From 96dc6b7d542c3298e22f800f153c51582c30bc9c Mon Sep 17 00:00:00 2001
From: AC <ac@fabtek.cn>
Date: Thu, 15 Oct 2026 13:21:32 +0800
Subject: [PATCH] Support current dtype shorthand in Array.pp

---
 bitstring/array_.py |  9 ++++-----
 doc/array.rst       |  6 ++++++
 release_notes.md    |  5 +++++
 tests/test_array.py | 31 +++++++++++++++++++++++++++++++
 4 files changed, 46 insertions(+), 5 deletions(-)

diff --git a/bitstring/array_.py b/bitstring/array_.py
index ad72bfca..5846b096 100644
--- a/bitstring/array_.py
+++ b/bitstring/array_.py
@@ -571,7 +571,8 @@ def pp(self, fmt: str | None = None, width: int = 120, sep: str = ' ',
            show_offset: bool = True, stream: TextIO | None = None, color: bool | None = None) -> None:
         """Pretty-print the Array contents.
 
-        fmt -- Data format string. Defaults to current Array dtype.
+        fmt -- Data format string. Defaults to current Array dtype. Use '_' to refer to
+               the current dtype within a format, e.g. '_, bin'.
         width -- Max width of printed lines in characters. Defaults to 120. A single group will always
                  be printed per line even if it exceeds the max width.
         sep -- A separator string to insert between groups. Defaults to a single space.
@@ -593,11 +594,9 @@ def pp(self, fmt: str | None = None, width: int = 120, sep: str = ' ',
             token_list = utils.preprocess_tokens(fmt)
             if len(token_list) not in [1, 2]:
                 raise ValueError(f"Only one or two tokens can be used in an Array.pp() format - '{fmt}' has {len(token_list)} tokens.")
-            name1, length1 = utils.parse_name_length_token(token_list[0])
-            dtype1 = Dtype(name1, length1)
+            dtype1 = self.dtype if token_list[0] == '_' else Dtype(token_list[0])
             if len(token_list) == 2:
-                name2, length2 = utils.parse_name_length_token(token_list[1])
-                dtype2 = Dtype(name2, length2)
+                dtype2 = self.dtype if token_list[1] == '_' else Dtype(token_list[1])
 
         token_length = dtype1.bitlength
         if dtype2 is not None:
diff --git a/doc/array.rst b/doc/array.rst
index ab6ee3e1..0f7748c7 100644
--- a/doc/array.rst
+++ b/doc/array.rst
@@ -304,6 +304,12 @@ Methods
 
     The format string `fmt` defaults to the Array's current :attr:`dtype`, but any other valid Array format string can be used.
 
+    Use ``_`` for the current :attr:`dtype` in either position of the format string. For example,
+    ``a.pp('_, bin')`` prints each element using its current dtype alongside its binary representation.
+    The dtype is resolved each time ``pp()`` is called, so changing ``a.dtype`` also changes
+    the interpretation used by ``_``. A second format with an explicit length must have the
+    same bit length as the current dtype.
+
     If a `fmt` doesn't have an explicit length, the Array's :attr:`itemsize` will be used.
 
     A pair of comma-separated format strings can also be used - if both formats specify a length they must be the same. For example ``'f, hex16'`` or ``'u4, bin4'``.
diff --git a/release_notes.md b/release_notes.md
index 9f0dc24d..3ccd053a 100644
--- a/release_notes.md
+++ b/release_notes.md
@@ -1,6 +1,11 @@
 
 # Release Notes
 
+## Unreleased
+
+* `Array.pp()` accepts `_` as shorthand for the current array dtype, including
+  in two-format output such as `a.pp('_, bin')`.
+
 ## October 2026: version 5.0.0 released
 
 A major release which completes the move to using the `tibs` Rust library as the core.
diff --git a/tests/test_array.py b/tests/test_array.py
index 6e4323b6..deb4870f 100644
--- a/tests/test_array.py
+++ b/tests/test_array.py
@@ -736,6 +736,37 @@ def test_pp_sep(self):
                                         " 0: c0403f804000\n" \
                                         "]\n"
 
+    @pytest.mark.parametrize('fmt', ['_, bin', 'bin, _', '_', '_, _'])
+    def test_pp_current_dtype(self, fmt):
+        a = Array('u8', [1, 255])
+        actual, expected = io.StringIO(), io.StringIO()
+        a.pp(fmt, stream=actual, color=False)
+        a.pp(fmt.replace('_', 'u8'), stream=expected, color=False)
+        assert actual.getvalue() == expected.getvalue()
+        assert a.tolist() == [1, 255]
+
+    def test_pp_current_dtype_after_change(self):
+        a = Array('u8', [255])
+        a.dtype = 'i8'
+        a.data += '0b11'
+        s = io.StringIO()
+        a.pp('_, hex', stream=s, color=False, show_offset=False)
+        assert s.getvalue() == "<Array fmt='i8, hex', length=1, itemsize=8 bits, total data size=2 bytes> [\n" \
+                               "  -1 : ff\n] + trailing_bits = 0b11\n"
+
+    def test_pp_current_dtype_bytes(self):
+        a = Array('bytes2', [b'ab'])
+        s = io.StringIO()
+        a.pp('_, hex', stream=s, color=False, show_offset=False)
+        assert "itemsize=16 bits" in s.getvalue()
+        assert "ab : 6162" in s.getvalue()
+
+    @pytest.mark.parametrize('fmt', ['_, hex16', 'hex16, _'])
+    def test_pp_current_dtype_mismatched_length(self, fmt):
+        a = Array('u8', [1])
+        with pytest.raises(ValueError, match='Two different format lengths'):
+            a.pp(fmt)
+
     def test_pp_signature_matches_bits(self):
         # The two pp() methods should stay positionally interchangeable.
         import inspect