Metadata-Version: 2.1
Name: parsr
Version: 0.4.0
Summary: Parsr is a simple parser combinator library in pure python.
Home-page: https://github.com/csams/parsr
License: Apache 2.0
Description: [![Documentation Status](https://readthedocs.org/projects/parsr/badge/?version=latest)](https://parsr.readthedocs.io/en/latest/?badge=latest)
        [![Test Status](https://travis-ci.org/csams/parsr.svg?branch=master)](https://travis-ci.org/csams/parsr.svg?branch=master)
        
        # parsr
        parsr is a little library for parsing simple, mostly context free grammars that
        might require knowledge of indentation or matching tags.
        
        It contains a small set of combinators that perform recursive decent with
        backtracking. Fancy tricks like rewriting left recursions and optimizations like
        [packrat](https://pdos.csail.mit.edu/~baford/packrat/thesis/thesis.pdf) are not
        implemented since the goal is a library that's small yet sufficient for parsing
        non-standard configuration files. It also includes a generic data model that
        parsers can target to take advantage of an embedded query system.
        
        To see how a handwritten parser might evolve to something like this project,
        check out the [lesson](https://github.com/csams/parsr/blob/master/parsr/lesson).
        
        [parser.query](https://github.com/csams/parsr/blob/master/parsr/query) contains 
        the common data model and query system.
        
        ## Install
        1. Ensure python2.7, python3.6, or python3.7 is installed.
        2. `python3.7 -m venv myproject && cd myproject`
        3. `source bin/activate`
        4. `pip install parsr`
        
        ## Examples
        * [Arithmetic](https://github.com/csams/parsr/blob/master/parsr/examples/arith.py)
        * [Generic Key/Value Pair configuration](https://github.com/csams/parsr/blob/master/parsr/examples/kvpairs.py)
        * [INI configuration](https://github.com/csams/parsr/blob/master/parsr/examples/iniparser.py) is an example of significant indentation.
        * [json](https://github.com/csams/parsr/blob/master/parsr/examples/json_parser.py)
        * [httpd configuration](https://github.com/csams/parsr/blob/master/parsr/examples/httpd_conf.py) is an example of matching starting and ending tags.
        * [nginx configuration](https://github.com/csams/parsr/blob/master/parsr/examples/nginx_conf.py)
        * [corosync configuration](https://github.com/csams/parsr/blob/master/parsr/examples/corosync_conf.py)
        * [multipath configuration](https://github.com/csams/parsr/blob/master/parsr/examples/multipath_conf.py)
        * [logrotate configuration](https://github.com/csams/parsr/blob/master/parsr/examples/logrotate_conf.py)
        
        ## Primitives
        These are the building blocks for matching individual characters, sets of
        characters, and a few convenient objects like numbers. All matching is case
        sensitive except for the `ignore_case` option with `Literal`.
        
        ### Char
        Match a single character.
        ```python
        a = Char("a")     # parses a single "a"
        val = a("a")      # produces an "a" from the data.
        val = a("b")      # raises an exception
        ```
        
        ### InSet
        Match any single character in a set.
        ```python
        vowel = InSet("aeiou")  # or InSet(set("aeiou"))
        val = vowel("a")  # okay
        val = vowel("e")  # okay
        val = vowel("i")  # okay
        val = vowel("o")  # okay
        val = vowel("u")  # okay
        val = vowel("y")  # raises an exception
        ```
        
        ### String
        Match one or more characters in a set. Matching is greedy.
        ```python
        vowels = String("aeiou")
        val = vowels("a")            # returns "a"
        val = vowels("u")            # returns "u"
        val = vowels("aaeiouuoui")   # returns "aaeiouuoui"
        val = vowels("uoiea")        # returns "uoiea"
        val = vowels("oouieaaea")    # returns "oouieaaea"
        val = vowels("ga")           # raises an exception
        ```
        
        ### StringUntil
        Matches any number of characters until a predicate is seen. You may set
        lower and upper bounds. Both are inclusive. The characters that match
        the predicate are not consumed.
        ```python
        su  = StringUntil(Char("="))  # parses any number of characters until '='
        val = su("ab=")               # produces "ab" from the data.
        val = su("ab")                # raises an exception
        
        su  = StringUntil(Char("="), lower=2)  # parses at least two characters until '='
        val = su("ab=")                        # produces "ab" from the data.
        val = su("a=")                         # raises an exception
        
        su  = StringUntil(Char("="), upper=2)  # parses at most two characters until '='
        val = su("ab=")                        # produces "ab" from the data.
        val = su("a=")                         # produces "a"
        val = su("abc=")                       # raises an exception
        ```
        
