PEP 813: The Pretty Print Protocol (#4690)

* PEP 813: The Pretty Print Protocol

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Carol Willing <carolcode@willingconsulting.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Eric V. Smith <eric@trueblade.com>

Author: 5 people

.github/CODEOWNERS

@@ -687,7 +687,7 @@ peps/pep-0808.rst  @FFY00
 peps/pep-0809.rst  @zooba
 peps/pep-0810.rst  @pablogsal @DinoV @Yhg1s
 peps/pep-0811.rst  @sethmlarson @gpshead
-# ...
+peps/pep-0813.rst  @warsaw @ericvsmith
 peps/pep-0814.rst  @vstinner @corona10
 peps/pep-0815.rst  @emmatyping
 peps/pep-0816.rst  @brettcannon

peps/pep-0813.rst

@@ -0,0 +1,255 @@
+PEP: 813
+Title: The Pretty Print Protocol
+Author: Barry Warsaw <barry@python.org>, Eric V. Smith <eric@trueblade.com>
+Discussions-To: Pending
+Status: Draft
+Type: Standards Track
+Created: 07-Nov-2025
+Python-Version: 3.15
+Post-History: Pending
+
+
+Abstract
+========
+
+This PEP describes the "pretty print protocol", a collection of changes proposed to make pretty printing more
+customizable and convenient.
+
+
+Motivation
+==========
+
+"Pretty printing" is a feature which provides a capability to format object representations for better
+readability.  The core functionality is implemented by the standard library :mod:`pprint` module.  ``pprint``
+includes a class and APIs which users can invoke to format and print more readable representations of objects,
+versus the standard ``repr()`` built-in function.  Important use cases include pretty printing large
+dictionaries and other complicated objects for debugging purposes.
+
+This PEP builds on the features of the module to provide more customization and user convenience.  It is also
+inspired by the `Rich library's pretty printing protocol <rich-repr-protocol_>`_.
+
+
+Rationale
+=========
+
+Pretty printing is very useful for displaying complex data structures, like dictionaries read from JSON
+content.  However, the existing :mod:`pprint` module can only format builtin objects that it knows about.
+By providing a way for classes to customize how their instances participate in pretty printing,
+users have more options for visually improving the display of their complex data, especially for debugging.
+
+By extending the built-in :func:`print` function to automatically pretty print its output, debugging with
+user-friendly display is made even more convenient.  Since no extra imports are required, users can easily
+just piggyback on well-worn "print debugging" patterns, at least for the most common use cases.
+
+These extensions work both independently and complimentary, to provide powerful new use cases.
+
+
+Specification
+=============
+
+There are several parts to this proposal.
+
+
+``__pprint__()`` methods
+------------------------
+
+Classes can implement a new dunder method, ``__pprint__()`` which if present, generates parts of their
+instance's pretty printed representation.  This augments ``__repr__()`` which, prior to this proposal, was the
+only method used to generate a custom representation of the object.  Since object reprs provide functionality
+distinct from pretty printing, some classes may want more control over their pretty display.  The
+:py:class:`python:pprint.PrettyPrinter` class is modified to respect an object's ``__pprint__()`` method if
+present.
+
+``__pprint__()`` is optional; if missing, the standard pretty printers fall back to ``__repr__()`` for full
+backward compatibility (technically speaking, :py:func:`python:pprint.saferepr` is used).  However, if defined
+on a class, ``__pprint__()`` takes a single argument, the object to be pretty printed (i.e. ``self``).
+
+The method is expected to return or yield a sequence of values, which are used to construct a pretty
+representation of the object.  These values are wrapped in standard class "chrome", such as the
+class name.  The printed representation will usually look like a class constructor, with positional,
+keyword, and default arguments.  The values can be any of the following formats:
+
+* A single value, representing a positional argument.  The value itself is used.
+* A 2-tuple of ``(name, value)`` representing a keyword argument.  A representation of
+  ``name=value`` is used.
+* A 3-tuple of ``(name, value, default_value)`` representing a keyword argument with a default
+  value.  If ``value`` equals ``default_value``, then this tuple is skipped, otherwise
+  ``name=value`` is used.
+
+.. note::
+
+   This protocol is compatible with the `Rich library's pretty printing protocol
+   <https://rich.readthedocs.io/en/latest/pretty.html#rich-repr-protocol>`_.
+
+
+A new argument to built-in ``print``
+------------------------------------
+
+Built-in :func:`print` takes a new optional argument, appended to the end of the argument list, called
+``pretty``, which can take one of the following values:
+
+* ``None`` - the default. No pretty printing is invoked.  Fully backward compatible.
+* ``True`` - use a temporary instance of the :py:class:`python:pprint.PrettyPrinter` class to get a
+  pretty representation of the object.
+* An instance with a ``pformat()`` method, which has the same signature as
+  :py:meth:`python:pprint.PrettyPrinter.pformat`.  When given, this will usually be an instance of a
+  subclass of ``PrettyPrinter`` with its ``pformat()`` method overridden.  Note that this form
