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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 3140 – str(container) should call str(item), not repr(item) | peps.python.org</title> <link rel="shortcut icon" href="../_static/py.png"> <link rel="canonical" href="https://peps.python.org/pep-3140/"> <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 3140 – str(container) should call str(item), not repr(item) | peps.python.org'> <meta property="og:description" content="This document discusses the advantages and disadvantages of the current implementation of str(container). 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Jewett <jimjjewett at gmail.com></dd> <dt class="field-even">Discussions-To<span class="colon">:</span></dt> <dd class="field-even"><a class="reference external" href="https://mail.python.org/pipermail/python-3000/">Python-3000 list</a></dd> <dt class="field-odd">Status<span class="colon">:</span></dt> <dd class="field-odd"><abbr title="Formally declined and will not be accepted">Rejected</abbr></dd> <dt class="field-even">Type<span class="colon">:</span></dt> <dd class="field-even"><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-odd">Created<span class="colon">:</span></dt> <dd class="field-odd">27-May-2008</dd> <dt class="field-even">Post-History<span class="colon">:</span></dt> <dd class="field-even">28-May-2008</dd> </dl> <hr class="docutils" /> <section id="contents"> <details><summary>Table of Contents</summary><ul class="simple"> <li><a class="reference internal" href="#rejection">Rejection</a></li> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#motivation">Motivation</a></li> <li><a class="reference internal" href="#current-situation">Current situation</a></li> <li><a class="reference internal" href="#a-different-approach-call-str-item">A different approach - call <code class="docutils literal notranslate"><span class="pre">str(item)</span></code></a></li> <li><a class="reference internal" href="#backward-compatibility">Backward compatibility</a></li> <li><a class="reference internal" href="#references">References</a></li> <li><a class="reference internal" href="#copyright">Copyright</a></li> </ul> </details></section> <section id="rejection"> <h2><a class="toc-backref" href="#rejection" role="doc-backlink">Rejection</a></h2> <p>Guido said this would cause too much disturbance too close to beta. See <a class="footnote-reference brackets" href="#id2" id="id1">[1]</a>.</p> </section> <section id="abstract"> <h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2> <p>This document discusses the advantages and disadvantages of the current implementation of <code class="docutils literal notranslate"><span class="pre">str(container)</span></code>. It also discusses the pros and cons of a different approach - to call <code class="docutils literal notranslate"><span class="pre">str(item)</span></code> instead of <code class="docutils literal notranslate"><span class="pre">repr(item)</span></code>.</p> </section> <section id="motivation"> <h2><a class="toc-backref" href="#motivation" role="doc-backlink">Motivation</a></h2> <p>Currently <code class="docutils literal notranslate"><span class="pre">str(container)</span></code> calls <code class="docutils literal notranslate"><span class="pre">repr</span></code> on items. Arguments for it:</p> <ul class="simple"> <li>containers refuse to guess what the user wants to see on <code class="docutils literal notranslate"><span class="pre">str(container)</span></code> - surroundings, delimiters, and so on;</li> <li><code class="docutils literal notranslate"><span class="pre">repr(item)</span></code> usually displays type information - apostrophes around strings, class names, etc.</li> </ul> <p>Arguments against:</p> <ul class="simple"> <li>it’s illogical; <code class="docutils literal notranslate"><span class="pre">str()</span></code> is expected to call <code class="docutils literal notranslate"><span class="pre">__str__</span></code> if it exists, not <code class="docutils literal notranslate"><span class="pre">__repr__</span></code>;</li> <li>there is no standard way to print a container’s content calling items’ <code class="docutils literal notranslate"><span class="pre">__str__</span></code>, that’s inconvenient in cases where <code class="docutils literal notranslate"><span class="pre">__str__</span></code> and <code class="docutils literal notranslate"><span class="pre">__repr__</span></code> return different results;</li> <li><code class="docutils literal notranslate"><span class="pre">repr(item)</span></code> sometimes do wrong things (hex-escapes non-ASCII strings, e.g.)</li> </ul> <p>This PEP proposes to change how <code class="docutils literal notranslate"><span class="pre">str(container)</span></code> works. It is proposed to mimic how <code class="docutils literal notranslate"><span class="pre">repr(container)</span></code> works except one detail - call <code class="docutils literal notranslate"><span class="pre">str</span></code> on items instead of <code class="docutils literal notranslate"><span class="pre">repr</span></code>. This allows a user to choose what results she want to get - from <code class="docutils literal notranslate"><span class="pre">item.__repr__</span></code> or <code class="docutils literal notranslate"><span class="pre">item.__str__</span></code>.</p> </section> <section id="current-situation"> <h2><a class="toc-backref" href="#current-situation" role="doc-backlink">Current situation</a></h2> <p>Most container types (tuples, lists, dicts, sets, etc.) do not implement <code class="docutils literal notranslate"><span class="pre">__str__</span></code> method, so <code class="docutils literal notranslate"><span class="pre">str(container)</span></code> calls <code class="docutils literal notranslate"><span class="pre">container.__repr__</span></code>, and <code class="docutils literal notranslate"><span class="pre">container.