tokenize — Tokenizer for Python source

Source code: Lib/

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 operator and delimiter tokens and Ellipsis are returned using the generic OP token type. The exact type can be determined by checking the exact_type property on the named tuple returned from tokenize.tokenize().


Note that the functions in this module are only designed to parse syntactically valid Python code (code that does not raise when parsed using ast.parse()). The behavior of the functions in this module is undefined when providing invalid Python code and it can change at any point.

Tokenizing Input

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

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

Changed in version 3.1: Added support for named tuples.

Changed in version 3.3: Added support for exact_type.

tokenize() determines the source encoding of the file by looking for a UTF-8 BOM or encoding cookie, according to PEP 263.


Tokenize a source reading unicode strings instead of bytes.

Like tokenize(), the readline argument is a callable returning a single line of input. However, generate_tokens() expects readline to return a str object rather than bytes.

The result is an iterator yielding named tuples, exactly like tokenize(). It does not yield an ENCODING token.

All constants from the token module are also exported from tokenize.

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(). If there is no encoding token in the input, it returns a str instead.

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. 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 open() to open Python source files: it uses detect_encoding() to detect the file encoding.

Open a file in read only mode using the encoding detected by detect_encoding().

Added in version 3.2.

exception tokenize.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



Command-Line Usage

Added in version 3.3.

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

python -m tokenize [-e] []

The following options are accepted:

-h, --help

show this help message and exit

-e, --exact

display token names using the exact type

If is specified its contents are tokenized to stdout. Otherwise, tokenization is performed on stdin.


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

    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')

Example of tokenizing from the command line. The script:

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


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)

$ python -m tokenize
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:

$ python -m tokenize -e
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      ''

Example of tokenizing a file programmatically, reading unicode strings instead of bytes with generate_tokens():

import tokenize

with'') as f:
    tokens = tokenize.generate_tokens(f.readline)
    for token in tokens:

Or reading bytes directly with tokenize():

import tokenize

with open('', 'rb') as f:
    tokens = tokenize.tokenize(f.readline)
    for token in tokens: