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<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta name="color-scheme" content="light dark"> <title>PEP 292 – Simpler String Substitutions | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-0292/"> <link rel="stylesheet" href="../_static/style.css" type="text/css"> <link rel="stylesheet" href="../_static/mq.css" type="text/css"> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" media="(prefers-color-scheme: light)" id="pyg-light"> <link rel="stylesheet" href="../_static/pygments_dark.css" type="text/css" media="(prefers-color-scheme: dark)" id="pyg-dark"> <link rel="alternate" type="application/rss+xml" title="Latest PEPs" href="https://peps.python.org/peps.rss"> <meta property="og:title" content='PEP 292 – Simpler String Substitutions | peps.python.org'> <meta property="og:description" content="This PEP describes a simpler string substitution feature, also known as string interpolation. 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This PEP is “simpler” in two respects:</p> <ol class="arabic simple"> <li>Python’s current string substitution feature (i.e. <code class="docutils literal notranslate"><span class="pre">%</span></code>-substitution) is complicated and error prone. This PEP is simpler at the cost of some expressiveness.</li> <li><a class="pep reference internal" href="../pep-0215/" title="PEP 215 – String Interpolation">PEP 215</a> proposed an alternative string interpolation feature, introducing a new <code class="docutils literal notranslate"><span class="pre">$</span></code> string prefix. <a class="pep reference internal" href="../pep-0292/" title="PEP 292 – Simpler String Substitutions">PEP 292</a> is simpler than this because it involves no syntax changes and has much simpler rules for what substitutions can occur in the string.</li> </ol> </section> <section id="rationale"> <h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2> <p>Python currently supports a string substitution syntax based on C’s <code class="docutils literal notranslate"><span class="pre">printf()</span></code> ‘<code class="docutils literal notranslate"><span class="pre">%</span></code>’ formatting character <a class="footnote-reference brackets" href="#id5" id="id1">[1]</a>. While quite rich, <code class="docutils literal notranslate"><span class="pre">%</span></code>-formatting codes are also error prone, even for experienced Python programmers. A common mistake is to leave off the trailing format character, e.g. the ‘<code class="docutils literal notranslate"><span class="pre">s</span></code>’ in <code class="docutils literal notranslate"><span class="pre">"%(name)s"</span></code>.</p> <p>In addition, the rules for what can follow a <code class="docutils literal notranslate"><span class="pre">%</span></code> sign are fairly complex, while the usual application rarely needs such complexity. Most scripts need to do some string interpolation, but most of those use simple ‘stringification’ formats, i.e. <code class="docutils literal notranslate"><span class="pre">%s</span></code> or <code class="docutils literal notranslate"><span class="pre">%(name)s</span></code> This form should be made simpler and less error prone.</p> </section> <section id="a-simpler-proposal"> <h2><a class="toc-backref" href="#a-simpler-proposal" role="doc-backlink">A Simpler Proposal</a></h2> <p>We propose the addition of a new class, called <code class="docutils literal notranslate"><span class="pre">Template</span></code>, which will live in the string module. The <code class="docutils literal notranslate"><span class="pre">Template</span></code> class supports new rules for string substitution; its value contains placeholders, introduced with the <code class="docutils literal notranslate"><span class="pre">$</span></code> character. The following rules for <code class="docutils literal notranslate"><span class="pre">$</span></code>-placeholders apply:</p> <ol class="arabic simple"> <li><code class="docutils literal notranslate"><span class="pre">$$</span></code> is an escape; it is replaced with a single <code class="docutils literal notranslate"><span class="pre">$</span></code></li> <li><code class="docutils literal notranslate"><span class="pre">$identifier</span></code> names a substitution placeholder matching a mapping key of “identifier”. By default, “identifier” must spell a Python identifier as defined in <a class="footnote-reference brackets" href="#id6" id="id2">[2]</a>. The first non-identifier character after the <code class="docutils literal notranslate"><span class="pre">$</span></code> character terminates this placeholder specification.</li> <li><code class="docutils literal notranslate"><span class="pre">${identifier}</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">$identifier</span></code>. It is required when valid identifier characters follow the placeholder but are not part of the placeholder, e.g. <code class="docutils literal notranslate"><span class="pre">"${noun}ification"</span></code>.</li> </ol> <p>If the <code class="docutils literal notranslate"><span class="pre">$</span></code> character appears at the end of the line, or is followed by any other character than those described above, a <code class="docutils literal notranslate"><span class="pre">ValueError</span></code> will be raised at interpolation time. Values in mapping are converted automatically to strings.</p> <p>No other characters have special meaning, however it is possible to derive from the <code class="docutils literal notranslate"><span class="pre">Template</span></code> class to define different substitution rules. For example, a derived class could allow for periods in the placeholder (e.g. to support a kind of dynamic namespace and attribute path lookup), or could define a delimiter character other than <code class="docutils literal notranslate"><span class="pre">$</span></code>.