CODEWITHSUNDEEP
pythonalgorithmparsinglexertokenize
Joel Cornett
Joel Cornett

Reputation: 24788

Creating a List Lexer/Parser

I need to create a lexer/parser which deals with input data of variable length and structure.

Say I have a list of reserved keywords:

keyWordList = ['command1', 'command2', 'command3']

and a user input string:

userInput = 'The quick brown command1 fox jumped over command2 the lazy dog command 3'
userInputList = userInput.split()

How would I go about writing this function:

INPUT:

tokenize(userInputList, keyWordList)

OUTPUT:
[['The', 'quick', 'brown'], 'command1', ['fox', 'jumped', 'over'], 'command 2', ['the', 'lazy', 'dog'], 'command3']

I've written a tokenizer that can identify keywords, but have been unable to figure out an efficent way to embed groups of non-keywords into lists that are a level deeper.

RE solutions are welcome, but I would really like to see the underlying algorithm as I am probably going to extend the application to lists of other objects and not just strings.

Upvotes: 2

Views: 948

Answers (4)

Nickle
Nickle

Reputation: 397

Or have a look at PyParsing. Quite a nice little lex parser combination

Upvotes: 1

Óscar López
Óscar López

Reputation: 236004

Try this:

keyWordList = ['command1', 'command2', 'command3']
userInput = 'The quick brown command1 fox jumped over command2 the lazy dog command3'
inputList = userInput.split()

def tokenize(userInputList, keyWordList):
    keywords = set(keyWordList)
    tokens, acc = [], []
    for e in userInputList:
        if e in keywords:
            tokens.append(acc)
            tokens.append(e)
            acc = []
        else:
            acc.append(e)
    if acc:
        tokens.append(acc)
    return tokens

tokenize(inputList, keyWordList)
> [['The', 'quick', 'brown'], 'command1', ['fox', 'jumped', 'over'], 'command2', ['the', 'lazy', 'dog'], 'command3']

Upvotes: 1

JBernardo
JBernardo

Reputation: 33397

That is easy to do with some regex:

>>> reg = r'(.+?)\s(%s)(?:\s|$)' % '|'.join(keyWordList)
>>> userInput = 'The quick brown command1 fox jumped over command2 the lazy dog command3'
>>> re.findall(reg, userInput)
[('The quick brown', 'command1'), ('fox jumped over', 'command2'), ('the lazy dog', 'command3')]

Now you just have to split the first element of each tuple.

For more than one level of deepness, regex may not be a good answer.

There are some nice parsers for you to choose on this page: http://wiki.python.org/moin/LanguageParsing

I think Lepl is a good one.

Upvotes: 2

Fred Foo
Fred Foo

Reputation: 363587

Something like this:

def tokenize(lst, keywords):
    cur = []
    for x in lst:
        if x in keywords:
            yield cur
            yield x
            cur = []
        else:
            cur.append(x)

This returns a generator, so wrap your call in one to list.

Upvotes: 5

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