如何使用jsonpathng算法？

Question

jsonpath-ng包声明支持基本算术（https://pypi.org/project/jsonpath-ng/），但解析器不接受算术语句。以下是其中之一：

from jsonpath_ng import parse

jsonpath_expr = parse('$.objects[*].cow + $.objects[*].cat')
obj = {'objects': [
  {'cow': 2, 'cat': 3},
  {'cow': 4, 'cat': 6}
]}
values = [match.value for match in jsonpath_expr.find(obj)]  
print(values)

这会引发一个错误：

Traceback (most recent call last):
  File "test.py", line 8, in <module>
    jsonpath_expr = parse('$.objects[*].cow + $.objects[*].cat')
  File "C:\Users\micha\AppData\Roaming\Python\Python38\site-packages\jsonpath_ng\parser.py", line 14, in parse
    return JsonPathParser().parse(string)
  File "C:\Users\micha\AppData\Roaming\Python\Python38\site-packages\jsonpath_ng\parser.py", line 32, in parse
    return self.parse_token_stream(lexer.tokenize(string))
  File "C:\Users\micha\AppData\Roaming\Python\Python38\site-packages\jsonpath_ng\parser.py", line 55, in parse_token_stream
    return new_parser.parse(lexer = IteratorToTokenStream(token_iterator))
  File "C:\Users\micha\AppData\Roaming\Python\Python38\site-packages\ply\yacc.py", line 333, in parse
    return self.parseopt_notrack(input, lexer, debug, tracking, tokenfunc)
  File "C:\Users\micha\AppData\Roaming\Python\Python38\site-packages\ply\yacc.py", line 1063, in parseopt_notrack
    lookahead = get_token()     # Get the next token
  File "C:\Users\micha\AppData\Roaming\Python\Python38\site-packages\jsonpath_ng\parser.py", line 179, in token
    return next(self.iterator)
  File "C:\Users\micha\AppData\Roaming\Python\Python38\site-packages\jsonpath_ng\lexer.py", line 35, in tokenize
    t = new_lexer.token()
  File "C:\Users\micha\AppData\Roaming\Python\Python38\site-packages\ply\lex.py", line 386, in token
    newtok = self.lexerrorf(tok)
  File "C:\Users\micha\AppData\Roaming\Python\Python38\site-packages\jsonpath_ng\lexer.py", line 167, in t_error
    raise JsonPathLexerError('Error on line %s, col %s: Unexpected character: %s ' % (t.lexer.lineno, t.lexpos - t.lexer.latest_newline, t.value[0]))
jsonpath_ng.lexer.JsonPathLexerError: Error on line 1, col 17: Unexpected character: +

我错过什么了吗？（我使用的是最新版本：1.5.2）

Answer 1

您需要使用扩展解析器使其工作：

#from jsonpath_ng import jsonpath
from jsonpath_ng.ext import parser

jsonpath_expr = parser.parse('$.objects[*].cow + $.objects[*].cat')
obj = {'objects': [
    {'cow': 2, 'cat': 3},
    {'cow': 4, 'cat': 6}
]}
print([match.value for match in jsonpath_expr.find(obj)])

这将打印：[5, 10]。所以它实际上是在每一行上加上每头牛和每只猫的值