shorthand.py

# Copyright 2015 Amazon.com, Inc. or its affiliates. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License"). You
# may not use this file except in compliance with the License. A copy of
# the License is located at
#
#     http://aws.amazon.com/apache2.0/
#
# or in the "license" file accompanying this file. This file is
# distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF
# ANY KIND, either express or implied. See the License for the specific
# language governing permissions and limitations under the License.
"""Module for parsing shorthand syntax.

This module parses any CLI options that use a "shorthand"
syntax::

    --foo A=b,C=d
         |------|
            |
            Shorthand syntax


This module provides two main classes to do this.
First, there's a ``ShorthandParser`` class.  This class works
on a purely syntactic level.  It looks only at the string value
provided to it in order to figure out how the string should be parsed.

However, because there was a pre-existing shorthand parser, we need
to remain backwards compatible with the previous parser.  One of the
things the previous parser did was use the associated JSON model to
control how the expression was parsed.

In order to accommodate this a post processing class is provided that
takes the parsed values from the ``ShorthandParser`` as well as the
corresponding JSON model for the CLI argument and makes any adjustments
necessary to maintain backwards compatibility.  This is done in the
``BackCompatVisitor`` class.

"""
import re
import string


_EOF = object()


class _NamedRegex(object):
    def __init__(self, name, regex_str):
        self.name = name
        self.regex = re.compile(regex_str, re.UNICODE)

    def match(self, value):
        return self.regex.match(value)


class ShorthandParseError(Exception):

    def _error_location(self):
        consumed, remaining, num_spaces = self.value, '', self.index
        if '\n' in self.value[:self.index]:
            # If there's newlines in the consumed expression, we want
            # to make sure we're only counting the spaces
            # from the last newline:
            # foo=bar,\n
            # bar==baz
            #     ^
            last_newline = self.value[:self.index].rindex('\n')
            num_spaces = self.index - last_newline - 1
        if '\n' in self.value[self.index:]:
            # If there's newline in the remaining, divide value
            # into consumed and remainig
            # foo==bar,\n
            #     ^
            # bar=baz
            next_newline = self.index + self.value[self.index:].index('\n')
            consumed = self.value[:next_newline]
            remaining = self.value[next_newline:]
        return '%s\n%s%s' % (consumed, (' ' * num_spaces) + '^', remaining)


class ShorthandParseSyntaxError(ShorthandParseError):
    def __init__(self, value, expected, actual, index):
        self.value = value
        self.expected = expected
        self.actual = actual
        self.index = index
        msg = self._construct_msg()
        super(ShorthandParseSyntaxError, self).__init__(msg)

    def _construct_msg(self):
        msg = (
            "Expected: '%s', received: '%s' for input:\n"
            "%s"
        ) % (self.expected, self.actual, self._error_location())
        return msg


class DuplicateKeyInObjectError(ShorthandParseError):
    def __init__(self, key, value, index):
        self.key = key
        self.value = value
        self.index = index
        msg = self._construct_msg()
        super(DuplicateKeyInObjectError, self).__init__(msg)

    def _construct_msg(self):
        msg = (
            "Second instance of key \"%s\" encountered for input:\n%s\n"
            "This is often because there is a preceeding \",\" instead of a "
            "space."
        ) % (self.key, self._error_location())
        return msg


class ShorthandParser(object):
    """Parses shorthand syntax in the CLI.

    Note that this parser does not rely on any JSON models to control
    how to parse the shorthand syntax.

    """

    _SINGLE_QUOTED = _NamedRegex('singled quoted', r'\'(?:\\\\|\\\'|[^\'])*\'')
    _DOUBLE_QUOTED = _NamedRegex('double quoted', r'"(?:\\\\|\\"|[^"])*"')
    _START_WORD = u'\!\#-&\(-\+\--\<\>-Z\\\\-z\u007c-\uffff'
    _FIRST_FOLLOW_CHARS = u'\s\!\#-&\(-\+\--\\\\\^-\|~-\uffff'
    _SECOND_FOLLOW_CHARS = u'\s\!\#-&\(-\+\--\<\>-\uffff'
    _ESCAPED_COMMA = '(\\\\,)'
    _FIRST_VALUE = _NamedRegex(
        'first',
        u'({escaped_comma}|[{start_word}])'
        u'({escaped_comma}|[{follow_chars}])*'.format(
            escaped_comma=_ESCAPED_COMMA,
            start_word=_START_WORD,
            follow_chars=_FIRST_FOLLOW_CHARS,
        ))
    _SECOND_VALUE = _NamedRegex(
        'second',
        u'({escaped_comma}|[{start_word}])'
        u'({escaped_comma}|[{follow_chars}])*'.format(
            escaped_comma=_ESCAPED_COMMA,
            start_word=_START_WORD,
            follow_chars=_SECOND_FOLLOW_CHARS,
        ))

    def __init__(self):
        self._tokens = []

    def parse(self, value):
        """Parse shorthand syntax.

