cpython/Doc/library/repr.rst

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

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


The :mod:`repr` 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` 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.


.. _repr-objects:

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

:class:`Repr` instances provide several members 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 a long 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
   ``string.join(string.split(type(obj).__name__, '_'))``. 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 repr
   import sys

   class MyRepr(repr.Repr):
       def repr_file(self, obj, level):
           if obj.name in ['<stdin>', '<stdout>', '<stderr>']:
               return obj.name
           else:
               return `obj`

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