__repr__</span></code>, once called, forgets it is called from <code class="docutils literal notranslate"><span class="pre">str</span></code> and always calls <code class="docutils literal notranslate"><span class="pre">repr</span></code> on the container’s items.</p> <p>This behaviour has advantages and disadvantages. One advantage is that most items are represented with type information - strings are surrounded by apostrophes, instances may have both class name and instance data:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">([</span><span class="mi">42</span><span class="p">,</span> <span class="s1">'42'</span><span class="p">])</span> <span class="go">[42, '42']</span> <span class="gp">>>> </span><span class="nb">print</span><span class="p">([</span><span class="n">Decimal</span><span class="p">(</span><span class="s1">'42'</span><span class="p">),</span> <span class="n">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">()])</span> <span class="go">[Decimal("42"), datetime.datetime(2008, 5, 27, 19, 57, 43, 485028)]</span> </pre></div> </div> <p>The disadvantage is that <code class="docutils literal notranslate"><span class="pre">__repr__</span></code> often returns technical data (like ‘<code class="docutils literal notranslate"><span class="pre"><object</span> <span class="pre">at</span> <span class="pre">address></span></code>’) or unreadable string (hex-encoded string if the input is non-ASCII string):</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">([</span><span class="s1">'тест'</span><span class="p">])</span> <span class="go">['\xd4\xc5\xd3\xd4']</span> </pre></div> </div> <p>One of the motivations for <a class="pep reference internal" href="../pep-3138/" title="PEP 3138 – String representation in Python 3000">PEP 3138</a> is that neither <code class="docutils literal notranslate"><span class="pre">repr</span></code> nor <code class="docutils literal notranslate"><span class="pre">str</span></code> will allow the sensible printing of dicts whose keys are non-ASCII text strings. Now that Unicode identifiers are allowed, it includes Python’s own attribute dicts. This also includes JSON serialization (and caused some hoops for the json lib).</p> <p><a class="pep reference internal" href="../pep-3138/" title="PEP 3138 – String representation in Python 3000">PEP 3138</a> proposes to fix this by breaking the “repr is safe ASCII” invariant, and changing the way <code class="docutils literal notranslate"><span class="pre">repr</span></code> (which is used for persistence) outputs some objects, with system-dependent failures.</p> <p>Changing how <code class="docutils literal notranslate"><span class="pre">str(container)</span></code> works would allow easy debugging in the normal case, and retain the safety of ASCII-only for the machine-readable case. The only downside is that <code class="docutils literal notranslate"><span class="pre">str(x)</span></code> and <code class="docutils literal notranslate"><span class="pre">repr(x)</span></code> would more often be different – but only in those cases where the current almost-the-same version is insufficient.</p> <p>It also seems illogical that <code class="docutils literal notranslate"><span class="pre">str(container)</span></code> calls <code class="docutils literal notranslate"><span class="pre">repr</span></code> on items instead of <code class="docutils literal notranslate"><span class="pre">str</span></code>. It’s only logical to expect following code:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">Test</span><span class="p">:</span> <span class="k">def</span><span class="w"> </span><span class="fm">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="s2">"STR"</span> <span class="k">def</span><span class="w"> </span><span class="fm">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="s2">"REPR"</span> <span class="n">test</span> <span class="o">=</span> <span class="n">Test</span><span class="p">()</span> <span class="nb">print</span><span class="p">(</span><span class="n">test</span><span class="p">)</span> <span class="nb">print</span><span class="p">(</span><span class="nb">repr</span><span class="p">(</span><span class="n">test</span><span class="p">))</span> <span class="nb">print</span><span class="p">([</span><span class="n">test</span><span class="p">])</span> <span class="nb">print</span><span class="p">(</span><span class="nb">str</span><span class="p">([</span><span class="n">test</span><span class="p">]))</span> </pre></div> </div> <p>to print:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">STR</span> <span class="n">REPR</span> <span class="p">[</span><span class="n">STR</span><span class="p">]</span> <span class="p">[</span><span class="n">STR</span><span class="p">]</span> </pre></div> </div> <p>where it actually prints:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">STR</span> <span class="n">REPR</span> <span class="p">[</span><span class="n">REPR</span><span class="p">]</span> <span class="p">[</span><span class="n">REPR</span><span class="p">]</span> </pre></div> </div> <p>Especially it is illogical to see that print in Python 2 uses <code class="docutils literal notranslate"><span class="pre">str</span></code> if it is called on what seems to be a tuple:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span> <span class="n">Decimal</span><span class="p">(</span><span class="s1">'42'</span><span class="p">),</span> <span class="n">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">()</span> <span class="go">42 2008-05-27 20:16:22.534285</span> </pre></div> </div> <p>where on an actual tuple it prints:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">((</span><span class="n">Decimal</span><span class="p">(</span><span class="s1">'42'</span><span class="p">),</span> <span class="n">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">()))</span> <span class="go">(Decimal("42"), datetime.datetime(2008, 5, 27, 20, 16, 27, 937911))</span> </pre></div> </div> </section> <section id="a-different-approach-call-str-item"> <h2><a class="toc-backref" href="#a-different-approach-call-str-item" role="doc-backlink">A different approach - call <code class="docutils literal notranslate"><span class="pre">str(item)</span></code></a></h2> <p>For example, with numbers it is often only the value that people care about.