PEP 813: The Pretty Print Protocol

Author: warsaw

peps/pep-0813.rst

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+PEP: 813
+Title: The Pretty Print Protocol
+Author: Barry Warsaw <barry@python.org>,
+        Eric V. Smith <eric at 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`.  ``pprint``
+includes a class and APIs which users can invoke to format and print more readable representations of objects.
+Important use cases include pretty printing large dictionaries and other complicated objects.
+
+The ``pprint`` module is great as far as it goes.  This PEP builds on the features of this module to provide
+more customization and convenience.
+
+
+Rationale
+=========
+
+Pretty printing is very useful for displaying complex data structures, like dictionaries read from JSON
+content.  By providing a way for classes to customize how their instances participate in pretty printing,
+users have more options for visually improving the display and debugging of their complex data.
+
+By extending the built-in :py:func:`print` function to automatically pretty print its output, this feature is
+made even more convenient, since no extra imports are required, and users can easily just piggyback on
+well-worn "print debugging" patterns, at least for the most common use cases.
+
+These two extensions work independently, but hand-in-hand can provide a powerful and convenient new feature.
+
+
+Specification
+=============
+
+There are two parts to this proposal.
+
+
+``__pretty__()`` methods
+------------------------
+
+Classes can implement a new dunder method, ``__pretty__()`` which if present, generates the pretty printed
+representation of their instances.  This augments ``__repr__()`` which, prior to this proposal, was the only
+method used to generate a pretty representation of the object.  Since object reprs provide functionality
+distinct from pretty printing, some classes may want more control over their pretty display.
+
+``__pretty__()`` is optional; if missing, the standard pretty printers fall back to ``__repr__()`` for full
+backward compatibility (technically speaking, :meth:`pprint.saferepr` is used).  However, if defined on a
+class, ``__pretty__()`` has the same argument signature as :py:func:`PrettyPrinter.format`, taking four
+arguments:
+
+* ``object`` - the object to print, which is effectively always ``self``
+* ``context`` - a dictionary mapping the ``id()`` of objects which are part of the current presentation
+  context
+* ``maxlevels`` - the requested limit to recursion
+* ``levels`` - the current recursion level
+
+Similarly, ``__pretty__()`` returns three values, the string to be used as the pretty printed representation,
+a boolean indicating whether the returned value is "readable", and a boolean indicating whether recursion has
+been detected.  In this context, "readable" means the same as :meth:`PrettyPrinter.isreadable`, i.e. that the
+returned value can be used to reconstruct the original object using ``eval()``.
+
+See :py:func:`PrettyPrinter.format` for details.
+
+
+A new argument to built-in ``print``
+------------------------------------
+
+Built-in :py: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; fully backward compatible
+* ``True`` - use a temporary instance of the :py:class:`PrettyPrinter` class to get a pretty representation of
+  the object.
+* An instance with a ``pformat()`` method, which has the same signature as
+  :meth:`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.
+
+
+Examples
+========
+
+A custom ``__pprint__()`` method can be used to customize the representation of the object:
+
+.. _code-block:
+
+    >>> class Custom:
+    ...     def __str__(self): return 'my str'
+    ...     def __repr__(self): return 'my repr'
+    ...     def __pprint__(self, context, maxlevels, level): return 'my pprint'
+
+    >>> pprint.pp(Custom())
+    my pprint
+
+Using the ``pretty`` argument to ``print()``:
+
+.. _code-block:
+
+    >>> 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
+===========
+
+TBD
+
+Acknowledgements
+================
+
+TBD
+
+
+Footnotes
+=========
+
+TBD
+
+
+Copyright
+=========
+
+This document is placed in the public domain or under the
+CC0-1.0-Universal license, whichever is more permissive.