:mod:!reprlib --- Alternate :func:repr implementation

.. module:: reprlib :synopsis: Alternate repr() implementation with size limits.

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(*, maxlevel=6, maxtuple=6, maxlist=6, maxarray=5, maxdict=4,
maxset=6, maxfrozenset=6, maxdeque=6, maxstring=30, maxlong=40,
maxother=30, fillvalue="...", indent=None)

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.

The keyword arguments of the constructor can be used as a shortcut to set the attributes of the :class:Repr instance. Which means that the following initialization::

  aRepr = reprlib.Repr(maxlevel=3)

Is equivalent to::

  aRepr = reprlib.Repr()
  aRepr.maxlevel = 3

See section Repr Objects_ for more information about :class:Repr attributes.

.. versionchanged:: 3.12 Allow attributes to be set via keyword arguments.

.. 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:~object.__repr__ and substituting a placeholder string instead.

.. index:: single: ...; placeholder

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

Decorator for :meth:~object.__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:

.. doctest::

  >>> from reprlib import recursive_repr
  >>> 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.fillvalue

This string is displayed for recursive references. It defaults to ....

.. versionadded:: 3.11

.. 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.

.. attribute:: Repr.indent

If this attribute is set to None (the default), the output is formatted with no line breaks or indentation, like the standard :func:repr. For example:

.. doctest:: indent

  >>> example = [
  ...     1, 'spam', {'a': 2, 'b': 'spam eggs', 'c': {3: 4.5, 6: []}}, 'ham']
  >>> import reprlib
  >>> aRepr = reprlib.Repr()
  >>> print(aRepr.repr(example))
  [1, 'spam', {'a': 2, 'b': 'spam eggs', 'c': {3: 4.5, 6: []}}, 'ham']

If :attr:~Repr.indent is set to a string, each recursion level is placed on its own line, indented by that string:

.. doctest:: indent

  >>> aRepr.indent = '-->'
  >>> print(aRepr.repr(example))
  [
  -->1,
  -->'spam',
  -->{
  -->-->'a': 2,
  -->-->'b': 'spam eggs',
  -->-->'c': {
  -->-->-->3: 4.5,
  -->-->-->6: [],
  -->-->},
  -->},
  -->'ham',
  ]

Setting :attr:~Repr.indent to a positive integer value behaves as if it was set to a string with that number of spaces:

.. doctest:: indent

  >>> aRepr.indent = 4
  >>> print(aRepr.repr(example))
  [
      1,
      'spam',
      {
          'a': 2,
          'b': 'spam eggs',
          'c': {
              3: 4.5,
              6: [],
          },
      },
      'ham',
  ]

.. versionadded:: 3.12

.. 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:

.. testcode::

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>'

.. testoutput::