source: python/trunk/Doc/library/tokenize.rst

:mod:`tokenize` --- Tokenizer for Python source
===============================================

.. module:: tokenize
   :synopsis: Lexical scanner for Python source code.
.. moduleauthor:: Ka Ping Yee
.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>

**Source code:** :source:`Lib/tokenize.py`

--------------

The :mod:`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.

To simplify token stream handling, all :ref:`operators` and :ref:`delimiters`
tokens are returned using the generic :data:`token.OP` token type.  The exact
type can be determined by checking the token ``string`` field on the
:term:`named tuple` returned from :func:`tokenize.tokenize` for the character
sequence that identifies a specific operator token.

The primary entry point is a :term:`generator`:

.. function:: generate_tokens(readline)

   The :func:`generate_tokens` generator requires one argument, *readline*,
   which must be a callable object which provides the same interface as the
   :meth:`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, *readline* may be a callable object that
   signals completion by raising :exc:`StopIteration`.

   The generator produces 5-tuples with these members: the token type; the token
   string; a 2-tuple ``(srow, scol)`` of ints specifying the row and column
   where the token begins in the source; a 2-tuple ``(erow, 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 (the last tuple item) is
   the *logical* line; continuation lines are included.

   .. versionadded:: 2.2

An older entry point is retained for backward compatibility:


.. function:: tokenize(readline[, tokeneater])

   The :func:`tokenize` function accepts two parameters: one representing the input
   stream, and one providing an output mechanism for :func:`tokenize`.

   The first parameter, *readline*, must be a callable object which provides the
   same interface as the :meth:`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, *readline* may be a callable object that
   signals completion by raising :exc:`StopIteration`.

   .. versionchanged:: 2.5
      Added :exc:`StopIteration` support.

   The second parameter, *tokeneater*, must also be a callable object.  It is
   called once for each token, with five arguments, corresponding to the tuples
   generated by :func:`generate_tokens`.

All constants from the :mod:`token` module are also exported from
:mod:`tokenize`, as are two additional token type values that might be passed to
the *tokeneater* function by :func:`tokenize`:


.. data:: COMMENT

   Token value used to indicate a comment.


.. data:: 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.

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.


.. function:: untokenize(iterable)

   Converts tokens back into Python source code.  The *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

Example of a script re-writer that transforms float literals into Decimal
objects::

   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)