Python: How best to parse a simple grammar?
- by Rosarch
Ok, so I've asked a bunch of smaller questions about this project, but I still don't have much confidence in the designs I'm coming up with, so I'm going to ask a question on a broader scale.
I am parsing pre-requisite descriptions for a course catalog. The descriptions almost always follow a certain form, which makes me think I can parse most of them.
From the text, I would like to generate a graph of course pre-requisite relationships. (That part will be easy, after I have parsed the data.)
Some sample inputs and outputs:
"CS 2110" => ("CS", 2110) # 0
"CS 2110 and INFO 3300" => [("CS", 2110), ("INFO", 3300)] # 1
"CS 2110, INFO 3300" => [("CS", 2110), ("INFO", 3300)] # 1
"CS 2110, 3300, 3140" => [("CS", 2110), ("CS", 3300), ("CS", 3140)] # 1
"CS 2110 or INFO 3300" => [[("CS", 2110)], [("INFO", 3300)]] # 2
"MATH 2210, 2230, 2310, or 2940" => [[("MATH", 2210), ("MATH", 2230), ("MATH", 2310)], [("MATH", 2940)]] # 3
If the entire description is just a course, it is output directly.
If the courses are conjoined ("and"), they are all output in the same list
If the course are disjoined ("or"), they are in separate lists
Here, we have both "and" and "or".
One caveat that makes it easier: it appears that the nesting of "and"/"or" phrases is never greater than as shown in example 3.
What is the best way to do this? I started with PLY, but I couldn't figure out how to resolve the reduce/reduce conflicts. The advantage of PLY is that it's easy to manipulate what each parse rule generates:
def p_course(p):
'course : DEPT_CODE COURSE_NUMBER'
p[0] = (p[1], int(p[2]))
With PyParse, it's less clear how to modify the output of parseString(). I was considering building upon @Alex Martelli's idea of keeping state in an object and building up the output from that, but I'm not sure exactly how that is best done.
def addCourse(self, str, location, tokens):
self.result.append((tokens[0][0], tokens[0][1]))
def makeCourseList(self, str, location, tokens):
dept = tokens[0][0]
new_tokens = [(dept, tokens[0][1])]
new_tokens.extend((dept, tok) for tok in tokens[1:])
self.result.append(new_tokens)
For instance, to handle "or" cases:
def __init__(self):
self.result = []
# ...
self.statement = (course_data + Optional(OR_CONJ + course_data)).setParseAction(self.disjunctionCourses)
def disjunctionCourses(self, str, location, tokens):
if len(tokens) == 1:
return tokens
print "disjunction tokens: %s" % tokens
How does disjunctionCourses() know which smaller phrases to disjoin? All it gets is tokens, but what's been parsed so far is stored in result, so how can the function tell which data in result corresponds to which elements of token? I guess I could search through the tokens, then find an element of result with the same data, but that feel convoluted...
What's a better way to approach this problem?
