Better error message if rows_from_file called with StringIO, closes #520

Refs #448

sqlite_utils/utils.py

@@ -303,7 +303,13 @@ def rows_from_file(
     elif format is None:
         # Detect the format, then call this recursively
         buffered = io.BufferedReader(cast(io.RawIOBase, fp), buffer_size=4096)
-        first_bytes = buffered.peek(2048).strip()
+        try:
+            first_bytes = buffered.peek(2048).strip()
+        except AttributeError:
+            # Likely the user passed a TextIO when this needs a BytesIO
+            raise TypeError(
+                "rows_from_file() requires a file-like object that supports peek(), such as io.BytesIO"
+            )
         if first_bytes.startswith(b"[") or first_bytes.startswith(b"{"):
             # TODO: Detect newline-JSON
             return rows_from_file(buffered, format=Format.JSON)

tests/test_rows_from_file.py

@@ -1,5 +1,5 @@
 from sqlite_utils.utils import rows_from_file, Format, RowError
-from io import BytesIO
+from io import BytesIO, StringIO
 import pytest
 
 
@@ -44,3 +44,11 @@ def test_rows_from_file_extra_fields_strategies(ignore_extras, extras_key, expec
             # We did not expect an error
             raise
     assert list_rows == expected
+
+
+def test_rows_from_file_error_on_string_io():
+    with pytest.raises(TypeError) as ex:
+        rows_from_file(StringIO("id,name\r\n1,Cleo"))
+    assert ex.value.args == (
+        "rows_from_file() requires a file-like object that supports peek(), such as io.BytesIO",
+    )