>>> chaine = "Il traque"
>>> # je veux changer "traque" pour "craque"
>>> chaine [3]
't'
>>> chaine [3] = "c"
Traceback (most recent call last):
  File "", line 1, in 
    chaine [3] = "c"
TypeError: 'str' object does not support item assignment
>>> chaine [:3]
'Il '
>>> chaine [:3] + "craque"
'Il craque'
>>> chaine [4:]
'raque'
>>> chaine [:3] + "c" + chaine [4:]
'Il craque'
>>> chaine2 = chaine [:3] + "c" + chaine [4:]
>>> chaine2
'Il craque'
>>>

Les chaînes de caractères (strings) sont immuables !