add a convert_to_undecoded_string preprocessor (#100)

This preprocessor is not used by default, and will have to be added by
the application at runtime as an argument to format_output().

It is designed to be run before other preprocessors, which one reason
why it preserves Nones.

Unlike the default convert_to_string(), it does not attempt to decode
some bytes as UTF-8.  It therefore treats all byte values the same way.

Author: rolandwalker

CHANGELOG

@@ -1,5 +1,11 @@
 # Changelog
 
+## Version 2.9.0
+
+(released on 2026-01-26)
+
+- Add optional `convert_to_undecoded_string` preprocessor.
+
 ## Version 2.8.2
 
 (released on 2026-01-26)

cli_helpers/tabular_output/preprocessors.py

@@ -55,6 +55,26 @@ def convert_to_string(data, headers, **_):
     )
 
 
+def convert_to_undecoded_string(data, headers, **_):
+    """Convert all *data* and *headers* to hex, if needed.
+
+    Binary data is converted to a hexadecimal representation via
+    :func:`binascii.hexlify`.
+
+    Unlike convert_to_string(), None values are left as Nones.
+
+    :param iterable data: An :term:`iterable` (e.g. list) of rows.
+    :param iterable headers: The column headers.
+    :return: The processed data and headers.
+    :rtype: tuple
+
+    """
+    return (
+        ([utils.to_undecoded_string(v) for v in row] for row in data),
+        [utils.to_undecoded_string(h) for h in headers],
+    )
+
+
 def override_missing_value(
     data,
     headers,

cli_helpers/utils.py

@@ -34,6 +34,17 @@ def bytes_to_string(b):
     return b
 
 
+def to_hex_if_bin(b):
+    """Convert bytes *b* to a string.
+
+    Pass b through if not bytes.
+
+    """
+    if isinstance(b, binary_type):
+        return "0x" + binascii.hexlify(b).decode("ascii")
+    return b
+
+
 def to_string(value):
     """Convert *value* to a string."""
     if isinstance(value, binary_type):
@@ -42,6 +53,19 @@ def to_string(value):
         return text_type(value)
 
 
+def to_undecoded_string(value):
+    """Convert *value* to an undecoded string, respecting Nones."""
+    # preserve Nones so that
+    # * this can run before override_missing_value when stringifying
+    # * Nones are preserved in formats such as CSV
+    if value is None:
+        return None
+    elif isinstance(value, binary_type):
+        return to_hex_if_bin(value)
+    else:
+        return text_type(value)
+
+
 def truncate_string(value, max_width=None, skip_multiline_string=True):
     """Truncate string values."""
     if skip_multiline_string and isinstance(value, text_type) and "\n" in value:

tests/tabular_output/test_preprocessors.py

@@ -11,6 +11,7 @@
     align_decimals,
     bytes_to_string,
     convert_to_string,
+    convert_to_undecoded_string,
     quote_whitespaces,
     override_missing_value,
     override_tab_value,
@@ -38,6 +39,16 @@ def test_convert_to_string():
     assert expected == (list(results[0]), results[1])
 
 
+def test_convert_to_undecoded_string():
+    """Test the convert_to_undecoded_string() function."""
+    data = [[1, "John"], [2, b"Jill"], [3, None]]
+    headers = [0, "name"]
+    expected = ([["1", "John"], ["2", "0x4a696c6c"], ["3", None]], ["0", "name"])
+    results = convert_to_undecoded_string(data, headers)
+
+    assert expected == (list(results[0]), results[1])
+
+
 def test_override_missing_values():
     """Test the override_missing_values() function."""
     data = [[1, None], [2, "Jill"]]