        ### Regex
        Match characters against a regular expression.
        ```python
        identifier = Regex("[a-zA-Z]([a-zA-Z0-9])*")
        identifier("abcd1") # returns "abcd1"
        identifier("1bcd1") # raises an exception
        ```
        
        ### Literal
        Match a literal string. The `value` keyword lets you return a python value
        instead of the matched input. The `ignore_case` keyword makes the match case
        insensitive.
        ```python
        lit = Literal("true")
        val = lit("true")  # returns "true"
        val = lit("True")  # raises an exception
        val = lit("one")   # raises an exception
        
        lit = Literal("true", ignore_case=True)
        val = lit("true")  # returns "true"
        val = lit("TRUE")  # returns "TRUE"
        val = lit("one")   # raises an exception
        
        t = Literal("true", value=True)
        f = Literal("false", value=False)
        val = t("true")  # returns the boolean True
        val = t("True")  # raises an exception
        
        val = f("false") # returns the boolean False
        val = f("False") # raises and exception
        
        t = Literal("true", value=True, ignore_case=True)
        f = Literal("false", value=False, ignore_case=True)
        val = t("true")  # returns the boolean True
        val = t("True")  # returns the boolean True
        
        val = f("false") # returns the boolean False
        val = f("False") # returns the boolean False
        ```
        
        ### Number
        Match a possibly negative integer or simple floating point number and return
        the python `int` or `float` for it.
        ```python
        val = Number("123")  # returns 123
        val = Number("-12")  # returns -12
        val = Number("12.4")  # returns 12.4
        val = Number("-12.4")  # returns -12.4
        ```
        
        parsr also provides SingleQuotedString, DoubleQuotedString, QuotedString, EOL,
        EOF, WS, AnyChar, and several other primitives. See the bottom of
        [parsr/\_\_init\_\_.py](https://github.com/csams/parsr/blob/master/parsr/__init__.py)
        
        ## Combinators
        There are several ways of combining primitives and their combinations.
        
        ### Sequence
        Require expressions to be in order.
        
        Sequences are optimized so only the first object maintains a list of itself and
        following objects. Be aware that using a sequence in other sequences will cause
        it to accumulate the elements of the new sequence onto it, which could affect it
        if it's used in multiple definitions. To ensure a sequence isn't "sticky" after
        its definition, wrap it in a `Wrapper` object.
        ```python
        a = Char("a")     # parses a single "a"
        b = Char("b")     # parses a single "b"
        c = Char("c")     # parses a single "c"
        
        ab = a + b        # parses a single "a" followed by a single "b"
                          # (a + b) creates a "Sequence" object. Using `ab` as an
                          # element in a later sequence would modify its original
                          # definition.
        
        abc = a + b + c   # parses "abc"
                          # (a + b) creates a "Sequence" object to which c is appended
        
        val = ab("ab")    # produces a list ["a", "b"]
        val = ab("a")     # raises an exception
        val = ab("b")     # raises an exception
        val = ab("ac")    # raises an exception
        val = ab("cb")    # raises an exception
        
        val = abc("abc")  # produces ["a", "b", "c"]
        ```
        
        ### Choice
        Accept one of several alternatives. Alternatives are checked from left to right,
        and checking stops with the first one to succeed.
        
