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: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>'
|
