Python lambda if statement re.sub

Question

So I am using the following regex to parse text and grab information from a specific dictionary:

re.sub(r'(<Q\d+>)',lambda m: quotes[m.group(1)][1],text)

What I want to do, is only have it replace if what it would replace with is a key in a separate dictionary. Logically it would look like this:

re.sub(r'(<Q\d+>)',lambda m: quotes[m.group(1)][1] if quotes[m.group(1)][1] in d,text)

now if I were to run the following, I get the following syntax error:

>>> re.sub(r'(<Q\d+>)',lambda m: quotes[m.group(1)][1] if quotes[m.group(1)][1] in d,text)
  File "<stdin>", line 1
    re.sub(r'(<Q\d+>)',lambda m: quotes[m.group(1)][1] if quotes[m.group(1)][1] in d,text)
                                                                                    ^
SyntaxError: invalid syntax

How can I only replace in this way?

Answer 1

The if expression always requires an else. You always have to replace the matched text. If you don't want to replace it, you just need to replace it with itself:

re.sub(r'(<Q\d+>)', 
  (lambda m: quotes[m.group(1)][1] if quotes[m.group(1)][1] in d else m.group(1)), text)