</p> <p>Once the <code class="docutils literal notranslate"><span class="pre">Template</span></code> has been created, substitutions can be performed by calling one of two methods:</p> <ul> <li><code class="docutils literal notranslate"><span class="pre">substitute()</span></code>. This method returns a new string which results when the values of a mapping are substituted for the placeholders in the <code class="docutils literal notranslate"><span class="pre">Template</span></code>. If there are placeholders which are not present in the mapping, a <code class="docutils literal notranslate"><span class="pre">KeyError</span></code> will be raised.</li> <li><code class="docutils literal notranslate"><span class="pre">safe_substitute()</span></code>. This is similar to the <code class="docutils literal notranslate"><span class="pre">substitute()</span></code> method, except that <code class="docutils literal notranslate"><span class="pre">KeyErrors</span></code> are never raised (due to placeholders missing from the mapping). When a placeholder is missing, the original placeholder will appear in the resulting string.<p>Here are some examples:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">string</span> <span class="kn">import</span> <span class="n">Template</span> <span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="n">Template</span><span class="p">(</span><span class="s1">'$</span><span class="si">{name}</span><span class="s1"> was born in $</span><span class="si">{country}</span><span class="s1">'</span><span class="p">)</span> <span class="gp">>>> </span><span class="nb">print</span> <span class="n">s</span><span class="o">.</span><span class="n">substitute</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s1">'Guido'</span><span class="p">,</span> <span class="n">country</span><span class="o">=</span><span class="s1">'the Netherlands'</span><span class="p">)</span> <span class="go">Guido was born in the Netherlands</span> <span class="gp">>>> </span><span class="nb">print</span> <span class="n">s</span><span class="o">.</span><span class="n">substitute</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s1">'Guido'</span><span class="p">)</span> <span class="gt">Traceback (most recent call last):</span> <span class="gr">[...]</span> <span class="gr">KeyError</span>: <span class="n">'country'</span> <span class="gp">>>> </span><span class="nb">print</span> <span class="n">s</span><span class="o">.</span><span class="n">safe_substitute</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s1">'Guido'</span><span class="p">)</span> <span class="go">Guido was born in ${country}</span> </pre></div> </div> </li> </ul> <p>The signature of <code class="docutils literal notranslate"><span class="pre">substitute()</span></code> and <code class="docutils literal notranslate"><span class="pre">safe_substitute()</span></code> allows for passing the mapping of placeholders to values, either as a single dictionary-like object in the first positional argument, or as keyword arguments as shown above. The exact details and signatures of these two methods is reserved for the standard library documentation.</p> </section> <section id="why-and-braces"> <h2><a class="toc-backref" href="#why-and-braces" role="doc-backlink">Why <code class="docutils literal notranslate"><span class="pre">$</span></code> and Braces?</a></h2> <p>The BDFL said it best <a class="footnote-reference brackets" href="#id7" id="id3">[3]</a>: “The <code class="docutils literal notranslate"><span class="pre">$</span></code> means “substitution” in so many languages besides Perl that I wonder where you’ve been. […] We’re copying this from the shell.”</p> <p>Thus the substitution rules are chosen because of the similarity with so many other languages. This makes the substitution rules easier to teach, learn, and remember.</p> </section> <section id="comparison-to-pep-215"> <h2><a class="toc-backref" href="#comparison-to-pep-215" role="doc-backlink">Comparison to PEP 215</a></h2> <p><a class="pep reference internal" href="../pep-0215/" title="PEP 215 – String Interpolation">PEP 215</a> describes an alternate proposal for string interpolation. Unlike that PEP, this one does not propose any new syntax for Python. All the proposed new features are embodied in a new library module. <a class="pep reference internal" href="../pep-0215/" title="PEP 215 – String Interpolation">PEP 215</a> proposes a new string prefix representation such as <code class="docutils literal notranslate"><span class="pre">$""</span></code> which signal to Python that a new type of string is present. <code class="docutils literal notranslate"><span class="pre">$</span></code>-strings would have to interact with the existing r-prefixes and u-prefixes, essentially doubling the number of string prefix combinations.</p> <p><a class="pep reference internal" href="../pep-0215/" title="PEP 215 – String Interpolation">PEP 215</a> also allows for arbitrary Python expressions inside the <code class="docutils literal notranslate"><span class="pre">$</span></code>-strings, so that you could do things like:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>import sys print $"sys = $sys, sys = $sys.modules['sys']" </pre></div> </div> <p>which would return:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">sys</span> <span class="o">=</span> <span class="o"><</span><span class="n">module</span> <span class="s1">'sys'</span> <span class="p">(</span><span class="n">built</span><span class="o">-</span><span class="ow">in</span><span class="p">)</span><span class="o">></span><span class="p">,</span> <span class="n">sys</span> <span class="o">=</span> <span class="o"><</span><span class="n">module</span> <span class="s1">'sys'</span> <span class="p">(</span><span class="n">built</span><span class="o">-</span><span class="ow">in</span><span class="p">)</span><span class="o">></span> </pre></div> </div> <p>It’s generally accepted that the rules in <a class="pep reference internal" href="../pep-0215/" title="PEP 215 – String Interpolation">PEP 215</a> are safe in the sense that they introduce no new security issues (see <a class="pep reference internal" href="../pep-0215/" title="PEP 215 – String Interpolation">PEP 215</a>, “Security Issues” for details). However, the rules are still quite complex, and make it more difficult to see the substitution placeholder in the original <code class="docutils literal notranslate"><span class="pre">$</span></code>-string.