo")
     assert "mode" in str(excinfo.value)
+
+
+def test_single_char_stripping() -> None:
+    """ "
+    Sanity check that a helper constant actually contains single-char strings.
+    """
+    assert all(len(s) == 1 for s in jsonlines.jsonlines.SKIPPABLE_SINGLE_INITIAL_CHARS)