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id="pep-page-section"> <header> <h1>Python Enhancement Proposals</h1> <ul class="breadcrumbs"> <li><a href="https://www.python.org/" title="The Python Programming Language">Python</a> » </li> <li><a href="../pep-0000/">PEP Index</a> » </li> <li>PEP 215</li> </ul> <button id="colour-scheme-cycler" onClick="setColourScheme(nextColourScheme())"> <svg aria-hidden="true" class="colour-scheme-icon-when-auto"><use href="#svg-sun-half"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-dark"><use href="#svg-moon"></use></svg> <svg aria-hidden="true" class="colour-scheme-icon-when-light"><use href="#svg-sun"></use></svg> <span class="visually-hidden">Toggle light / dark / auto colour theme</span> </button> </header> <article> <section id="pep-content"> <h1 class="page-title">PEP 215 – String Interpolation</h1> <dl class="rfc2822 field-list simple"> <dt class="field-odd">Author<span class="colon">:</span></dt> <dd class="field-odd">Ka-Ping Yee <ping at zesty.ca></dd> <dt class="field-even">Status<span class="colon">:</span></dt> <dd class="field-even"><abbr title="Replaced by another succeeding PEP">Superseded</abbr></dd> <dt class="field-odd">Type<span class="colon">:</span></dt> <dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd> <dt class="field-even">Created<span class="colon">:</span></dt> <dd class="field-even">24-Jul-2000</dd> <dt class="field-odd">Python-Version<span class="colon">:</span></dt> <dd class="field-odd">2.1</dd> <dt class="field-even">Post-History<span class="colon">:</span></dt> <dd class="field-even"><p></p></dd> <dt class="field-odd">Superseded-By<span class="colon">:</span></dt> <dd class="field-odd"><a class="reference external" href="../pep-0292/">292</a></dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> <li><a class="reference internal" href="#specification">Specification</a></li> <li><a class="reference internal" href="#examples">Examples</a></li> <li><a class="reference internal" href="#discussion">Discussion</a></li> <li><a class="reference internal" href="#security-issues">Security Issues</a></li> <li><a class="reference internal" href="#implementation">Implementation</a></li> <li><a class="reference internal" href="#references">References</a></li> </ul> </details></section> <div class="pep-banner sticky-banner deprecated superseded admonition important"> <p class="admonition-title">Important</p> <p>This PEP has been superseded by <a class="pep reference internal" href="../pep-0292/" title="PEP 292 – Simpler String Substitutions">PEP 292</a>.</p> <p class="close-button">×</p> <p></p> </div> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> <p>This document proposes a string interpolation feature for Python to allow easier string formatting. The suggested syntax change is the introduction of a ‘$’ prefix that triggers the special interpretation of the ‘$’ character within a string, in a manner reminiscent to the variable interpolation found in Unix shells, awk, Perl, or Tcl.</p> </section> <section id="copyright"> <h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2> <p>This document is in the public domain.</p> </section> <section id="specification"> <h2><a class="toc-backref" href="#specification" role="doc-backlink">Specification</a></h2> <p>Strings may be preceded with a ‘$’ prefix that comes before the leading single or double quotation mark (or triplet) and before any of the other string prefixes (‘r’ or ‘u’). Such a string is processed for interpolation after the normal interpretation of backslash-escapes in its contents. The processing occurs just before the string is pushed onto the value stack, each time the string is pushed. In short, Python behaves exactly as if ‘$’ were a unary operator applied to the string. The operation performed is as follows:</p> <p>The string is scanned from start to end for the ‘$’ character (<code class="docutils literal notranslate"><span class="pre">\x24</span></code> in 8-bit strings or <code class="docutils literal notranslate"><span class="pre">\u0024</span></code> in Unicode strings). If there are no ‘$’ characters present, the string is returned unchanged.</p> <p>Any ‘$’ found in the string, followed by one of the two kinds of expressions described below, is replaced with the value of the expression as evaluated in the current namespaces. The value is converted with <code class="docutils literal notranslate"><span class="pre">str()</span></code> if the containing string is an 8-bit string, or with <code class="docutils literal notranslate"><span class="pre">unicode()</span></code> if it is a Unicode string.