What is wrong with my attempt to do a string replace operation in Python?

Question

What am I doing wrong here?

import re
x = "The sky is red"
r = re.compile ("red")
y = r.sub(x, "blue")
print x  # Prints "The sky is red"
print y  # Prints "blue"

How do i get it to print "The sky is blue"?

Answer 1

Try:

x = r.sub("blue", x)

Answer 2

The problem with your code is that there are two sub functions in the re module. One is the general one and there's one tied to regular expression objects. Your code is not following either one:

The two methods are:

re.sub(pattern, repl, string[, count]) (docs here)

Used like so:

>>> y = re.sub(r, 'blue', x)
>>> y
'The sky is blue'

And for when you compile it before hand, as you tried, you can use:

RegexObject.sub(repl, string[, count=0]) (docs here)

Used like so:

>>> z = r.sub('blue', x)
>>> z
'The sky is blue'

Answer 3

You read the API wrong

http://docs.python.org/library/re.html#re.sub

pattern.sub(repl, string[, count])¶

r.sub(x, "blue")
# should be
r.sub("blue", x)

Answer 4

You have the arguments to your call to sub the wrong way round it should be:

import re
x = "The sky is red"
r = re.compile ("red")
y = r.sub("blue", x)
print x  # Prints "The sky is red"
print y  # Prints "The sky is blue"

Answer 5

By the way, for such a simple example, the re module is overkill:

x= "The sky is red"
y= x.replace("red", "blue")
print y