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PyParsing: What does Combine() do?

Asked 2010-05-30T21:51:04.777
9

What is the difference between:

foo = TOKEN1 + TOKEN2

and

foo = Combine(TOKEN1 + TOKEN2)

Thanks.

UPDATE: Based on my experimentation, it seems like Combine() is for terminals, where you're trying to build an expression to match on, whereas plain + is for non-terminals. But I'm not sure.

python parsing nlp pyparsing
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21

Combine has 2 effects:

  • it concatenates all the tokens into a single string

  • it requires the matching tokens to all be adjacent with no intervening whitespace

If you create an expression like 

realnum = Word(nums) + "." + Word(nums)

Then realnum.parseString("3.14") will return a list of 3 tokens: the leading '3', the '.', and the trailing '14'. But if you wrap this in Combine, as in:

realnum = Combine(Word(nums) + "." + Word(nums))

then realnum.parseString("3.14") will return '3.14' (which you could then convert to a float using a parse action). And since Combine suppresses pyparsing's default whitespace skipping between tokens, you won't accidentally find "3.14" in "The answer is 3. 14 is the next answer."

answered 2010-05-31T00:30:25.080

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