</p> <ol class="arabic simple"> <li>A Python identifier optionally followed by any number of trailers, where a trailer consists of: - a dot and an identifier, - an expression enclosed in square brackets, or - an argument list enclosed in parentheses (This is exactly the pattern expressed in the Python grammar by “<code class="docutils literal notranslate"><span class="pre">NAME</span> <span class="pre">trailer*</span></code>”, using the definitions in <code class="docutils literal notranslate"><span class="pre">Grammar/Grammar</span></code>.)</li> <li>Any complete Python expression enclosed in curly braces.</li> </ol> <p>Two dollar-signs (“$$”) are replaced with a single “$”.</p> </section> <section id="examples"> <h2><a class="toc-backref" href="#examples" role="doc-backlink">Examples</a></h2> <p>Here is an example of an interactive session exhibiting the expected behaviour of this feature.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>>>> a, b = 5, 6 >>> print $'a = $a, b = $b' a = 5, b = 6 >>> $u'uni${a}ode' u'uni5ode' >>> print $'\$a' 5 >>> print $r'\$a' \5 >>> print $'$$$a.$b' $5.6 >>> print $'a + b = ${a + b}' a + b = 11 >>> import sys >>> print $'References to $a: $sys.getrefcount(a)' References to 5: 15 >>> print $"sys = $sys, sys = $sys.modules['sys']" sys = <module 'sys' (built-in)>, sys = <module 'sys' (built-in)> >>> print $'BDFL = $sys.copyright.split()[4].upper()' BDFL = GUIDO </pre></div> </div> </section> <section id="discussion"> <h2><a class="toc-backref" href="#discussion" role="doc-backlink">Discussion</a></h2> <p>‘$’ is chosen as the interpolation character within the string for the sake of familiarity, since it is already used for this purpose in many other languages and contexts.</p> <p>It is then natural to choose ‘$’ as a prefix, since it is a mnemonic for the interpolation character.</p> <p>Trailers are permitted to give this interpolation mechanism even more power than the interpolation available in most other languages, while the expression to be interpolated remains clearly visible and free of curly braces.</p> <p>‘$’ works like an operator and could be implemented as an operator, but that prevents the compile-time optimization and presents security issues. So, it is only allowed as a string prefix.</p> </section> <section id="security-issues"> <h2><a class="toc-backref" href="#security-issues" role="doc-backlink">Security Issues</a></h2> <p>“$” has the power to eval, but only to eval a literal. As described here (a string prefix rather than an operator), it introduces no new security issues since the expressions to be evaluated must be literally present in the code.</p> </section> <section id="implementation"> <h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2> <p>The <code class="docutils literal notranslate"><span class="pre">Itpl</span></code> module at <a class="footnote-reference brackets" href="#id2" id="id1">[1]</a> provides a prototype of this feature. It uses the <code class="docutils literal notranslate"><span class="pre">tokenize</span></code> module to find the end of an expression to be interpolated, then calls <code class="docutils literal notranslate"><span class="pre">eval()</span></code> on the expression each time a value is needed. In the prototype, the expression is parsed and compiled again each time it is evaluated.</p> <p>As an optimization, interpolated strings could be compiled directly into the corresponding bytecode; that is,</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$'a = $a, b = $b' </pre></div> </div> <p>could be compiled as though it were the expression</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="s1">'a = '</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="n">a</span><span class="p">)</span> <span class="o">+</span> <span class="s1">', b = '</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="n">b</span><span class="p">))</span> </pre></div> </div> <p>so that it only needs to be compiled once.</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="id2" role="doc-footnote"> <dt class="label" id="id2">[<a href="#id1">1</a>]</dt> <dd><a class="reference external" href="http://www.lfw.org/python/Itpl.py">http://www.lfw.org/python/Itpl.py</a></aside> </aside> </section> </section> <hr class="docutils" /> <p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0215.rst">https://github.com/python/peps/blob/main/peps/pep-0215.rst</a></p> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0215.rst">2024-04-14 13:35:25 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="#copyright">Copyright</a></li> <li><a class="reference internal" href="#specification">Specification</a></li> <li><a class="reference internal" href="#examples">Examples</a></li> <li><a class="reference internal" href="#discussion">Discussion</a></li> <li><a class="reference internal" href="#security-issues">Security Issues</a></li> <li><a class="reference internal" href="#implementation">Implementation</a></li> <li><a class="reference internal" href="#references">References</a></li> </ul> <br> <a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0215.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>