211 lines
8.2 KiB
TeX
211 lines
8.2 KiB
TeX
\section{\module{pprint} ---
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Data pretty printer}
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\declaremodule{standard}{pprint}
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\modulesynopsis{Data pretty printer.}
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\moduleauthor{Fred L. Drake, Jr.}{fdrake@acm.org}
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\sectionauthor{Fred L. Drake, Jr.}{fdrake@acm.org}
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The \module{pprint} module provides a capability to ``pretty-print''
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arbitrary Python data structures in a form which can be used as input
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to the interpreter. If the formatted structures include objects which
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are not fundamental Python types, the representation may not be
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loadable. This may be the case if objects such as files, sockets,
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classes, or instances are included, as well as many other builtin
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objects which are not representable as Python constants.
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The formatted representation keeps objects on a single line if it can,
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and breaks them onto multiple lines if they don't fit within the
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allowed width. Construct \class{PrettyPrinter} objects explicitly if
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you need to adjust the width constraint.
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\versionchanged[Dictionaries are sorted by key before the display is
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computed; before 2.5, a dictionary was sorted only if its display
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required more than one line, although that wasn't documented]{2.5}
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The \module{pprint} module defines one class:
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% First the implementation class:
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\begin{classdesc}{PrettyPrinter}{...}
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Construct a \class{PrettyPrinter} instance. This constructor
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understands several keyword parameters. An output stream may be set
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using the \var{stream} keyword; the only method used on the stream
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object is the file protocol's \method{write()} method. If not
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specified, the \class{PrettyPrinter} adopts \code{sys.stdout}. Three
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additional parameters may be used to control the formatted
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representation. The keywords are \var{indent}, \var{depth}, and
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\var{width}. The amount of indentation added for each recursive level
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is specified by \var{indent}; the default is one. Other values can
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cause output to look a little odd, but can make nesting easier to
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spot. The number of levels which may be printed is controlled by
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\var{depth}; if the data structure being printed is too deep, the next
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contained level is replaced by \samp{...}. By default, there is no
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constraint on the depth of the objects being formatted. The desired
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output width is constrained using the \var{width} parameter; the
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default is eighty characters. If a structure cannot be formatted
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within the constrained width, a best effort will be made.
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\begin{verbatim}
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>>> import pprint, sys
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>>> stuff = sys.path[:]
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>>> stuff.insert(0, stuff[:])
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>>> pp = pprint.PrettyPrinter(indent=4)
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>>> pp.pprint(stuff)
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[ [ '',
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'/usr/local/lib/python1.5',
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'/usr/local/lib/python1.5/test',
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'/usr/local/lib/python1.5/sunos5',
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'/usr/local/lib/python1.5/sharedmodules',
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'/usr/local/lib/python1.5/tkinter'],
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'',
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'/usr/local/lib/python1.5',
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'/usr/local/lib/python1.5/test',
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'/usr/local/lib/python1.5/sunos5',
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'/usr/local/lib/python1.5/sharedmodules',
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'/usr/local/lib/python1.5/tkinter']
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>>>
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>>> import parser
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>>> tup = parser.ast2tuple(
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... parser.suite(open('pprint.py').read()))[1][1][1]
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>>> pp = pprint.PrettyPrinter(depth=6)
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>>> pp.pprint(tup)
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(266, (267, (307, (287, (288, (...))))))
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\end{verbatim}
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\end{classdesc}
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% Now the derivative functions:
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The \class{PrettyPrinter} class supports several derivative functions:
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\begin{funcdesc}{pformat}{object\optional{, indent\optional{,
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width\optional{, depth}}}}
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Return the formatted representation of \var{object} as a string. \var{indent},
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\var{width} and \var{depth} will be passed to the \class{PrettyPrinter}
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constructor as formatting parameters.
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\versionchanged[The parameters \var{indent}, \var{width} and \var{depth}
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were added]{2.4}
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\end{funcdesc}
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\begin{funcdesc}{pprint}{object\optional{, stream\optional{,
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indent\optional{, width\optional{, depth}}}}}
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Prints the formatted representation of \var{object} on \var{stream},
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followed by a newline. If \var{stream} is omitted, \code{sys.stdout}
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is used. This may be used in the interactive interpreter instead of a
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\keyword{print} statement for inspecting values. \var{indent},
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\var{width} and \var{depth} will be passed to the \class{PrettyPrinter}
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constructor as formatting parameters.
