30.7. tokenize — Tokenizer for Python source

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.

The primary entry point is a generator:


The tokenize() generator requires one argument, readline, which must be a callable object which provides the same interface as the 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 named tuple with the field names: type string start end line.

Changed in version 3.1: Added support for named tuples.

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 token module are also exported from tokenize, as are three additional token type values:

Token value used to indicate a comment.
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.
Token value that indicates the encoding used to decode the source bytes into text. The first token returned by 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.


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 tokenize().

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


The 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 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.

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

Example of a script re-writer 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

    Output from calculations with Decimal should be identical across all

    >>> exec(decistmt(s))
    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
                (NAME, 'Decimal'),
                (OP, '('),
                (STRING, repr(tokval)),
                (OP, ')')
            result.append((toknum, tokval))
    return untokenize(result).decode('utf-8')

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