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205 lines
12 KiB
HTML
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<title>Kapitel 04: __str__ und Darstellung</title>
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<a href="../index.html">🏠 Zur Kursübersicht</a>
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<span>›</span>
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<a href="../03_methoden_und_verhalten/index.html">Kapitel 03</a>
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<span>›</span>
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<span style="color: white; font-weight: 600;">Kapitel 04</span>
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<div class="header-badge">Kapitel 04</div>
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<h1>Schöne Ausgaben mit <code>__str__</code> & Objektlisten 🧾</h1>
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<p>Lerne anhand eines Pizzeria-Beispiels, wie du <code>print(objekt)</code> formatierst und Objekte in Listen sammelst.</p>
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</div>
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</header>
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<main class="container">
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<!-- 1. Das Problem mit Standard print -->
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<section class="card">
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<h2>1. Warum sieht <code>print(objekt)</code> standardmäßig so kryptisch aus?</h2>
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<p>
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Wenn du versuchst, ein Objekt ohne die Dunder-Methode <code>__str__</code> auszudrucken:
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</p>
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<div class="code-container">
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<pre><span class="var">mein_buch</span> = <span class="fn">Buch</span>(<span class="str">"Harry Potter"</span>, <span class="str">"J.K. Rowling"</span>)
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<span class="fn">print</span>(<span class="var">mein_buch</span>) <span class="cm"># Ausgabe: <__main__.Buch object at 0x7fa2b0485d90></span></pre>
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</div>
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<p>
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Python weiß nicht, welche Attribute für einen Menschen interessant sind.
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</p>
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<div class="box box-tipp">
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<div class="box-title">✨ Die Magie von <code>__str__</code>:</div>
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Sobald du in deiner Klasse die Methode <code>def __str__(self):</code> definierst, ruft Python diese automatisch auf, sobald das Objekt in Text umgewandelt werden soll (bei <code>print()</code>, <code>str()</code> oder in f-Strings <code>f"{objekt}"</code>)!
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</div>
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</section>
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<!-- 2. Vollständiges Beispiel: Pizza & Pizzeria -->
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<section class="card">
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<h2>2. 🍕 Vollständiges Beispiel: Pizza & Pizzeria-Bestellung</h2>
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<p>Hier siehst du zwei vollständige Klassen, die zusammenarbeiten: <code>Pizza</code> (mit <code>__str__</code>) und <code>Bestellung</code> (die eine Liste von Pizzen verwaltet):</p>
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<div class="code-container">
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<div class="code-header">
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<div class="code-dots">
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<span class="code-dot dot-red"></span>
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<span class="code-dot dot-yellow"></span>
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<span class="code-dot dot-green"></span>
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</div>
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<span>Python (Komplettes Beispiel: pizzeria.py)</span>
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</div>
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<pre><span class="cm"># ==============================================================================</span>
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<span class="cm"># KLASSE 1: Einzelne Pizza mit schöner __str__ Ausgabe</span>
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<span class="cm"># ==============================================================================</span>
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<span class="kw">class</span> <span class="fn">Pizza</span>:
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<span class="kw">def</span> <span class="fn">__init__</span>(<span class="var">self</span>, <span class="var">name</span>, <span class="var">preis</span>, <span class="var">extra_kaese</span>=<span class="kw">False</span>):
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<span class="var">self</span>.<span class="attr">name</span> = <span class="var">name</span>
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<span class="var">self</span>.<span class="attr">preis</span> = <span class="var">preis</span>
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<span class="var">self</span>.<span class="attr">extra_kaese</span> = <span class="var">extra_kaese</span>
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<span class="kw">if</span> <span class="var">extra_kaese</span>:
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<span class="var">self</span>.<span class="attr">preis</span> += <span class="num">1.50</span> <span class="cm"># Extra Käse kostet 1.50€ mehr</span>
