L'utilisateur récalcitrant

Faire confiance à l'utilisateur

Parfois, il faut demander une valeur à l'utilisateur en cours d'exécution et il faut que cette valeur vérifie certaines contraintes, comme par exemple être un entier positif. Il est possible de faire confiance à l'utilisateur, mais la confiance n'exclut pas le contrôle.

Vérifier les pré-conditions avec des assertions

Les pré-conditions sont des conditions sur les arguments données aux fonctions. Par exemple, pour la fonction division ci-dessous, il ne faut pas que la valeur du deuxième paramètre soit nul :

def division(a, b):
    assert b != 0, "On ne peut pas diviser par 0"
    return a / b

Pour vérifier cette pré-condition, on utilise une assertion. En Python on utilise assert selon la syntaxe suivante :

assert CONDITION  # Version sans message
assert CONDITION, MESSAGE  # Version avec message

Si la condition est vérifiée, il ne se passe rien. Sinon, un message d'erreur est affiché et l'exécution du programme s'arrête. S'il y a un message en plus, il est également affiché, pour expliquer pourquoi l'erreur s'est produite.

Vous pouvez tester cette fonction ci-dessous :

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Interaction avec l'utilisateur

Dans le cas d'un programme où on demande des valeurs à l'utilisateur avec input, il faut parfois s'assurer que l'utilisateur donne bien une réponse valide pour ne pas provoquer une erreur lors de la suite de l'exécution du programme. On pourrait utiliser des assertions, mais cela veut dire que si l'utilisateur se trompe le programme s'arrête net et il faut tout recommencer.

Il vaut mieux redemander une nouvelle valeur comme dans l'exemple suivant :

def donner_entier_pos():
    n = -1  # (1)!
    while n < 0:  # (2)!
        n = int(input("Donner un entier positif. "))  # (3)!
    print("merci")
    return n  # (4)!

1. On commence par donner une valeur incorrecte pour être sûr de rentrer dans la boucle.
2. On continue tant que la valeur n'est pas bonne.
3. On (re)demande une valeur.
4. On renvoie la dernière réponse donnée qui est forcément positive.

Vous pouvez tester la fonction ci-dessous

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La fonction int

La fonction input renvoyant forcément un texte, il faut le convertir en entier pour le comparer à 0. C'est le rôle de la fonction int. Par contre, cela ne marche pas si le texte ne correspond pas à un entier :

>>> int("4")
4
>>> int("-4")
-4
>>> int("2.4")
Traceback (most recent call last):
  File "<pyshell>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '2.4'

On pourrait vérifier que la valeur donnée par l'utilisateur correspond à un entier, mais ce serait plus compliqué.

Exercice 2

Écrire la fonction oui_ou_non qui ne prend pas de paramètre et qui demande à l'utilisateur de répondre O ou N. Aucune autre réponse n'est acceptée. Tant que l'utilisateur n'a pas donné de réponse valide, on lui repose la question. La première réponse valide est renvoyée par la fonction.

Deux versions du code

Il y a deux versions du code pour cet exercice.

• La version vide contient le minimum. Il faut écrire tous le code de la fonction. Cette version est plutôt pour les élèves qui veulent réviser et qui ont déjà fait cet exercice.
• La version avec code à trous permet de n'avoir qu'une partie de la fonction à écrire. C'est plus simple pour la première fois que vous faites l'exercice.

Exemples

>>> oui_ou_non()
Oui ou non (O/N) ? a  # Pas une réponse acceptée
Oui ou non (O/N) ? o  # On n'accepte que les réponses en majuscule
Oui ou non (O/N) ? n
Oui ou non (O/N) ? N  # C'est une bonne réponse.
'N'

Indices sur la condition de la boucle

Indice 1

Il faut probablement comparer rep avec "O" et avec "N".

Indice 2

Il faut piocher parmi rep == "O", rep == "N", rep != "O" et rep != "N".

Indice 3

Les opérateurs booléens en Python sont COND1 and COND2 pour et, COND1 or COND2 pour ou et not COND pour non.

Indices sur la valeur initiale de rep

Indice 1

Pour assurer de rentrer dans la boucle, il serait bien d'avoir une valeur initiale de rep qui vérifie la condition.

Indice 2

Il vaut mieux mettre une valeur qui n'est pas une réponse valide.

Version videVersion avec code à trous

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

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