160 lines
5.1 KiB
ReStructuredText
160 lines
5.1 KiB
ReStructuredText
:mod:`reprlib` --- Alternate :func:`repr` implementation
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========================================================
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.. module:: reprlib
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:synopsis: Alternate repr() implementation with size limits.
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.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
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**Source code:** :source:`Lib/reprlib.py`
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--------------
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The :mod:`reprlib` 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
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debugger and may be useful in other contexts as well.
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This module provides a class, an instance, and a function:
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.. class:: Repr()
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Class which provides formatting services useful in implementing functions
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similar to the built-in :func:`repr`; size limits for different object types
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are added to avoid the generation of representations which are excessively long.
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.. data:: aRepr
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This is an instance of :class:`Repr` which is used to provide the
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:func:`.repr` function described below. Changing the attributes of this
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object will affect the size limits used by :func:`.repr` and the Python
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debugger.
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.. function:: repr(obj)
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This is the :meth:`~Repr.repr` method of ``aRepr``. It returns a string
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similar to that returned by the built-in function of the same name, but with
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limits on most sizes.
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In addition to size-limiting tools, the module also provides a decorator for
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detecting recursive calls to :meth:`__repr__` and substituting a placeholder
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string instead.
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.. decorator:: recursive_repr(fillvalue="...")
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Decorator for :meth:`__repr__` methods to detect recursive calls within the
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same thread. If a recursive call is made, the *fillvalue* is returned,
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otherwise, the usual :meth:`__repr__` call is made. For example:
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>>> class MyList(list):
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... @recursive_repr()
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... def __repr__(self):
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... return '<' + '|'.join(map(repr, self)) + '>'
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...
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>>> m = MyList('abc')
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>>> m.append(m)
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>>> m.append('x')
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>>> print(m)
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<'a'|'b'|'c'|...|'x'>
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.. versionadded:: 3.2
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.. _repr-objects:
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Repr Objects
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------------
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:class:`Repr` instances provide several attributes which can be used to provide
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size limits for the representations of different object types, and methods
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which format specific object types.
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.. attribute:: Repr.maxlevel
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Depth limit on the creation of recursive representations. The default is ``6``.
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.. attribute:: Repr.maxdict
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Repr.maxlist
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Repr.maxtuple
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Repr.maxset
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Repr.maxfrozenset
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Repr.maxdeque
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Repr.maxarray
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Limits on the number of entries represented for the named object type. The
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default is ``4`` for :attr:`maxdict`, ``5`` for :attr:`maxarray`, and ``6`` for
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the others.
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.. attribute:: Repr.maxlong
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Maximum number of characters in the representation for an integer. Digits
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are dropped from the middle. The default is ``40``.
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.. attribute:: Repr.maxstring
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Limit on the number of characters in the representation of the string. Note
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that the "normal" representation of the string is used as the character source:
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if escape sequences are needed in the representation, these may be mangled when
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the representation is shortened. The default is ``30``.
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.. attribute:: Repr.maxother
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This limit is used to control the size of object types for which no specific
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formatting method is available on the :class:`Repr` object. It is applied in a
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similar manner as :attr:`maxstring`. The default is ``20``.
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.. method:: Repr.repr(obj)
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The equivalent to the built-in :func:`repr` that uses the formatting imposed by
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the instance.
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.. method:: Repr.repr1(obj, level)
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Recursive implementation used by :meth:`.repr`. This uses the type of *obj* to
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determine which formatting method to call, passing it *obj* and *level*. The
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type-specific methods should call :meth:`repr1` to perform recursive formatting,
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with ``level - 1`` for the value of *level* in the recursive call.
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.. method:: Repr.repr_TYPE(obj, level)
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:noindex:
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Formatting methods for specific types are implemented as methods with a name
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based on the type name. In the method name, **TYPE** is replaced by
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``'_'.join(type(obj).__name__.split())``. Dispatch to these methods is
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handled by :meth:`repr1`. Type-specific methods which need to recursively
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format a value should call ``self.repr1(subobj, level - 1)``.
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.. _subclassing-reprs:
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Subclassing Repr Objects
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------------------------
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The use of dynamic dispatching by :meth:`Repr.repr1` allows subclasses of
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:class:`Repr` to add support for additional built-in object types or to modify
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the handling of types already supported. This example shows how special support
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for file objects could be added::
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import reprlib
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import sys
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class MyRepr(reprlib.Repr):
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def repr_file(self, obj, level):
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if obj.name in ['<stdin>', '<stdout>', '<stderr>']:
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return obj.name
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else:
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return repr(obj)
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aRepr = MyRepr()
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print(aRepr.repr(sys.stdin)) # prints '<stdin>'
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