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<span class="cm"># Diese Methode bestimmt, wie die Pizza als Text aussieht:</span>
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<span class="kw">def</span> <span class="fn">__str__</span>(<span class="var">self</span>):
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<span class="var">kaese_hinweis</span> = <span class="str">" (mit Extra-Käse 🧀)"</span> <span class="kw">if</span> <span class="var">self</span>.<span class="attr">extra_kaese</span> <span class="kw">else</span> <span class="str">""</span>
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<span class="kw">return</span> <span class="str">f"Pizza {self.name}{kaese_hinweis} - {self.preis:.2f} €"</span>
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<span class="cm"># ==============================================================================</span>
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<span class="cm"># KLASSE 2: Bestellung verwaltet eine Liste von Pizzen</span>
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<span class="cm"># ==============================================================================</span>
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<span class="kw">class</span> <span class="fn">Bestellung</span>:
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<span class="kw">def</span> <span class="fn">__init__</span>(<span class="var">self</span>, <span class="var">kunden_name</span>):
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<span class="var">self</span>.<span class="attr">kunden_name</span> = <span class="var">kunden_name</span>
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<span class="var">self</span>.<span class="attr">pizzen</span> = [] <span class="cm"># Startet mit einer leeren Liste</span>
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<span class="kw">def</span> <span class="fn">pizza_hinzufuegen</span>(<span class="var">self</span>, <span class="var">pizza</span>):
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<span class="var">self</span>.<span class="attr">pizzen</span>.<span class="fn">append</span>(<span class="var">pizza</span>)
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<span class="kw">def</span> <span class="fn">gesamtpreis</span>(<span class="var">self</span>):
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<span class="kw">return</span> <span class="fn">sum</span>(<span class="var">p</span>.<span class="attr">preis</span> <span class="kw">for</span> <span class="var">p</span> <span class="kw">in</span> <span class="var">self</span>.<span class="attr">pizzen</span>)
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<span class="kw">def</span> <span class="fn">rechnung_drucken</span>(<span class="var">self</span>):
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<span class="var">zeilen</span> = [<span class="str">f"=== RECHNUNG FÜR {self.kunden_name.upper()} ==="</span>]
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<span class="kw">for</span> <span class="var">p</span> <span class="kw">in</span> <span class="var">self</span>.<span class="attr">pizzen</span>:
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<span class="var">zeilen</span>.<span class="fn">append</span>(<span class="str">f"- {p}"</span>) <span class="cm"># Hier wird automatisch p.__str__() aufgerufen!</span>
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<span class="var">zeilen</span>.<span class="fn">append</span>(<span class="str">"--------------------------------"</span>)
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<span class="var">zeilen</span>.<span class="fn">append</span>(<span class="str">f"Gesamtsumme: {self.gesamtpreis():.2f} €"</span>)
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<span class="kw">return</span> <span class="str">"\n"</span>.<span class="fn">join</span>(<span class="var">zeilen</span>)
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<span class="cm"># Ausprobieren:</span>
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<span class="kw">if</span> <span class="var">__name__</span> == <span class="str">"__main__"</span>:
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<span class="var">bestellung</span> = <span class="fn">Bestellung</span>(<span class="str">"Lisa"</span>)
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<span class="var">p1</span> = <span class="fn">Pizza</span>(<span class="str">"Margherita"</span>, <span class="num">8.50</span>)
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<span class="var">p2</span> = <span class="fn">Pizza</span>(<span class="str">"Salami"</span>, <span class="num">9.50</span>, <span class="var">extra_kaese</span>=<span class="kw">True</span>)
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<span class="var">bestellung</span>.<span class="fn">pizza_hinzufuegen</span>(<span class="var">p1</span>)
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<span class="var">bestellung</span>.<span class="fn">pizza_hinzufuegen</span>(<span class="var">p2</span>)
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<span class="fn">print</span>(<span class="var">bestellung</span>.<span class="fn">rechnung_drucken</span>())</pre>
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</div>
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</section>
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<!-- 3. Stolperfallen -->
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<section class="card">
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<h2>3. ⚠️ Wichtig bei <code>__str__</code></h2>
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<div class="box box-danger">
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<div class="box-title">1. Immer 'return', niemals 'print'!</div>
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<code>def __str__(self): return f"..."</code> ✓<br>
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<code>def __str__(self): print(f"...")</code> ✗ (Falsch! <code>__str__</code> MUSS einen String mit <code>return</code> zurückgeben!)
