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Tutur33
128 lines
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128 lines
6.9 KiB
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<h1 id="topic_header_text">Portée de variables: variables globales et locales</h1>
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<p></p>
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<p class="rvps7"><span class="rvts49">La </span><span class="rvts88">portée d'une variable</span><span class="rvts49"> est l'endroit du programme où on peut accéder à la variable.</span></p>
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<p class="rvps7"><span class="rvts49">Observons le script suivant :</span></p>
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<p class="rvps8"><span class="rvts38">a = 10 </span><span class="rvts39"># variable globale au programme</span><br/><span class="rvts46">def</span><span class="rvts38"> </span><span class="rvts41">mafonction</span><span class="rvts38">():</span><br/><span class="rvts38"> a = 20 </span><span class="rvts39"># variable locale à la fonction</span><br/><span class="rvts38"> </span><span class="rvts40">print</span><span class="rvts38">(a)</span><br/><span class="rvts38"> </span><span class="rvts46">return</span></p>
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<p class="rvps8"><span class="rvts37">>>></span><span class="rvts38"> </span><span class="rvts40">print</span><span class="rvts38">(a) </span><span class="rvts39"># nous sommmes dans l'espace global du programme</span><br/><img alt="" style="padding : 1px;" src="lib/NouvelElement7.png"><br/><span class="rvts37">>>></span><span class="rvts38"> mafonction() </span><span class="rvts39"># nous sommes dans l'espace local de la fonction</span><br/><img alt="" style="padding : 1px;" src="lib/NouvelElement7.png"><br/><span class="rvts37">>>></span><span class="rvts38"> </span><span class="rvts40">print</span><span class="rvts38">(a) </span><span class="rvts39"># de retour dans l'espace global</span><br/><img alt="" style="padding : 1px;" src="lib/NouvelElement7.png"></p>
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<p class="rvps7"><span class="rvts11">Nous avons deux variables différentes qui portent le même nom </span><span class="rvts17">a</span></p>
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<p class="rvps7"><span class="rvts11">Une variable </span><span class="rvts17">a</span><span class="rvts11"> de valeur 20 est créée dans la fonction : c'est une </span><span class="rvts43">variable locale</span><span class="rvts11"> à la fonction.</span><br/><span class="rvts11">Elle est détruite dès que l'on sort de la fonction.</span></p>
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<h4 class="rvps13"><span class="rvts0"><span class="rvts80">global</span></span></h4>
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<p class="rvps7"><span class="rvts49">L'instruction </span><span class="rvts76">global</span><span class="rvts49"> rend une variable globale :</span></p>
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<p class="rvps8"><span class="rvts38">a = 10 </span><span class="rvts39"># variable globale</span><br/><span class="rvts46">def</span><span class="rvts38"> </span><span class="rvts41">mafonction</span><span class="rvts38">():</span><br/><span class="rvts38"> </span><span class="rvts46">global</span><span class="rvts38"> a </span><span class="rvts39"># la variable est maintenant globale</span><br/><span class="rvts38"> a = 20</span><br/><span class="rvts38"> </span><span class="rvts40">print</span><span class="rvts38">(a)</span><br/><span class="rvts38"> </span><span class="rvts46">return</span></p>
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<p class="rvps8"><span class="rvts37">>>></span><span class="rvts38"> </span><span class="rvts40">print</span><span class="rvts38">(a)</span><br/><img alt="" style="padding : 1px;" src="lib/NouvelElement7.png"><br/><span class="rvts37">>>></span><span class="rvts38"> mafonction()</span><br/><img alt="" style="padding : 1px;" src="lib/NouvelElement7.png"><br/><span class="rvts37">>>></span><span class="rvts38"> </span><span class="rvts40">print</span><span class="rvts38">(a)</span><br/><img alt="" style="padding : 1px;" src="lib/NouvelElement7.png"></p>
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<p class="rvps7"><span class="rvts11">Remarque : il est préférable d'éviter l'utilisation de l'instruction </span><span class="rvts17">global</span><span class="rvts11"> car c'est une source d'erreurs (on peut ainsi modifier le contenu d'une variable globale en croyant agir sur une variable locale).</span><br/><span class="rvts11">La sagesse recommande donc de suivre la règle suivante :</span></p>
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<ul style="text-indent: 0px; padding: 0; margin: 0 0 0 24px; list-style-position: outside; list-style-type: square;">
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<li class="rvps2"><span class="rvts11">ne jamais affecter dans un bloc de code local une variable de même nom qu'une variable globale</span></li>
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</ul>
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<p class="rvps4" style="clear: both;"><span class="rvts15">Créé avec HelpNDoc Personal Edition: </span><a class="rvts16" href="https://www.helpndoc.com/fr/outils-creation-aide">Environnement de création d'aide complet</a></p>
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