source: trunk/essentials/dev-lang/python/Doc/lib/libtokenize.tex@ 3368

\section{\module{tokenize} ---
         Tokenizer for Python source}

\declaremodule{standard}{tokenize}
\modulesynopsis{Lexical scanner for Python source code.}
\moduleauthor{Ka Ping Yee}{}
\sectionauthor{Fred L. Drake, Jr.}{[email protected]}


The \module{tokenize} module provides a lexical scanner for Python
source code, implemented in Python.  The scanner in this module
returns comments as tokens as well, making it useful for implementing
``pretty-printers,'' including colorizers for on-screen displays.

The primary entry point is a generator:

\begin{funcdesc}{generate_tokens}{readline}
  The \function{generate_tokens()} generator requires one argment,
  \var{readline}, which must be a callable object which
  provides the same interface as the \method{readline()} method of
  built-in file objects (see section~\ref{bltin-file-objects}).  Each
  call to the function should return one line of input as a string.

  The generator produces 5-tuples with these members:
  the token type;
  the token string;
  a 2-tuple \code{(\var{srow}, \var{scol})} of ints specifying the
  row and column where the token begins in the source;
  a 2-tuple \code{(\var{erow}, \var{ecol})} of ints specifying the
  row and column where the token ends in the source;
  and the line on which the token was found.
  The line passed is the \emph{logical} line;
  continuation lines are included.
  \versionadded{2.2}
\end{funcdesc}

An older entry point is retained for backward compatibility:

\begin{funcdesc}{tokenize}{readline\optional{, tokeneater}}
  The \function{tokenize()} function accepts two parameters: one
  representing the input stream, and one providing an output mechanism
  for \function{tokenize()}.

  The first parameter, \var{readline}, must be a callable object which
  provides the same interface as the \method{readline()} method of
  built-in file objects (see section~\ref{bltin-file-objects}).  Each
  call to the function should return one line of input as a string.
  Alternately, \var{readline} may be a callable object that signals
  completion by raising \exception{StopIteration}.
  \versionchanged[Added \exception{StopIteration} support]{2.5}

  The second parameter, \var{tokeneater}, must also be a callable
  object.  It is called once for each token, with five arguments,
  corresponding to the tuples generated by \function{generate_tokens()}.
\end{funcdesc}


All constants from the \refmodule{token} module are also exported from
\module{tokenize}, as are two additional token type values that might be
passed to the \var{tokeneater} function by \function{tokenize()}:

\begin{datadesc}{COMMENT}
  Token value used to indicate a comment.
\end{datadesc}
\begin{datadesc}{NL}
  Token value used to indicate a non-terminating newline.  The NEWLINE
  token indicates the end of a logical line of Python code; NL tokens
  are generated when a logical line of code is continued over multiple
  physical lines.
\end{datadesc}

Another function is provided to reverse the tokenization process.
This is useful for creating tools that tokenize a script, modify
the token stream, and write back the modified script.

\begin{funcdesc}{untokenize}{iterable}
  Converts tokens back into Python source code.  The \var{iterable}
  must return sequences with at least two elements, the token type and
  the token string.  Any additional sequence elements are ignored.

  The reconstructed script is returned as a single string.  The
  result is guaranteed to tokenize back to match the input so that
  the conversion is lossless and round-trips are assured.  The
  guarantee applies only to the token type and token string as
  the spacing between tokens (column positions) may change.
  \versionadded{2.5}
\end{funcdesc}

Example of a script re-writer that transforms float literals into
Decimal objects:
\begin{verbatim}
def decistmt(s):
    """Substitute Decimals for floats in a string of statements.

    >>> from decimal import Decimal
    >>> s = 'print +21.3e-5*-.1234/81.7'
    >>> decistmt(s)
    "print +Decimal ('21.3e-5')*-Decimal ('.1234')/Decimal ('81.7')"

    >>> exec(s)
    -3.21716034272e-007
    >>> exec(decistmt(s))
    -3.217160342717258261933904529E-7

    """
    result = []
    g = generate_tokens(StringIO(s).readline)   # tokenize the string
    for toknum, tokval, _, _, _  in g:
        if toknum == NUMBER and '.' in tokval:  # replace NUMBER tokens
            result.extend([
                (NAME, 'Decimal'),
                (OP, '('),
                (STRING, repr(tokval)),
                (OP, ')')
            ])
        else:
            result.append((toknum, tokval))
    return untokenize(result)
\end{verbatim}