        For example::

            parser = ShorthandParser()
            parser.parse('a=b')  # {'a': 'b'}
            parser.parse('a=b,c')  # {'a': ['b', 'c']}

        :tpye value: str
        :param value: Any value that needs to be parsed.

        :return: Parsed value, which will be a dictionary.
        """
        self._input_value = value
        self._index = 0
        return self._parameter()

    def _parameter(self):
        # parameter = keyval *("," keyval)
        params = {}
        key, val = self._keyval()
        params[key] = val
        last_index = self._index
        while self._index < len(self._input_value):
            self._expect(',', consume_whitespace=True)
            key, val = self._keyval()
            # If a key is already defined, it is likely an incorrectly written
            # shorthand argument. Raise an error to inform the user.
            if key in params:
                raise DuplicateKeyInObjectError(
                    key, self._input_value, last_index + 1
                )
            params[key] = val
            last_index = self._index
        return params

    def _keyval(self):
        # keyval = key "=" [values]
        key = self._key()
        self._expect('=', consume_whitespace=True)
        values = self._values()
        return key, values

    def _key(self):
        # key = 1*(alpha / %x30-39 / %x5f / %x2e / %x23)  ; [a-zA-Z0-9\-_.#/]
        valid_chars = string.ascii_letters + string.digits + '-_.#/:'
        start = self._index
        while not self._at_eof():
            if self._current() not in valid_chars:
                break
            self._index += 1
        return self._input_value[start:self._index]

    def _values(self):
        # values = csv-list / explicit-list / hash-literal
        if self._at_eof():
            return ''
        elif self._current() == '[':
            return self._explicit_list()
        elif self._current() == '{':
            return self._hash_literal()
        else:
            return self._csv_value()

    def _csv_value(self):
        # Supports either:
        # foo=bar     -> 'bar'
        #     ^
        # foo=bar,baz -> ['bar', 'baz']
        #     ^
        first_value = self._first_value()
        self._consume_whitespace()
        if self._at_eof() or self._input_value[self._index] != ',':
            return first_value
        self._expect(',', consume_whitespace=True)
        csv_list = [first_value]
        # Try to parse remaining list values.
        # It's possible we don't parse anything:
        # a=b,c=d
        #     ^-here
        # In the case above, we'll hit the ShorthandParser,
        # backtrack to the comma, and return a single scalar
        # value 'b'.
        while True:
            try:
                current = self._second_value()
                self._consume_whitespace()
                if self._at_eof():
                    csv_list.append(current)
                    break
                self._expect(',', consume_whitespace=True)
                csv_list.append(current)
            except ShorthandParseSyntaxError:
                # Backtrack to the previous comma.
                # This can happen when we reach this case:
                # foo=a,b,c=d,e=f
                #     ^-start
                # foo=a,b,c=d,e=f
                #          ^-error, "expected ',' received '='
                # foo=a,b,c=d,e=f
                #        ^-backtrack to here.
                if self._at_eof():
                    raise
                self._backtrack_to(',')
                break
        if len(csv_list) == 1:
            # Then this was a foo=bar case, so we expect
            # this to parse to a scalar value 'bar', i.e
            # {"foo": "bar"} instead of {"bar": ["bar"]}
            return first_value
        return csv_list

    def _value(self):
        result = self._FIRST_VALUE.match(self._input_value[self._index:])
        if result is not None:
            consumed = self._consume_matched_regex(result)
            return consumed.replace('\\,', ',').rstrip()
        return ''

    def _explicit_list(self):
        # explicit-list = "[" [value *(",' value)] "]"
        self._expect('[', consume_whitespace=True)
        values = []
        while self._current() != ']':
            val = self._explicit_values()
            values.append(val)
            self._consume_whitespace()
            if self._current() != ']':
                self._expect(',')
                self._consume_whitespace()
        self._expect(']')
        return values

    def _explicit_values(self):
        # values = csv-list / explicit-list / hash-literal
        if self._current() == '[':
            return self._explicit_list()
        elif self._current() == '{':
            return self._hash_literal()
        else:
            return self._first_value()

    def _hash_literal(self):
        self._expect('{', consume_whitespace=True)
        keyvals = {}
        while self._current() != '}':
            key = self._key()
            self._expect('=', consume_whitespace=True)
            v = self._explicit_values()
            self._consume_whitespace()
            if self._current() != '}':
                self._expect(',')
                self._consume_whitespace()
            keyvals[key] = v
        self._expect('}')
        return keyvals

