Tout est un booléen

Conversions en booléen

Dans Python, n'importe quelle valeur ou objet peut être converti en booléen à l'aide de la fonction bool.

>>> bool(0)
False
>>> bool(1)
True
>>> bool(2)
True
>>> bool(-1)
True
>>> bool(0.0)
False
>>> bool("False")
True
>>> bool("")
False

Pour les nombres, seul 0 correspond à False. Toute valeur non nulle correspond à True. L'expression bool(a) est donc équivalente à a != 0.

Pour les chaînes de caractères, bool(texte) est donc équivalente à texte != "".

Exercice 3 : conversions implicites

La conversion peut également se faire de façon implicite, mais attention à ne pas rendre son programme illisible dans cet exemple :

###(Dés-)Active le code après la ligne # Tests (insensible à la casse)
(Ctrl+I)

Tronquer ou non le feedback dans les terminaux (sortie standard & stacktrace / relancer le code pour appliquer)

Exécuter le code
(Ctrl+S)TéléchargerTéléverserRéinitialiser l'éditeurSauvegarder dans le navigateurÉvaluations restantes : ∞/∞

Déterminer une valeur pour nom permettant d'afficher le premier message et une autre permettant d'afficher le deuxième.

Solution

Pour avoir le premier message, il suffit de mettre n'importe quel nom qui n'est pas le texte vide.

Pour avoir le deuxième message, il faut mettre le texte vide "".

En fait, ma ligne if nom: est équivalente à if bool(nom):, qui est équivalente à if nom != "":. C'est une convertion implicite.

Les convertions implicites peuvent rendre la recherche de bugs très compliquée. Il vaut mieux les éviter, à moins de savoir très bien ce que l'on fait.

Exercice 4 (sur papier)

On considère la fonction ci-dessous :

def valide(x):
    if x == 1 or 2 or 3:
        print("bien")
    else:
        print("mal")

Sans vérifier le résultat avec l'ordinateur, quel doit être le message affiché par :

1. valide(1)
2. valide(2)
3. valide(3)
4. valide(4)

Exercice 5

Vérifier les réponses de l'exercice précédent et proposer une correction de cette fonction.

###(Dés-)Active le code après la ligne # Tests (insensible à la casse)
(Ctrl+I)

Tronquer ou non le feedback dans les terminaux (sortie standard & stacktrace / relancer le code pour appliquer)

Exécuter le code
(Ctrl+S)Valider
(Ctrl+Enter)TéléchargerTéléverserRéinitialiser l'éditeurSauvegarder dans le navigateurÉvaluations restantes : 5/5

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