        Choices are optimized so only the first object maintains a list of alternatives.
        Be aware that using a choice object as an element in other choices will
        cause it to accumulate the elemtents of the new choice onto it, which could
        affect it if it's used in multiple definitions. To ensure a Choice isn't
        "sticky" after its definition, wrap it in a `Wrapper` object.
        ```python
        abc = a | b | c   # alternation or choice.
        val = abc("a")    # parses a single "a"
        val = abc("b")    # parses a single "b"
        val = abc("c")    # parses a single "c"
        val = abc("d")    # raises an exception
        ```
        
        ### Many
        Match zero or more occurences of an expression. Matching is greedy.
        
        Since `Many` can match zero occurences, it always succeeds. Keep this in mind
        when using it in a list of alternatives or with `FollowedBy` or `NotFollowedBy`.
        ```python
        x = Char("x")
        xs = Many(x)      # parses many (or no) x's in a row
        val = xs("")      # returns []
        val = xs("a")     # returns []
        val = xs("x")     # returns ["x"]
        val = xs("xxxxx") # returns ["x", "x", "x", "x", "x"]
        val = xs("xxxxb") # returns ["x", "x", "x", "x"]
        
        ab = Many(a + b)  # parses "abab..."
        val = ab("")      # produces []
        val = ab("ab")    # produces [["a", b"]]
        val = ab("ba")    # produces []
        val = ab("ababab")# produces [["a", b"], ["a", "b"], ["a", "b"]]
        
        ab = Many(a | b)  # parses any combination of "a" and "b" like "aababbaba..."
        val = ab("aababb")# produces ["a", "a", "b", "a", "b", "b"]
        
        xs = Many(x, lower=1)     # parses many (or no) x's in a row
        val = xs("")      # raises an exception
        val = xs("a")     # raises an exception
        val = xs("x")     # returns ["x"]
        val = xs("xxxxx") # returns ["x", "x", "x", "x", "x"]
        val = xs("xxxxb") # returns ["x", "x", "x", "x"]
        
        ab = Many(a + b, lower=1) # parses "abab..."
        val = ab("")      # raises an exception
        val = ab("ab")    # produces [["a", "b"]]
        val = ab("ba")    # raises an exception
        val = ab("ababab")# produces [["a", "b"], ["a", "b"], ["a", "b"]]
        
        ab = Many(a | b, lower=1) # parses any combination of "a" and "b" like "aababbaba..."
        val = ab("aababb")# produces ["a", "a", "b", "a", "b", "b"]
        
        ab = Many(a | b, upper=2) # parses any combination of "a" and "b" like "aababbaba..."
        val = ab("ab")    # produces ["a", "b"]
        val = ab("aab")   # raises an exception
        ```
        
        ### Until
        Match zero or more occurences of an expression until a predicate matches.
        Matching is greedy.
        
        Since `Until` can match zero occurences, it always succeeds. Keep this in mind
        when using it in a list of alternatives or with `FollowedBy` or `NotFollowedBy`.
        ```python
        cs = AnyChar.until(Char("y")) # parses many (or no) characters until a "y" is
                                      # encountered.
        
        val = cs("")                  # returns []
        val = cs("a")                 # returns ["a"]
        val = cs("x")                 # returns ["x"]
        val = cs("ccccc")             # returns ["c", "c", "c", "c", "c"]
        val = cs("abcdycc")           # returns ["a", "b", "c", "d"]
        ```
        
        ### Followed by
        Require an expression to be followed by another, but don't consume the input
        that matches the latter expression.
        ```python
        ab = Char("a") & Char("b") # matches an "a" followed by a "b", but the "b"
                                   # isn't consumed from the input.
        val = ab("ab")             # returns "a" and leaves "b" to be consumed.
        val = ab("ac")             # raises an exception and doesn't consume "a".
        ```
        
        ### Not followed by
        Require an expression to *not* be followed by another.
        ```python
        anb = Char("a") / Char("b") # matches an "a" not followed by a "b".
        val = anb("ac")             # returns "a" and leaves "c" to be consumed
        val = anb("ab")             # raises an exception and doesn't consume "a".
        ```
        
        ### Keep Left / Keep Right
        `KeepLeft` (`<<`) and `KeepRight` (`>>`) match adjacent expressions but ignore
        one of their results.
        ```python
        a = Char("a")
        q = Char('"')
        
        qa = a << q      # like a + q except only the result of a is returned
        val = qa('a"')   # returns "a". Keeps the thing on the left of the << 
        
        qa = q >> a      # like q + a except only the result of a is returned
        val = qa('"a')   # returns "a". Keeps the thing on the right of the >> 
        
        qa = q >> a << q # like q + a + q except only the result of the a is returned
        val = qa('"a"')  # returns "a".
        ```
        
