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<div class="section" id="module-tokenize">
<span id="tokenize-tokenizer-for-python-source"></span><h1>30.7. <a class="reference internal" href="#module-tokenize" title="tokenize: Lexical scanner for Python source code."><tt class="xref py py-mod docutils literal"><span class="pre">tokenize</span></tt></a> — Tokenizer for Python source<a class="headerlink" href="#module-tokenize" title="Permalink to this headline">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="http://hg.python.org/cpython/file/3.2/Lib/tokenize.py">Lib/tokenize.py</a></p>
<hr class="docutils" />
<p>The <a class="reference internal" href="#module-tokenize" title="tokenize: Lexical scanner for Python source code."><tt class="xref py py-mod docutils literal"><span class="pre">tokenize</span></tt></a> 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 internal" href="../glossary.html#term-generator"><em class="xref std std-term">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 class="reference internal" href="#module-tokenize" title="tokenize: Lexical scanner for Python source code."><tt class="xref py py-func 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
<a class="reference internal" href="io.html#io.IOBase.readline" title="io.IOBase.readline"><tt class="xref py py-meth docutils literal"><span class="pre">io.IOBase.readline()</span></tt></a> method of file objects. 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 internal" href="../glossary.html#term-named-tuple"><em class="xref std std-term">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 class="versionchanged">
<span class="versionmodified">Changed in version 3.1: </span>Added support for named tuples.</p>
<p><a class="reference internal" href="#module-tokenize" title="tokenize: Lexical scanner for Python source code."><tt class="xref py py-func 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-0"></span><a class="pep 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 class="reference internal" href="token.html#module-token" title="token: Constants representing terminal nodes of the parse tree."><tt class="xref py py-mod docutils literal"><span class="pre">token</span></tt></a> module are also exported from
<a class="reference internal" href="#module-tokenize" title="tokenize: Lexical scanner for Python source code."><tt class="xref py py-mod docutils literal"><span class="pre">tokenize</span></tt></a>, 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><p>Token value used to indicate a comment.</p>
</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><p>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.</p>
</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><p>Token value that indicates the encoding used to decode the source bytes
into text. The first token returned by <a class="reference internal" href="#module-tokenize" title="tokenize: Lexical scanner for Python source code."><tt class="xref py py-func docutils literal"><span class="pre">tokenize()</span></tt></a> will always be an
ENCODING token.</p>
</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 class="reference internal" href="#module-tokenize" title="tokenize: Lexical scanner for Python source code."><tt class="xref py py-func docutils literal"><span class="pre">tokenize()</span></tt></a>.</p>
</dd></dl>
<p><a class="reference internal" href="#module-tokenize" title="tokenize: Lexical scanner for Python source code."><tt class="xref py py-func 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 class="reference internal" href="#tokenize.detect_encoding" title="tokenize.detect_encoding"><tt class="xref py py-func 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 class="reference internal" href="#module-tokenize" title="tokenize: Lexical scanner for Python source code."><tt class="xref py py-func 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 <span class="target" id="index-1"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0263"><strong>PEP 263</strong></a>. If both a BOM and a cookie are present,
but disagree, a SyntaxError will be raised. Note that if the BOM is found,
<tt class="docutils literal"><span class="pre">'utf-8-sig'</span></tt> will be returned as an encoding.</p>
<p>If no encoding is specified, then the default of <tt class="docutils literal"><span class="pre">'utf-8'</span></tt> will be
returned.</p>
<p>Use <a class="reference internal" href="functions.html#open" title="open"><tt class="xref py py-func docutils literal"><span class="pre">open()</span></tt></a> to open Python source files: it uses
<a class="reference internal" href="#tokenize.detect_encoding" title="tokenize.detect_encoding"><tt class="xref py py-func docutils literal"><span class="pre">detect_encoding()</span></tt></a> to detect the file encoding.</p>
