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<div class="section" id="module-tokenize">
<h1>30.7. <tt class="xref docutils literal"><span class="pre">tokenize</span></tt> — Tokenizer for Python source<a class="headerlink" href="#module-tokenize" title="Permalink to this headline">¶</a></h1>
<p>The <tt class="xref docutils literal"><span class="pre">tokenize</span></tt> 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.</p>
<p>The primary entry point is a <a class="reference external" href="../glossary.html#term-generator"><em class="xref">generator</em></a>:</p>
<dl class="function">
<dt id="tokenize.tokenize">
<tt class="descclassname">tokenize.</tt><tt class="descname">tokenize</tt><big>(</big><em>readline</em><big>)</big><a class="headerlink" href="#tokenize.tokenize" title="Permalink to this definition">¶</a></dt>
<dd><p>The <a title="tokenize.tokenize" class="reference internal" href="#tokenize.tokenize"><tt class="xref docutils literal"><span class="pre">tokenize()</span></tt></a> generator requires one argument, <em>readline</em>, which
must be a callable object which provides the same interface as the
<tt class="xref docutils literal"><span class="pre">readline()</span></tt> method of built-in file objects (see section
<a class="reference external" href="stdtypes.html#bltin-file-objects"><em>File Objects</em></a>). Each call to the function should return one
line of input as bytes.</p>
<p>The generator produces 5-tuples with these members: the token type; the
token string; a 2-tuple <tt class="docutils literal"><span class="pre">(srow,</span> <span class="pre">scol)</span></tt> of ints specifying the row and
column where the token begins in the source; a 2-tuple <tt class="docutils literal"><span class="pre">(erow,</span> <span class="pre">ecol)</span></tt> 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 <em>logical</em> line; continuation lines are included. The 5 tuple is
returned as a <a class="reference external" href="../glossary.html#term-named-tuple"><em class="xref">named tuple</em></a> with the field names:
<tt class="docutils literal"><span class="pre">type</span> <span class="pre">string</span> <span class="pre">start</span> <span class="pre">end</span> <span class="pre">line</span></tt>.</p>
<p>
<span class="versionmodified">Changed in version 3.1: </span>Added support for named tuples.</p>
<p><a title="tokenize.tokenize" class="reference internal" href="#tokenize.tokenize"><tt class="xref docutils literal"><span class="pre">tokenize()</span></tt></a> determines the source encoding of the file by looking for a
UTF-8 BOM or encoding cookie, according to <span class="target" id="index-609"></span><a class="reference external" href="http://www.python.org/dev/peps/pep-0263"><strong>PEP 263</strong></a>.</p>
</dd></dl>
<p>All constants from the <a title="Constants representing terminal nodes of the parse tree." class="reference external" href="token.html#module-token"><tt class="xref docutils literal"><span class="pre">token</span></tt></a> module are also exported from
<tt class="xref docutils literal"><span class="pre">tokenize</span></tt>, as are three additional token type values:</p>
<dl class="data">
<dt id="tokenize.COMMENT">
<tt class="descclassname">tokenize.</tt><tt class="descname">COMMENT</tt><a class="headerlink" href="#tokenize.COMMENT" title="Permalink to this definition">¶</a></dt>
<dd>Token value used to indicate a comment.</dd></dl>
<dl class="data">
<dt id="tokenize.NL">
<tt class="descclassname">tokenize.</tt><tt class="descname">NL</tt><a class="headerlink" href="#tokenize.NL" title="Permalink to this definition">¶</a></dt>
<dd>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.</dd></dl>
<dl class="data">
<dt id="tokenize.ENCODING">
<tt class="descclassname">tokenize.</tt><tt class="descname">ENCODING</tt><a class="headerlink" href="#tokenize.ENCODING" title="Permalink to this definition">¶</a></dt>
<dd>Token value that indicates the encoding used to decode the source bytes
into text. The first token returned by <a title="tokenize.tokenize" class="reference internal" href="#tokenize.tokenize"><tt class="xref docutils literal"><span class="pre">tokenize()</span></tt></a> will always be an
ENCODING token.</dd></dl>
<p>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.</p>
<dl class="function">
<dt id="tokenize.untokenize">
<tt class="descclassname">tokenize.</tt><tt class="descname">untokenize</tt><big>(</big><em>iterable</em><big>)</big><a class="headerlink" href="#tokenize.untokenize" title="Permalink to this definition">¶</a></dt>
<dd><p>Converts tokens back into Python source code. The <em>iterable</em> must return
sequences with at least two elements, the token type and the token string.
Any additional sequence elements are ignored.</p>
<p>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.</p>
<p>It returns bytes, encoded using the ENCODING token, which is the first
token sequence output by <a title="tokenize.tokenize" class="reference internal" href="#tokenize.tokenize"><tt class="xref docutils literal"><span class="pre">tokenize()</span></tt></a>.</p>
</dd></dl>
<p><a title="tokenize.tokenize" class="reference internal" href="#tokenize.tokenize"><tt class="xref docutils literal"><span class="pre">tokenize()</span></tt></a> needs to detect the encoding of source files it tokenizes. The
function it uses to do this is available:</p>
<dl class="function">
<dt id="tokenize.detect_encoding">
<tt class="descclassname">tokenize.</tt><tt class="descname">detect_encoding</tt><big>(</big><em>readline</em><big>)</big><a class="headerlink" href="#tokenize.detect_encoding" title="Permalink to this definition">¶</a></dt>
<dd><p>The <a title="tokenize.detect_encoding" class="reference internal" href="#tokenize.detect_encoding"><tt class="xref docutils literal"><span class="pre">detect_encoding()</span></tt></a> 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 <a title="tokenize.tokenize" class="reference internal" href="#tokenize.tokenize"><tt class="xref docutils literal"><span class="pre">tokenize()</span></tt></a> generator.</p>
<p>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.</p>
<p>It detects the encoding from the presence of a utf-8 bom or an encoding
cookie as specified in pep-0263. If both a bom and a cookie are present,
but disagree, a SyntaxError will be raised.</p>
<p>If no encoding is specified, then the default of ‘utf-8’ will be returned.</p>
</dd></dl>
<p>Example of a script re-writer that transforms float literals into Decimal
objects:</p>
<div class="highlight-python3"><pre>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')</pre>
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