mirror of https://github.com/python/cpython.git
128 lines
4.3 KiB
TeX
128 lines
4.3 KiB
TeX
\section{\module{repr} ---
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Alternate \function{repr()} implementation}
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\sectionauthor{Fred L. Drake, Jr.}{fdrake@acm.org}
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\declaremodule{standard}{repr}
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\modulesynopsis{Alternate \function{repr()} implementation with size limits.}
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The \module{repr} module provides a means for producing object
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representations with limits on the size of the resulting strings.
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This is used in the Python debugger and may be useful in other
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contexts as well.
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This module provides a class, an instance, and a function:
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\begin{classdesc}{Repr}{}
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Class which provides formatting services useful in implementing
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functions similar to the built-in \function{repr()}; size limits for
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different object types are added to avoid the generation of
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representations which are excessively long.
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\end{classdesc}
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\begin{datadesc}{aRepr}
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This is an instance of \class{Repr} which is used to provide the
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\function{repr()} function described below. Changing the attributes
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of this object will affect the size limits used by \function{repr()}
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and the Python debugger.
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\end{datadesc}
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\begin{funcdesc}{repr}{obj}
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This is the \method{repr()} method of \code{aRepr}. It returns a
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string similar to that returned by the built-in function of the same
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name, but with limits on most sizes.
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\end{funcdesc}
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\subsection{Repr Objects \label{Repr-objects}}
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\class{Repr} instances provide several members which can be used to
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provide size limits for the representations of different object types,
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and methods which format specific object types.
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\begin{memberdesc}{maxlevel}
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Depth limit on the creation of recursive representations. The
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default is \code{6}.
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\end{memberdesc}
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\begin{memberdesc}{maxdict}
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\memberline{maxlist}
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\memberline{maxtuple}
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Limits on the number of entries represented for the named object
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type. The default for \member{maxdict} is \code{4}, for the others,
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\code{6}.
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\end{memberdesc}
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\begin{memberdesc}{maxlong}
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Maximum number of characters in the representation for a long
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integer. Digits are dropped from the middle. The default is
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\code{40}.
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\end{memberdesc}
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\begin{memberdesc}{maxstring}
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Limit on the number of characters in the representation of the
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string. Note that the ``normal'' representation of the string is
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used as the character source: if escape sequences are needed in the
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representation, these may be mangled when the representation is
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shortened. The default is \code{30}.
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\end{memberdesc}
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\begin{memberdesc}{maxother}
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This limit is used to control the size of object types for which no
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specific formatting method is available on the \class{Repr} object.
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It is applied in a similar manner as \member{maxstring}. The
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default is \code{20}.
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\end{memberdesc}
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\begin{methoddesc}{repr}{obj}
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The equivalent to the built-in \function{repr()} that uses the
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formatting imposed by the instance.
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\end{methoddesc}
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\begin{methoddesc}{repr1}{obj, level}
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Recursive implementation used by \method{repr()}. This uses the
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type of \var{obj} to determine which formatting method to call,
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passing it \var{obj} and \var{level}. The type-specific methods
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should call \method{repr1()} to perform recursive formatting, with
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\code{\var{level} - 1} for the value of \var{level} in the recursive
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call.
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\end{methoddesc}
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\begin{methoddescni}{repr_\var{type}}{obj, level}
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Formatting methods for specific types are implemented as methods
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with a name based on the type name. In the method name, \var{type}
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is replaced by
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\code{string.join(string.split(type(\var{obj}).__name__, '_')}.
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Dispatch to these methods is handled by \method{repr1()}.
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Type-specific methods which need to recursively format a value
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should call \samp{self.repr1(\var{subobj}, \var{level} - 1)}.
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\end{methoddescni}
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\subsection{Subclassing Repr Objects \label{subclassing-reprs}}
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The use of dynamic dispatching by \method{Repr.repr1()} allows
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subclasses of \class{Repr} to add support for additional built-in
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object types or to modify the handling of types already supported.
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This example shows how special support for file objects could be
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added:
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\begin{verbatim}
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import repr
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import sys
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class MyRepr(repr.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 `obj`
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aRepr = MyRepr()
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print aRepr.repr(sys.stdin) # prints '<stdin>'
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\end{verbatim}
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