</dd></dl>
<dl class="function">
<dt id="tokenize.open">
<tt class="descclassname">tokenize.</tt><tt class="descname">open</tt><big>(</big><em>filename</em><big>)</big><a class="headerlink" href="#tokenize.open" title="Permalink to this definition">¶</a></dt>
<dd><p>Open a file in read only mode using the encoding detected by
<a class="reference internal" href="#tokenize.detect_encoding" title="tokenize.detect_encoding"><tt class="xref py py-func docutils literal"><span class="pre">detect_encoding()</span></tt></a>.</p>
<p class="versionadded">
<span class="versionmodified">New in version 3.2.</span></p>
</dd></dl>
<p>Example of a script rewriter that transforms float literals into Decimal
objects:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">tokenize</span> <span class="k">import</span> <span class="n">tokenize</span><span class="p">,</span> <span class="n">untokenize</span><span class="p">,</span> <span class="n">NUMBER</span><span class="p">,</span> <span class="n">STRING</span><span class="p">,</span> <span class="n">NAME</span><span class="p">,</span> <span class="n">OP</span>
<span class="kn">from</span> <span class="nn">io</span> <span class="k">import</span> <span class="n">BytesIO</span>
<span class="k">def</span> <span class="nf">decistmt</span><span class="p">(</span><span class="n">s</span><span class="p">):</span>
<span class="sd">"""Substitute Decimals for floats in a string of statements.</span>
<span class="sd"> >>> from decimal import Decimal</span>
<span class="sd"> >>> s = 'print(+21.3e-5*-.1234/81.7)'</span>
<span class="sd"> >>> decistmt(s)</span>
<span class="sd"> "print (+Decimal ('21.3e-5')*-Decimal ('.1234')/Decimal ('81.7'))"</span>
<span class="sd"> The format of the exponent is inherited from the platform C library.</span>
<span class="sd"> Known cases are "e-007" (Windows) and "e-07" (not Windows). Since</span>
<span class="sd"> we're only showing 12 digits, and the 13th isn't close to 5, the</span>
<span class="sd"> rest of the output should be platform-independent.</span>
<span class="sd"> >>> exec(s) #doctest: +ELLIPSIS</span>
<span class="sd"> -3.21716034272e-0...7</span>
<span class="sd"> Output from calculations with Decimal should be identical across all</span>
<span class="sd"> platforms.</span>
<span class="sd"> >>> exec(decistmt(s))</span>
<span class="sd"> -3.217160342717258261933904529E-7</span>
<span class="sd"> """</span>
<span class="n">result</span> <span class="o">=</span> <span class="p">[]</span>
<span class="n">g</span> <span class="o">=</span> <span class="n">tokenize</span><span class="p">(</span><span class="n">BytesIO</span><span class="p">(</span><span class="n">s</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="s">'utf-8'</span><span class="p">))</span><span class="o">.</span><span class="n">readline</span><span class="p">)</span> <span class="c"># tokenize the string</span>
<span class="k">for</span> <span class="n">toknum</span><span class="p">,</span> <span class="n">tokval</span><span class="p">,</span> <span class="n">_</span><span class="p">,</span> <span class="n">_</span><span class="p">,</span> <span class="n">_</span> <span class="ow">in</span> <span class="n">g</span><span class="p">:</span>
<span class="k">if</span> <span class="n">toknum</span> <span class="o">==</span> <span class="n">NUMBER</span> <span class="ow">and</span> <span class="s">'.'</span> <span class="ow">in</span> <span class="n">tokval</span><span class="p">:</span> <span class="c"># replace NUMBER tokens</span>
<span class="n">result</span><span class="o">.</span><span class="n">extend</span><span class="p">([</span>
<span class="p">(</span><span class="n">NAME</span><span class="p">,</span> <span class="s">'Decimal'</span><span class="p">),</span>
<span class="p">(</span><span class="n">OP</span><span class="p">,</span> <span class="s">'('</span><span class="p">),</span>
<span class="p">(</span><span class="n">STRING</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">tokval</span><span class="p">)),</span>
<span class="p">(</span><span class="n">OP</span><span class="p">,</span> <span class="s">')'</span><span class="p">)</span>
<span class="p">])</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">result</span><span class="o">.</span><span class="n">append</span><span class="p">((</span><span class="n">toknum</span><span class="p">,</span> <span class="n">tokval</span><span class="p">))</span>
<span class="k">return</span> <span class="n">untokenize</span><span class="p">(</span><span class="n">result</span><span class="p">)</span><span class="o">.</span><span class="n">decode</span><span class="p">(</span><span class="s">'utf-8'</span><span class="p">)</span>
</pre></div>
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