Why string isn't changed ?

Question

soz = "Python"
soz.replace("P" , "7" )
print(soz)

Answer 1

You need to understand that string are NOT mutable : contrary to lists, for instance, you cannot change its content.
All methods that "change" a string actually return a new string with changes applied (the old one isn't modified). So when working with strings, always store the result!

Answer 2

You need to store or use the return value of replace-method. It does not apply it's return value automatically on the object it was executed with.

Answer 3

Théophile , Perfect Answer to the question.