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span> <span class="n">Decimal</span><span class="p">(</span><span class="s1">'3'</span><span class="p">)</span> <span class="go">3</span> </pre></div> </div> <p>But putting the value in a list forces users to read the type information, exactly as if <code class="docutils literal notranslate"><span class="pre">repr</span></code> had been called for the benefit of a machine:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span> <span class="p">[</span><span class="n">Decimal</span><span class="p">(</span><span class="s1">'3'</span><span class="p">)]</span> <span class="go">[Decimal("3")]</span> </pre></div> </div> <p>After this change, the type information would not clutter the <code class="docutils literal notranslate"><span class="pre">str</span></code> output:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span> <span class="s2">"</span><span class="si">%s</span><span class="s2">"</span><span class="o">.</span><span class="n">format</span><span class="p">([</span><span class="n">Decimal</span><span class="p">(</span><span class="s1">'3'</span><span class="p">)])</span> <span class="go">[3]</span> <span class="gp">>>> </span><span class="nb">str</span><span class="p">([</span><span class="n">Decimal</span><span class="p">(</span><span class="s1">'3'</span><span class="p">)])</span> <span class="c1"># ==</span> <span class="go">[3]</span> </pre></div> </div> <p>But it would still be available if desired:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span> <span class="s2">"</span><span class="si">%r</span><span class="s2">"</span><span class="o">.</span><span class="n">format</span><span class="p">([</span><span class="n">Decimal</span><span class="p">(</span><span class="s1">'3'</span><span class="p">)])</span> <span class="go">[Decimal('3')]</span> <span class="gp">>>> </span><span class="nb">repr</span><span class="p">([</span><span class="n">Decimal</span><span class="p">(</span><span class="s1">'3'</span><span class="p">)])</span> <span class="c1"># ==</span> <span class="go">[Decimal('3')]</span> </pre></div> </div> <p>There is a number of strategies to fix the problem. The most radical is to change <code class="docutils literal notranslate"><span class="pre">__repr__</span></code> so it accepts a new parameter (flag) “called from <code class="docutils literal notranslate"><span class="pre">str</span></code>, so call <code class="docutils literal notranslate"><span class="pre">str</span></code> on items, not <code class="docutils literal notranslate"><span class="pre">repr</span></code>”. The drawback of the proposal is that every <code class="docutils literal notranslate"><span class="pre">__repr__</span></code> implementation must be changed. Introspection could help a bit (inspect <code class="docutils literal notranslate"><span class="pre">__repr__</span></code> before calling if it accepts 2 or 3 parameters), but introspection doesn’t work on classes written in C, like all built-in containers.</p> <p>Less radical proposal is to implement <code class="docutils literal notranslate"><span class="pre">__str__</span></code> methods for built-in container types. The obvious drawback is a duplication of effort - all those <code class="docutils literal notranslate"><span class="pre">__str__</span></code> and <code class="docutils literal notranslate"><span class="pre">__repr__</span></code> implementations are only differ in one small detail - if they call <code class="docutils literal notranslate"><span class="pre">str</span></code> or <code class="docutils literal notranslate"><span class="pre">repr</span></code> on items.</p> <p>The most conservative proposal is not to change str at all but to allow developers to implement their own application- or library-specific pretty-printers. The drawback is again a multiplication of effort and proliferation of many small specific container-traversal algorithms.</p> </section> <section id="backward-compatibility"> <h2><a class="toc-backref" href="#backward-compatibility" role="doc-backlink">Backward compatibility</a></h2> <p>In those cases where type information is more important than usual, it will still be possible to get the current results by calling <code class="docutils literal notranslate"><span class="pre">repr</span></code> explicitly.</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>Guido van Rossum, PEP: str(container) should call str(item), not repr(item) <a class="reference external" href="https://mail.python.org/pipermail/python-3000/2008-May/013876.html">https://mail.python.org/pipermail/python-3000/2008-May/013876.html</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-3140.rst">https://github.com/python/peps/blob/main/peps/pep-3140.rst</a></p> <p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-3140.rst">2025-02-01 08:59:27 GMT</a></p> </article> <nav id="pep-sidebar"> <h2>Contents</h2> <ul> <li><a class="reference internal" href="#rejection">Rejection</a></li> <li><a class="reference internal" href="#abstract">Abstract</a></li> <li><a class="reference internal" href="#motivation">Motivation</a></li> <li><a class="reference internal" href="#current-situation">Current situation</a></li> <li><a class="reference internal" href="#a-different-approach-call-str-item">A different approach - call <code class="docutils literal notranslate"><span class="pre">str(item)</span></code></a></li> <li><a class="reference internal" href="#backward-compatibility">Backward compatibility</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-3140.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>