Double replace or single re.sub?

[EP]

How does Python execute something like the following

oldPhrase="My dog has fleas on his knees"
newPhrase=oldPhrase.replace("fleas",
"wrinkles").replace("knees","face")

Does it do two iterations of the replace method on the initial and then
an intermediate string (my guess) -- or does it compile to something
more efficient (I doubt it, unless it's Christmas in Pythonville... but
I thought I'd query)