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</div>
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<div class="box box-warning">
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<div class="box-title">2. Rückgabewert muss zwingend ein String sein:</div>
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Wenn du eine Zahl zurückgibst (z.B. <code>return self.preis</code>), stürzt Python ab mit <code>TypeError: __str__ returned non-string</code>. Nutze immer f-Strings <code>f"{self.preis}"</code> oder <code>str(...)</code>.
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</div>
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</section>
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<!-- 4. Aufgaben Card -->
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<section class="card">
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<h2>4. 🎯 Deine Aufgaben in <code>aufgabe.py</code></h2>
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<ul class="task-list">
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<li class="task-item">
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<input type="checkbox" class="task-checkbox" id="t4_1">
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<label for="t4_1" class="task-label">
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<strong>TODO 1:</strong> <code>Artikel.__init__(self, name, preis, anzahl=1)</code>.
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</label>
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</li>
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<li class="task-item">
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<input type="checkbox" class="task-checkbox" id="t4_2">
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<label for="t4_2" class="task-label">
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<strong>TODO 2:</strong> <code>Artikel.gesamtpreis(self)</code> -> <code>self.preis * self.anzahl</code>.
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</label>
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</li>
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<li class="task-item">
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<input type="checkbox" class="task-checkbox" id="t4_3">
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<label for="t4_3" class="task-label">
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<strong>TODO 3:</strong> <code>Artikel.__str__(self)</code> -> <code>"{anzahl}x {name} (je {preis:.2f} €) = {gesamtpreis:.2f} €"</code>.
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</label>
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</li>
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<li class="task-item">
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<input type="checkbox" class="task-checkbox" id="t4_4">
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<label for="t4_4" class="task-label">
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<strong>TODO 4-7:</strong> <code>Warenkorb</code> mit <code>artikel_hinzufuegen</code>, <code>gesamtsumme</code> und <code>bon_text</code>.
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</label>
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</li>
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</ul>
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<h3>💡 Lösungshinweise (Aufklappen bei Bedarf)</h3>
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<details>
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<summary>🔍 Tipp zu TODO 7 (Kassenzettel zusammensetzen)</summary>
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<div class="details-content">
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<div class="code-container">
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<pre><span class="kw">def</span> <span class="fn">bon_text</span>(<span class="var">self</span>):
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<span class="var">zeilen</span> = [<span class="str">"--- KASSENZETTEL ---"</span>]
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<span class="kw">for</span> <span class="var">artikel</span> <span class="kw">in</span> <span class="var">self</span>.<span class="attr">artikel_liste</span>:
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<span class="var">zeilen</span>.<span class="fn">append</span>(<span class="fn">str</span>(<span class="var">artikel</span>)) <span class="cm"># Nutzt automatisch __str__ des Artikels!</span>
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<span class="var">zeilen</span>.<span class="fn">append</span>(<span class="str">"--------------------"</span>)
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<span class="var">zeilen</span>.<span class="fn">append</span>(<span class="str">f"Gesamtsumme: {self.gesamtsumme():.2f} €"</span>)
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<span class="kw">return</span> <span class="str">"\n"</span>.<span class="fn">join</span>(<span class="var">zeilen</span>)</pre>
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</div>
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</div>
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</details>
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</section>
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<!-- 5. Testen & Navigation -->
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<section class="card">
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<h2>5. 🧪 Lösung überprüfen</h2>
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<p>Führe im Terminal im Ordner <code>04_str_und_darstellung</code> folgenden Befehl aus:</p>
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<div class="code-container">
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<div class="code-header">
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<span>Terminal</span>
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</div>
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<pre><code>python3 test_aufgabe.py</code></pre>
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</div>
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