I want to replace only specific word in one string. However, some other words have that word inside but I don't want them to be changed.

For example, for the below string I only want to replace x with y in z string. how to do that?

x = "the"
y = "a"
z = "This is the thermometer"

import repattern=r'\bthe\b' # \b - start and end of the  word
repl='a'
string = 'This is the thermometer'string=re.sub(pattern, repl, string)