You could investigate some of the options described in this StackOverflow post.
Here is the equivalent of the first example in the post using a Transform node.

#### ConfigureFields Script
from difflib import SequenceMatcher
#Configure all fields from input 'in1' to be mapped
#to the corresponding fields on the output 'out1'
out1 += in1
out1.MatchRatio = float
def strSimilar(a, b):
if (a is Null or b is Null):
return Null
else:
return SequenceMatcher(None, a, b).ratio()
#### End ConfigureFields Script
#### ProcessRecords Script
#Copy all fields from input 'in1' to the corresponding output fields
#in output 'out1'. Copies the values of the fields that have been setup
#in the mapping defined in the ConfigureFields property
out1 += in1
out1.MatchRatio = strSimilar(fields.StringA, fields.StringB)
#### End ProcessRecords Script
Note, you may need to use the Python node rather than the Transform node if you are interested in using 3rd party Python packages.