path: root/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 ``exact_type`` property on the
:term:`named tuple` returned from :func:`tokenize.tokenize`.

Tokenizing Input
----------------

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

.. function:: tokenize(readline)

   The :func:`tokenize` generator requires one argument, *readline*, which
   must be a callable object which provides the same interface as the
   :meth:`io.IOBase.readline` method of file objects.  Each call to the
   function should return one line of input as bytes.

   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.  The 5 tuple is
   returned as a :term:`named tuple` with the field names:
   ``type string start end line``.

   The returned :term:`named tuple` has an additional property named
   ``exact_type`` that contains the exact operator type for
   :data:`token.OP` tokens.  For all other token types ``exact_type``
   equals the named tuple ``type`` field.

   .. versionchanged:: 3.1
      Added support for named tuples.

   .. versionchanged:: 3.3
      Added support for ``exact_type``.

   :func:`tokenize` determines the source encoding of the file by looking for a
   UTF-8 BOM or encoding cookie, according to :pep:`263`.


All constants from the :mod:`token` module are also exported from
:mod:`tokenize`, as are three additional token type values:

.. 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.


.. data:: ENCODING

    Token value that indicates the encoding used to decode the source bytes
    into text. The first token returned by :func:`tokenize` will always be an
    ENCODING token.


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.

    It returns bytes, encoded using the ENCODING token, which is the first
    token sequence output by :func:`tokenize`.


:func:`tokenize` needs to detect the encoding of source files it tokenizes. The
function it uses to do this is available:

.. function:: detect_encoding(readline)

    The :func:`detect_encoding` function is used to detect the encoding that
    should be used to decode a Python source file. It requires one argument,
    readline, in the same way as the :func:`tokenize` generator.

    It will call readline a maximum of twice, and return the encoding used
    (as a string) and a list of any lines (not decoded from bytes) it has read
    in.

    It detects the encoding from the presence of a UTF-8 BOM or an encoding
    cookie as specified in :pep:`263`. If both a BOM and a cookie are present,
    but disagree, a SyntaxError will be raised. Note that if the BOM is found,
    ``'utf-8-sig'`` will be returned as an encoding.

    If no encoding is specified, then the default of ``'utf-8'`` will be
    returned.

    Use :func:`open` to open Python source files: it uses
    :func:`detect_encoding` to detect the file encoding.


.. function:: open(filename)

   Open a file in read only mode using the encoding detected by
   :func:`detect_encoding`.

   .. versionadded:: 3.2

.. exception:: TokenError

   Raised when either a docstring or expression that may be split over several
   lines is not completed anywhere in the file, for example::

      """Beginning of
      docstring

   or::

      [1,
       2,
       3

Note that unclosed single-quoted strings do not cause an error to be
raised. They are tokenized as ``ERRORTOKEN``, followed by the tokenization of
their contents.


.. _tokenize-cli:

Command-Line Usage
------------------

.. versionadded:: 3.3

The :mod:`tokenize` module can be executed as a script from the command line.
It is as simple as:

.. code-block:: sh

   python -m tokenize [-e] [filename.py]

The following options are accepted:

.. program:: tokenize

.. cmdoption:: -h, --help

   show this help message and exit

.. cmdoption:: -e, --exact

   display token names using the exact type

If :file:`filename.py` is specified its contents are tokenized to stdout.
Otherwise, tokenization is performed on stdin.

Examples
------------------

Example of a script rewriter that transforms float literals into Decimal
objects::

    from tokenize import tokenize, untokenize, NUMBER, STRING, NAME, OP
    from io import BytesIO

    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'))"

        The format of the exponent is inherited from the platform C library.
        Known cases are "e-007" (Windows) and "e-07" (not Windows).  Since
        we're only showing 12 digits, and the 13th isn't close to 5, the
        rest of the output should be platform-independent.

        >>> exec(s) #doctest: +ELLIPSIS
        -3.21716034272e-0...7

        Output from calculations with Decimal should be identical across all
        platforms.

        >>> exec(decistmt(s))
        -3.217160342717258261933904529E-7
        """
        result = []
        g = tokenize(BytesIO(s.encode('utf-8')).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).decode('utf-8')

Example of tokenizing from the command line.  The script::

    def say_hello():
        print("Hello, World!")

    say_hello()

will be tokenized to the following output where the first column is the range
of the line/column coordinates where the token is found, the second column is
the name of the token, and the final column is the value of the token (if any)

.. code-block:: sh

    $ python -m tokenize hello.py
    0,0-0,0:            ENCODING       'utf-8'
    1,0-1,3:            NAME           'def'
    1,4-1,13:           NAME           'say_hello'
    1,13-1,14:          OP             '('
    1,14-1,15:          OP             ')'
    1,15-1,16:          OP             ':'
    1,16-1,17:          NEWLINE        '\n'
    2,0-2,4:            INDENT         '    '
    2,4-2,9:            NAME           'print'
    2,9-2,10:           OP             '('
    2,10-2,25:          STRING         '"Hello, World!"'
    2,25-2,26:          OP             ')'
    2,26-2,27:          NEWLINE        '\n'
    3,0-3,1:            NL             '\n'
    4,0-4,0:            DEDENT         ''
    4,0-4,9:            NAME           'say_hello'
    4,9-4,10:           OP             '('
    4,10-4,11:          OP             ')'
    4,11-4,12:          NEWLINE        '\n'
    5,0-5,0:            ENDMARKER      ''

The exact token type names can be displayed using the ``-e`` option:

.. code-block:: sh

    $ python -m tokenize -e hello.py
    0,0-0,0:            ENCODING       'utf-8'
    1,0-1,3:            NAME           'def'
    1,4-1,13:           NAME           'say_hello'
    1,13-1,14:          LPAR           '('
    1,14-1,15:          RPAR           ')'
    1,15-1,16:          COLON          ':'
    1,16-1,17:          NEWLINE        '\n'
    2,0-2,4:            INDENT         '    '
    2,4-2,9:            NAME           'print'
    2,9-2,10:           LPAR           '('
    2,10-2,25:          STRING         '"Hello, World!"'
    2,25-2,26:          RPAR           ')'
    2,26-2,27:          NEWLINE        '\n'
    3,0-3,1:            NL             '\n'
    4,0-4,0:            DEDENT         ''
    4,0-4,9:            NAME           'say_hello'
    4,9-4,10:           LPAR           '('
    4,10-4,11:          RPAR           ')'
    4,11-4,12:          NEWLINE        '\n'
    5,0-5,0:            ENDMARKER      ''