cpython/Doc/library/repr.rst

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:mod:`repr` --- Alternate :func:`repr` implementation
=====================================================
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.. module:: repr
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:synopsis: Alternate repr() implementation with size limits.
.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
.. note::
The :mod:`repr` module has been renamed to :mod:`reprlib` in
Python 3.0.
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The :mod:`repr` module provides a means for producing object representations
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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.
.. versionadded:: 2.4
:attr:`maxset`, :attr:`maxfrozenset`, and :attr:`set`.
.. 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>'