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\begin{verbatim}
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>>> stuff = sys.path[:]
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>>> stuff.insert(0, stuff)
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>>> pprint.pprint(stuff)
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[<Recursion on list with id=869440>,
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'',
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'/usr/local/lib/python1.5',
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'/usr/local/lib/python1.5/test',
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'/usr/local/lib/python1.5/sunos5',
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'/usr/local/lib/python1.5/sharedmodules',
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'/usr/local/lib/python1.5/tkinter']
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\end{verbatim}
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\versionchanged[The parameters \var{indent}, \var{width} and \var{depth}
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were added]{2.4}
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\end{funcdesc}
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\begin{funcdesc}{isreadable}{object}
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Determine if the formatted representation of \var{object} is
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``readable,'' or can be used to reconstruct the value using
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\function{eval()}\bifuncindex{eval}. This always returns false for
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recursive objects.
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\begin{verbatim}
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>>> pprint.isreadable(stuff)
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False
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\end{verbatim}
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\end{funcdesc}
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\begin{funcdesc}{isrecursive}{object}
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Determine if \var{object} requires a recursive representation.
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\end{funcdesc}
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One more support function is also defined:
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\begin{funcdesc}{saferepr}{object}
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Return a string representation of \var{object}, protected against
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recursive data structures. If the representation of \var{object}
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exposes a recursive entry, the recursive reference will be represented
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as \samp{<Recursion on \var{typename} with id=\var{number}>}. The
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representation is not otherwise formatted.
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\end{funcdesc}
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% This example is outside the {funcdesc} to keep it from running over
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% the right margin.
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\begin{verbatim}
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>>> pprint.saferepr(stuff)
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"[<Recursion on list with id=682968>, '', '/usr/local/lib/python1.5', '/usr/loca
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l/lib/python1.5/test', '/usr/local/lib/python1.5/sunos5', '/usr/local/lib/python
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1.5/sharedmodules', '/usr/local/lib/python1.5/tkinter']"
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\end{verbatim}
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\subsection{PrettyPrinter Objects}
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\label{PrettyPrinter Objects}
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\class{PrettyPrinter} instances have the following methods:
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\begin{methoddesc}{pformat}{object}
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Return the formatted representation of \var{object}. This takes into
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account the options passed to the \class{PrettyPrinter} constructor.
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\end{methoddesc}
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\begin{methoddesc}{pprint}{object}
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Print the formatted representation of \var{object} on the configured
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stream, followed by a newline.
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\end{methoddesc}
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The following methods provide the implementations for the
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corresponding functions of the same names. Using these methods on an
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instance is slightly more efficient since new \class{PrettyPrinter}
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objects don't need to be created.
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\begin{methoddesc}{isreadable}{object}
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Determine if the formatted representation of the object is
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``readable,'' or can be used to reconstruct the value using
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\function{eval()}\bifuncindex{eval}. Note that this returns false for
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recursive objects. If the \var{depth} parameter of the
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\class{PrettyPrinter} is set and the object is deeper than allowed,
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this returns false.
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\end{methoddesc}
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\begin{methoddesc}{isrecursive}{object}
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Determine if the object requires a recursive representation.
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\end{methoddesc}
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This method is provided as a hook to allow subclasses to modify the
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way objects are converted to strings. The default implementation uses
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the internals of the \function{saferepr()} implementation.
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\begin{methoddesc}{format}{object, context, maxlevels, level}
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Returns three values: the formatted version of \var{object} as a
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string, a flag indicating whether the result is readable, and a flag
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indicating whether recursion was detected. The first argument is the
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object to be presented. The second is a dictionary which contains the
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\function{id()} of objects that are part of the current presentation
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context (direct and indirect containers for \var{object} that are
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affecting the presentation) as the keys; if an object needs to be
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presented which is already represented in \var{context}, the third
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return value should be true. Recursive calls to the \method{format()}
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method should add additional entries for containers to this
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dictionary. The third argument, \var{maxlevels}, gives the requested
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limit to recursion; this will be \code{0} if there is no requested
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limit. This argument should be passed unmodified to recursive calls.
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The fourth argument, \var{level}, gives the current level; recursive
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calls should be passed a value less than that of the current call.
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\versionadded{2.3}
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\end{methoddesc}
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