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# 31.7. `tokenize` — Tokenizer for Python source

Source code: Lib/tokenize.py

The `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 Operators and Delimiters tokens are returned using the generic `token.OP` token type. The exact type can be determined by checking the token `string` field on the named tuple returned from `tokenize.tokenize()` for the character sequence that identifies a specific operator token.

The primary entry point is a generator:

`tokenize.``generate_tokens`(readline)

  The `generate_tokens()` generator requires one argument, readline, which must be a callable object which provides the same interface as the `readline()` method of built-in file objects (see section 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 `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.

  New in version 2.2.

An older entry point is retained for backward compatibility:

`tokenize.``tokenize`(readline[, tokeneater])

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

  The first parameter, readline, must be a callable object which provides the same interface as the `readline()` method of built-in file objects (see section 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 `StopIteration`.

  Changed in version 2.5: Added `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 `generate_tokens()`.

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

`tokenize.``COMMENT`

  Token value used to indicate a comment.

`tokenize.``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.

`tokenize.``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.

  New in version 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)

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