        ### Opt
        `Opt` wraps a parser and returns a default value of `None` if it fails. That
        value can be changed with the `default` keyword. Input is consumed if the
        wrapped parser succeeds but not otherwise.
        ```python
        a = Char("a")
        o = Opt(a)      # matches an "a" if its available. Still succeeds otherwise but
                        # doesn't advance the read pointer.
        val = o("a")    # returns "a"
        val = o("b")    # returns None. Read pointer is not advanced.
        
        o = Opt(a, default="x") # matches an "a" if its available. Returns "x" otherwise.
        val = o("a")    # returns "a"
        val = o("b")    # returns "x". Read pointer is not advanced.
        ```
        
        ### map
        All parsers have a `.map` function that allows you to pass a function to
        evaluate the input they've matched.
        ```python
        def to_number(val):
            # val is like [non_zero_digit, [other_digits]]
            first, rest = val
            s = first + "".join(rest)
            return int(s)
        
        m = NonZeroDigit + Many(Digit)  # returns [nzd, [other digits]]
        n = m.map(to_number)  # converts the match to an actual integer
        val = n("15")  # returns the int 15
        ```
        
        ### Lift
        Allows a multiple parameter function to work on parsers.
        ```python
        def comb(a, b, c):
            """ a, b, and c should be strings. Returns their concatenation."""
            return "".join([a, b, c])
        
        # You'd normally invoke comb like comb("x", "y", "z"), but you can "lift" it for
        # use with parsers like this:
        
        x = Char("x")
        y = Char("y")
        z = Char("z")
        p = Lift(comb) * x * y * z
        
        # The * operator separates parsers whose results will go into the arguments of
        # the lifted function. I've used Char above, but x, y, and z can be arbitrarily
        # complex.
        
        val = p("xyz")  # would return "xyz"
        val = p("xyx")  # raises an exception. nothing would be consumed
        ```
        
        ### Forward
        `Forward` allows recursive grammars where a nonterminal's definition includes
        itself directly or indirectly. You initially create a `Forward` nonterminal
        with regular assignment.
        ```python
        expr = Forward()
        ```
        
        You later give it its real definition with the `<=` operator.
        ```python
        expr <= (term + Many(LowOps + term)).map(op)
        ```
        
        ### Arithmetic
        Here's an arithmetic parser that ties several concepts together. A progression
        of this parser from a simple imperative style to what you see below is in the
        [repo](https://github.com/csams/parsr/blob/master/parsr/lesson).
        
        ```python
        from parsr import EOF, Forward, InSet, LeftParen, Many, Number, RightParen, WS
        
        
        def op(args):
            ans, rest = args
            for op, arg in rest:
                if op == "+":
                    ans += arg
                elif op == "-":
                    ans -= arg
                elif op == "*":
                    ans *= arg
                elif op == "/":
                    ans /= arg
            return ans
        
        
        # high precedence operations
        HighOps = InSet("*/")
        
        # low precedence operations
        LowOps = InSet("+-")
        
        # Operator precedence is handled by having different declarations for each
        # prededence level. expr handles low level operations, term handles high level
        # operations, and factor handles simple numbers or subexpressions between
        # parentheses. Since the first element in expr is term and the first element in
        # term is factor, factors are evaluated first, then terms, and then exprs.
        
        # We have to declare expr before its definition since it's used recursively
        # through the definition of factor.
        expr = Forward()
        
        # A factor is a simple number or a subexpression between parentheses.
        factor = WS >> (Number | (LeftParen >> expr << RightParen)) << WS
        
        # A term handles strings of multiplication and division. As written, it would
        # convert "1 + 2 - 3 + 4" into [1, [['+', 2], ['-', 3], ['+', 4]]]. The first
        # element in the outer list is the initial factor. The second element of the
        # outer list is another list, which is the result of the Many. The Many's list
        # contains several two-element lists generated from each match of
        # (HighOps + factor). We pass the entire structure into the op function with
        # map.
        term = (factor + Many(HighOps + factor)).map(op)
        
        # expr has the same form and behavior as term.
        # Notice that we assign to expr with "<=" instead of "=". This is how you assign
        # to nonterminals that have been declared previously as Forward.
        expr <= (term + Many(LowOps + term)).map(op)
        
        val = expr("2*(3+4)/3+4")  # returns 8.666666666666668
        ```
        
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