+  requires **an instance** of a pretty printer, not a class, as only ``print(..., pretty=True)``
+  performs implicit instantiation.
+
+
+Additions to ``f-strings`` and ``str.format()``
+-----------------------------------------------
+
+In addition to the existing ``!s``, ``!r``, and ``!a`` conversion specifiers, an additional ``!p`` conversion
+will be added.  The effect of this specifier with an expression ``value`` will be to call
+:py:func:`python:pprint.pformat`, passing ``value`` as the only argument. In this initial specification, it
+will be an error to provide any format specifier if ``!p`` is used.
+
+Examples
+========
+
+A custom ``__pprint__()`` method can be used to customize the representation of the object, such as with this
+class:
+
+.. code-block:: python
+
+    class Bass:
+        def __init__(self, strings: int, pickups: str, active: bool=False):
+            self._strings = strings
+            self._pickups = pickups
+            self._active = active
+
+        def __pprint__(self):
+            yield self._strings
+            yield 'pickups', self._pickups
+            yield 'active', self._active, False
+
+Now let's create a couple of instances, and pretty print them:
+
+.. code-block:: pycon
+
+    >>> precision = Bass(4, 'split coil P', active=False)
+    >>> stingray = Bass(5, 'humbucker', active=True)
+
+    >>> pprint.pprint(precision)
+    Bass(4, pickups='split coil P')
+    >>> pprint.pprint(stingray)
+    Bass(5, pickups='humbucker', active=True)
+
+Here's an example of using the ``pretty`` argument to built-in ``print()``:
+
+.. code-block:: pycon
+
+    >>> import os
+    >>> print(os.pathconf_names)
+    {'PC_ASYNC_IO': 17, 'PC_CHOWN_RESTRICTED': 7, 'PC_FILESIZEBITS': 18, 'PC_LINK_MAX': 1, 'PC_MAX_CANON': 2, 'PC_MAX_INPUT': 3, 'PC_NAME_MAX': 4, 'PC_NO_TRUNC': 8, 'PC_PATH_MAX': 5, 'PC_PIPE_BUF': 6, 'PC_PRIO_IO': 19, 'PC_SYNC_IO': 25, 'PC_VDISABLE': 9, 'PC_MIN_HOLE_SIZE': 27, 'PC_ALLOC_SIZE_MIN': 16, 'PC_REC_INCR_XFER_SIZE': 20, 'PC_REC_MAX_XFER_SIZE': 21, 'PC_REC_MIN_XFER_SIZE': 22, 'PC_REC_XFER_ALIGN': 23, 'PC_SYMLINK_MAX': 24}
+
+    >>> print(os.pathconf_names, pretty=True)
+    {'PC_ALLOC_SIZE_MIN': 16,
+     'PC_ASYNC_IO': 17,
+     'PC_CHOWN_RESTRICTED': 7,
+     'PC_FILESIZEBITS': 18,
+     'PC_LINK_MAX': 1,
+     'PC_MAX_CANON': 2,
+     'PC_MAX_INPUT': 3,
+     'PC_MIN_HOLE_SIZE': 27,
+     'PC_NAME_MAX': 4,
+     'PC_NO_TRUNC': 8,
+     'PC_PATH_MAX': 5,
+     'PC_PIPE_BUF': 6,
+     'PC_PRIO_IO': 19,
+     'PC_REC_INCR_XFER_SIZE': 20,
+     'PC_REC_MAX_XFER_SIZE': 21,
+     'PC_REC_MIN_XFER_SIZE': 22,
+     'PC_REC_XFER_ALIGN': 23,
+     'PC_SYMLINK_MAX': 24,
+     'PC_SYNC_IO': 25,
+     'PC_VDISABLE': 9}
+
+
+Backwards Compatibility
+=======================
+
+When none of the new features are used, this PEP is fully backward compatible, both for built-in
+``print()`` and the ``pprint`` module.
+
+
+Security Implications
+=====================
+
+There are no known security implications for this proposal.
+
+
+How to Teach This
+=================
+
+Documentation and examples are added to the ``pprint`` module and the ``print()`` function.
+Beginners don't need to be taught these new features until they want prettier representations of
+their objects.
+
+
+Reference Implementation
+========================
+
+The reference implementation is currently available as a `PEP author branch of the CPython main
+branch <https://github.com/warsaw/cpython/tree/pprint>`__.
+
+
+Rejected Ideas
+==============
+
+None at this time.
+
+
+Open Issues
+===========
+
+The output format and APIs are heavily inspired by `Rich
+<rich-repr-protocol_>`_. The idea is that Rich could
+implement an API compatible with ``print(..., pretty=RichPrinter)`` fairly easily.  Rich's API is designed to
+print constructor-like representations of instances, which means that it's not possible to control much of the
+"class chrome" around the arguments.  Rich does support using angle brackets (i.e. ``<...>``) instead of
+parentheses by setting the attribute ``.angular=True`` on the rich repr method.  This PEP does not support
+that feature, although it likely could in the future.
+
+This also means that there's no way to control the pretty printed format of built-in types like strings,
+dicts, lists, etc.  This seems fine as ``pprint`` is not intended to be as feature-rich (pun intended!) as
+Rich.  This PEP purposefully deems such fancy features as out-of-scope.
+
+One consequence of ``print(..., pretty=True)`` is that it can be more less obvious if you wanted to print
+multiple objects with, say a newline between the object representations.  Compare these two outputs:
+
+.. code-block:: pycon
+
+    >>> print(precision, '\n', stingray, pretty=True)
+    Bass(4, pickups='split coil P') '\n' Bass(5, pickups='humbucker', active=True)
+
+    >>> print(precision, stingray, sep='\n', pretty=True)
+    Bass(4, pickups='split coil P')
+    Bass(5, pickups='humbucker', active=True)
+
+It's likely you'll want the second output, but more complicated multi-object displays could get even less
+convenient and/or more verbose.
+
+
+Acknowledgments
+===============
+
+TBD
+
+
+Footnotes
+=========
+
+TBD
+
+
+Copyright
+=========
+
+This document is placed in the public domain or under the
+CC0-1.0-Universal license, whichever is more permissive.
+
+
+.. _rich-repr-protocol: https://rich.readthedocs.io/en/stable/pretty.html#rich-repr-protocol