Going character by character in a string and swapping whitespaces with python

Question

Okay so I have to switch ' ' to *s. I came up with the following

def characterSwitch(ch,ca1,ca2,start = 0, end = len(ch)):
    while start < end:
        if ch[start] == ca1:
            ch[end] == ca2
        start = start + 1
sentence = "Ceci est une toute petite phrase."
print characterSwitch(sentence, ' ', '*')
print characterSwitch(sentence, ' ', '*', 8, 12)
print characterSwitch(sentence, ' ', '*', 12)
print characterSwitch(sentence, ' ', '*', end = 12)

Assigning len(ch) doesn't seem to work and also I'm pretty sure this isn't the most efficient way of doing this. The following is the output I'm aiming for:

Ceci*est*une*toute*petite*phrase.
Ceci est*une*toute petite phrase.
Ceci est une*toute*petite*phrase.
Ceci*est*une*toute petite phrase.

Answer 1

Are you looking for replace() ?

sentence = "Ceci est une toute petite phrase."
sentence = sentence.replace(' ', '*')
print sentence
# Ceci*sest*sune*stoute*spetite*sphrase.

See a demo on ideone.com additionally.

For your second requirement (to replace only from the 8th to the 12th character), you could do:

sentence = sentence[8:12].replace(' ', '*')

Answer 2

Assuming you have to do it character by character you could do it this way:

sentence = "this is a sentence."
replaced = ""

for c in sentence:
    if c == " ":
        replaced += "*"
    else:
        replaced += c

print replaced