</p> <p>The interesting thing is that the <code class="docutils literal notranslate"><span class="pre">Template</span></code> class defined in this PEP is designed for inheritance and, with a little extra work, it’s possible to support <a class="pep reference internal" href="../pep-0215/" title="PEP 215 – String Interpolation">PEP 215</a>’s functionality using existing Python syntax.</p> <p>For example, one could define subclasses of <code class="docutils literal notranslate"><span class="pre">Template</span></code> and dict that allowed for a more complex placeholder syntax and a mapping that evaluated those placeholders.</p> </section> <section id="internationalization"> <h2><a class="toc-backref" href="#internationalization" role="doc-backlink">Internationalization</a></h2> <p>The implementation supports internationalization by recording the original template string in the <code class="docutils literal notranslate"><span class="pre">Template</span></code> instance’s <code class="docutils literal notranslate"><span class="pre">template</span></code> attribute. This attribute would serve as the lookup key in an gettext-based catalog. It is up to the application to turn the resulting string back into a <code class="docutils literal notranslate"><span class="pre">Template</span></code> for substitution.</p> <p>However, the <code class="docutils literal notranslate"><span class="pre">Template</span></code> class was designed to work more intuitively in an internationalized application, by supporting the mixing-in of <code class="docutils literal notranslate"><span class="pre">Template</span></code> and unicode subclasses. Thus an internationalized application could create an application-specific subclass, multiply inheriting from <code class="docutils literal notranslate"><span class="pre">Template</span></code> and unicode, and using instances of that subclass as the gettext catalog key. Further, the subclass could alias the special <code class="docutils literal notranslate"><span class="pre">__mod__()</span></code> method to either <code class="docutils literal notranslate"><span class="pre">.substitute()</span></code> or <code class="docutils literal notranslate"><span class="pre">.safe_substitute()</span></code> to provide a more traditional string/unicode like <code class="docutils literal notranslate"><span class="pre">%</span></code>-operator substitution syntax.</p> </section> <section id="reference-implementation"> <h2><a class="toc-backref" href="#reference-implementation" role="doc-backlink">Reference Implementation</a></h2> <p>The implementation <a class="footnote-reference brackets" href="#id8" id="id4">[4]</a> has been committed to the Python 2.4 source tree.</p> </section> <section id="references"> <h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2> <aside class="footnote-list brackets"> <aside class="footnote brackets" id="id5" role="doc-footnote"> <dt class="label" id="id5">[<a href="#id1">1</a>]</dt> <dd>String Formatting Operations <a class="reference external" href="https://docs.python.org/release/2.6/library/stdtypes.html#string-formatting-operations">https://docs.python.org/release/2.6/library/stdtypes.html#string-formatting-operations</a></aside> <aside class="footnote brackets" id="id6" role="doc-footnote"> <dt class="label" id="id6">[<a href="#id2">2</a>]</dt> <dd>Identifiers and Keywords <a class="reference external" href="https://docs.python.org/release/2.6/reference/lexical_analysis.html#identifiers-and-keywords">https://docs.python.org/release/2.6/reference/lexical_analysis.html#identifiers-and-keywords</a></aside> <aside class="footnote brackets" id="id7" role="doc-footnote"> <dt class="label" id="id7">[<a href="#id3">3</a>]</dt> <dd><a class="reference external" href="https://mail.python.org/pipermail/python-dev/2002-June/025652.html">https://mail.python.org/pipermail/python-dev/2002-June/025652.html</a></aside> <aside class="footnote brackets" id="id8" role="doc-footnote"> <dt class="label" id="id8">[<a href="#id4">4</a>]</dt> <dd>Reference Implementation <a class="reference external" href="http://sourceforge.net/tracker/index.php?func=detail&aid=1014055&group_id=5470&atid=305470">http://sourceforge.net/tracker/index.php?func=detail&aid=1014055&group_id=5470&atid=305470</a></aside> </aside> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> <p>This document has been placed in the public domain.</p> </section> </section> <hr class="docutils" /> <p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0292.rst">https://github.com/python/peps/blob/main/peps/pep-0292.rst</a></p> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0292.rst">2023-09-09 17:39:29 GMT</a></p> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#rationale">Rationale</a></li> <li><a class="reference internal" href="#a-simpler-proposal">A Simpler Proposal</a></li> <li><a class="reference internal" href="#why-and-braces">Why <code class="docutils literal notranslate"><span class="pre">$</span></code> and Braces?</a></li> <li><a class="reference internal" href="#comparison-to-pep-215">Comparison to PEP 215</a></li> <li><a class="reference internal" href="#internationalization">Internationalization</a></li> <li><a class="reference internal" href="#reference-implementation">Reference Implementation</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> <br> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0292.rst">Page Source (GitHub)</a> </nav> </section> <script src="../_static/colour_scheme.js"></script> <script src="../_static/wrap_tables.js"></script> <script src="../_static/sticky_banner.js"></script> </body> </html>