cpython/Doc/library/reprlib.rst

:mod:`reprlib` --- Alternate :func:`repr` implementation
========================================================

.. module:: reprlib
   :synopsis: Alternate repr() implementation with size limits.
.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>

**Source code:** :source:`Lib/reprlib.py`

--------------

The :mod:`reprlib` module provides a means for producing object representations
with limits on the size of the resulting strings. This is used in the Python
debugger and may be useful in other contexts as well.

This module provides a class, an instance, and a function:


.. class:: Repr()

   Class which provides formatting services useful in implementing functions
   similar to the built-in :func:`repr`; size limits for  different object types
   are added to avoid the generation of representations which are excessively long.


.. data:: aRepr

   This is an instance of :class:`Repr` which is used to provide the
   :func:`.repr` function described below.  Changing the attributes of this
   object will affect the size limits used by :func:`.repr` and the Python
   debugger.


.. function:: repr(obj)

   This is the :meth:`~Repr.repr` method of ``aRepr``.  It returns a string
   similar to that returned by the built-in function of the same name, but with
   limits on most sizes.

In addition to size-limiting tools, the module also provides a decorator for
detecting recursive calls to :meth:`__repr__` and substituting a placeholder
string instead.

.. decorator:: recursive_repr(fillvalue="...")

   Decorator for :meth:`__repr__` methods to detect recursive calls within the
   same thread.  If a recursive call is made, the *fillvalue* is returned,
   otherwise, the usual :meth:`__repr__` call is made.  For example:

        >>> class MyList(list):
        ...     @recursive_repr()
        ...     def __repr__(self):
        ...         return '<' + '|'.join(map(repr, self)) + '>'
        ...
        >>> m = MyList('abc')
        >>> m.append(m)
        >>> m.append('x')
        >>> print(m)
        <'a'|'b'|'c'|...|'x'>

   .. versionadded:: 3.2


.. _repr-objects:

Repr Objects
------------

:class:`Repr` instances provide several attributes which can be used to provide
size limits for the representations of different object types,  and methods
which format specific object types.


.. attribute:: Repr.maxlevel

   Depth limit on the creation of recursive representations.  The default is ``6``.


.. attribute:: Repr.maxdict
               Repr.maxlist
               Repr.maxtuple
               Repr.maxset
               Repr.maxfrozenset
               Repr.maxdeque
               Repr.maxarray

   Limits on the number of entries represented for the named object type.  The
   default is ``4`` for :attr:`maxdict`, ``5`` for :attr:`maxarray`, and  ``6`` for
   the others.


.. attribute:: Repr.maxlong

   Maximum number of characters in the representation for an integer.  Digits
   are dropped from the middle.  The default is ``40``.


.. attribute:: Repr.maxstring

   Limit on the number of characters in the representation of the string.  Note
   that the "normal" representation of the string is used as the character source:
   if escape sequences are needed in the representation, these may be mangled when
   the representation is shortened.  The default is ``30``.


.. attribute:: Repr.maxother

   This limit is used to control the size of object types for which no specific
   formatting method is available on the :class:`Repr` object. It is applied in a
   similar manner as :attr:`maxstring`.  The default is ``20``.


.. method:: Repr.repr(obj)

   The equivalent to the built-in :func:`repr` that uses the formatting imposed by
   the instance.


.. method:: Repr.repr1(obj, level)

   Recursive implementation used by :meth:`.repr`.  This uses the type of *obj* to
   determine which formatting method to call, passing it *obj* and *level*.  The
   type-specific methods should call :meth:`repr1` to perform recursive formatting,
   with ``level - 1`` for the value of *level* in the recursive  call.


.. method:: Repr.repr_TYPE(obj, level)
   :noindex:

   Formatting methods for specific types are implemented as methods with a name
   based on the type name.  In the method name, **TYPE** is replaced by
   ``'_'.join(type(obj).__name__.split())``. Dispatch to these methods is
   handled by :meth:`repr1`. Type-specific methods which need to recursively
   format a value should call ``self.repr1(subobj, level - 1)``.


.. _subclassing-reprs:

Subclassing Repr Objects
------------------------

The use of dynamic dispatching by :meth:`Repr.repr1` allows subclasses of
:class:`Repr` to add support for additional built-in object types or to modify
the handling of types already supported. This example shows how special support
for file objects could be added::

   import reprlib
   import sys

   class MyRepr(reprlib.Repr):

       def repr_TextIOWrapper(self, obj, level):
           if obj.name in {'<stdin>', '<stdout>', '<stderr>'}:
               return obj.name
           return repr(obj)

   aRepr = MyRepr()
   print(aRepr.repr(sys.stdin))         # prints '<stdin>'