    def _first_value(self):
        # first-value = value / single-quoted-val / double-quoted-val
        if self._current() == "'":
            return self._single_quoted_value()
        elif self._current() == '"':
            return self._double_quoted_value()
        return self._value()

    def _single_quoted_value(self):
        # single-quoted-value = %x27 *(val-escaped-single) %x27
        # val-escaped-single  = %x20-26 / %x28-7F / escaped-escape /
        #                       (escape single-quote)
        return self._consume_quoted(self._SINGLE_QUOTED, escaped_char="'")

    def _consume_quoted(self, regex, escaped_char=None):
        value = self._must_consume_regex(regex)[1:-1]
        if escaped_char is not None:
            value = value.replace("\\%s" % escaped_char, escaped_char)
            value = value.replace("\\\\", "\\")
        return value

    def _double_quoted_value(self):
        return self._consume_quoted(self._DOUBLE_QUOTED, escaped_char='"')

    def _second_value(self):
        if self._current() == "'":
            return self._single_quoted_value()
        elif self._current() == '"':
            return self._double_quoted_value()
        else:
            consumed = self._must_consume_regex(self._SECOND_VALUE)
            return consumed.replace('\\,', ',').rstrip()

    def _expect(self, char, consume_whitespace=False):
        if consume_whitespace:
            self._consume_whitespace()
        if self._index >= len(self._input_value):
            raise ShorthandParseSyntaxError(self._input_value, char,
                                            'EOF', self._index)
        actual = self._input_value[self._index]
        if actual != char:
            raise ShorthandParseSyntaxError(self._input_value, char,
                                            actual, self._index)
        self._index += 1
        if consume_whitespace:
            self._consume_whitespace()

    def _must_consume_regex(self, regex):
        result = regex.match(self._input_value[self._index:])
        if result is not None:
            return self._consume_matched_regex(result)
        raise ShorthandParseSyntaxError(self._input_value, '<%s>' % regex.name,
                                        '<none>', self._index)

    def _consume_matched_regex(self, result):
        start, end = result.span()
        v = self._input_value[self._index+start:self._index+end]
        self._index += (end - start)
        return v

    def _current(self):
        # If the index is at the end of the input value,
        # then _EOF will be returned.
        if self._index < len(self._input_value):
            return self._input_value[self._index]
        return _EOF

    def _at_eof(self):
        return self._index >= len(self._input_value)

    def _backtrack_to(self, char):
        while self._index >= 0 and self._input_value[self._index] != char:
            self._index -= 1

    def _consume_whitespace(self):
        while self._current() != _EOF and self._current() in string.whitespace:
            self._index += 1


class ModelVisitor(object):
    def visit(self, params, model):
        self._visit({}, model, '', params)

    def _visit(self, parent, shape, name, value):
        method = getattr(self, '_visit_%s' % shape.type_name,
                         self._visit_scalar)
        method(parent, shape, name, value)

    def _visit_structure(self, parent, shape, name, value):
        if not isinstance(value, dict):
            return
        for member_name, member_shape in shape.members.items():
            self._visit(value, member_shape, member_name,
                        value.get(member_name))

    def _visit_list(self, parent, shape, name, value):
        if not isinstance(value, list):
            return
        for i, element in enumerate(value):
            self._visit(value, shape.member, i, element)

    def _visit_map(self, parent, shape, name, value):
        if not isinstance(value, dict):
            return
        value_shape = shape.value
        for k, v in value.items():
            self._visit(value, value_shape, k, v)

    def _visit_scalar(self, parent, shape, name, value):
        pass


class BackCompatVisitor(ModelVisitor):
    def _visit_list(self, parent, shape, name, value):
        if not isinstance(value, list):
            # Convert a -> [a] because they specified
            # "foo=bar", but "bar" should really be ["bar"].
            if value is not None:
                parent[name] = [value]
        else:
            return super(BackCompatVisitor, self)._visit_list(
                parent, shape, name, value)

    def _visit_scalar(self, parent, shape, name, value):
        if value is None:
            return
        type_name = shape.type_name
        if type_name in ['integer', 'long']:
            parent[name] = int(value)
        elif type_name in ['double', 'float']:
            parent[name] = float(value)
        elif type_name == 'boolean':
            # We want to make sure we only set a value
            # only if "true"/"false" is specified.
            if value.lower() == 'true':
                parent[name] = True
            elif value.lower() == 'false':
                parent[name] = False