NOOB! Replacing part of a string using a For loop : Hi i am a complete python and coding NOOB. recently started a college course. currently on For loops and have a question where i need to get a user to enter a sentence. using a FOR loop i then need to replace all spaces with %20 in order to prep the string to be used as a URL. cannot for the life of me figure it out. any help would be well received.

sentence = str(input("Please enter sentence:"))
space = (" ")

for space in sentence:
    space.replace(sentence, space, "%20", 500)
print(sentence)

this is what i have entered so far but it is completely wrong and i  really just cant fathom how to get around it. 