Xpath like query for nested python dictionaries

Question

Is there a way to define a XPath type query for nested python dictionaries.

Something like this:

foo = {
  'spam':'eggs',
  'morefoo': {
               'bar':'soap',
               'morebar': {'bacon' : 'foobar'}
              }
   }

print( foo.select("/morefoo/morebar") )

>> {'bacon' : 'foobar'}

I also needed to select nested lists ;)

This can be done easily with @jellybean's solution:

def xpath_get(mydict, path):
    elem = mydict
    try:
        for x in path.strip("/").split("/"):
            try:
                x = int(x)
                elem = elem[x]
            except ValueError:
                elem = elem.get(x)
    except:
        pass

    return elem

foo = {
  'spam':'eggs',
  'morefoo': [{
               'bar':'soap',
               'morebar': {
                           'bacon' : {
                                       'bla':'balbla'
                                     }
                           }
              },
              'bla'
              ]
   }

print xpath_get(foo, "/morefoo/0/morebar/bacon")

[EDIT 2016] This question and the accepted answer are ancient. The newer answers may do the job better than the original answer. However I did not test them so I won't change the accepted answer.

Answer 1

Not exactly beautiful, but you might use sth like

def xpath_get(mydict, path):
    elem = mydict
    try:
        for x in path.strip("/").split("/"):
            elem = elem.get(x)
    except:
        pass

    return elem

This doesn't support xpath stuff like indices, of course ... not to mention the / key trap unutbu indicated.

Answer 2

One of the best libraries I've been able to identify, which, in addition, is very actively developed, is an extracted project from boto: JMESPath. It has a very powerful syntax of doing things that would normally take pages of code to express.

Here are some examples:

search('foo | bar', {"foo": {"bar": "baz"}}) -> "baz"
search('foo[*].bar | [0]', {
    "foo": [{"bar": ["first1", "second1"]},
            {"bar": ["first2", "second2"]}]}) -> ["first1", "second1"]
search('foo | [0]', {"foo": [0, 1, 2]}) -> [0]

Answer 3

There is an easier way to do this now.

http://github.com/akesterson/dpath-python

$ easy_install dpath
>>> dpath.util.search(YOUR_DICTIONARY, "morefoo/morebar")

... done. Or if you don't like getting your results back in a view (merged dictionary that retains the paths), yield them instead:

$ easy_install dpath
>>> for (path, value) in dpath.util.search(YOUR_DICTIONARY, "morefoo/morebar", yielded=True)

... and done. 'value' will hold {'bacon': 'foobar'} in that case.

Answer 4

There is the newer jsonpath-rw library supporting a JSONPATH syntax but for python dictionaries and arrays, as you wished.

So your 1st example becomes:

from jsonpath_rw import parse

print( parse('$.morefoo.morebar').find(foo) )

And the 2nd:

print( parse("$.morefoo[0].morebar.bacon").find(foo) )

---

PS: An alternative simpler library also supporting dictionaries is python-json-pointer with a more XPath-like syntax.

Answer 5

If terseness is your fancy:

def xpath(root, path, sch='/'):
    return reduce(lambda acc, nxt: acc[nxt],
                  [int(x) if x.isdigit() else x for x in path.split(sch)],
                  root)

Of course, if you only have dicts, then it's simpler:

def xpath(root, path, sch='/'):
    return reduce(lambda acc, nxt: acc[nxt],
                  path.split(sch),
                  root)

Good luck finding any errors in your path spec tho ;-)

Answer 6

More work would have to be put into how the XPath-like selector would work. '/' is a valid dictionary key, so how would

foo={'/':{'/':'eggs'},'//':'ham'}

be handled?

foo.select("///")

would be ambiguous.

Answer 7

Another alternative (besides that suggested by jellybean) is this:

def querydict(d, q):
  keys = q.split('/')
  nd = d
  for k in keys:
    if k == '':
      continue
    if k in nd:
      nd = nd[k]
    else:
      return None
  return nd

foo = {
  'spam':'eggs',
  'morefoo': {
               'bar':'soap',
               'morebar': {'bacon' : 'foobar'}
              }
   }
print querydict(foo, "/morefoo/morebar")

Answer 8

Is there any reason for you to the query it the way like the XPath pattern? As the commenter to your question suggested, it just a dictionary, so you can access the elements in a nest manner. Also, considering that data is in the form of JSON, you can use simplejson module to load it and access the elements too.

There is this project JSONPATH, which is trying to help people do opposite of what you intend to do (given an XPATH, how to make it easily accessible via python objects), which seems more useful.

Answer 9

dict > json > jmespath

You can use JMESPath which is a query language for JSON, and which has a python implementation.

import jmespath # pip install jmespath

data = {'root': {'section': {'item1': 'value1', 'item2': 'value2'}}}

jmespath.search('root.section.item2', data)
Out[42]: 'value2'

The jmespath query syntax and live examples: http://jmespath.org/tutorial.html

dict > xml > xpath

Another option would be converting your dictionaries to XML using something like dicttoxml and then use regular XPath expressions e.g. via lxml or whatever other library you prefer.

from dicttoxml import dicttoxml  # pip install dicttoxml
from lxml import etree  # pip install lxml

data = {'root': {'section': {'item1': 'value1', 'item2': 'value2'}}}
xml_data = dicttoxml(data, attr_type=False)
Out[43]: b'<?xml version="1.0" encoding="UTF-8" ?><root><root><section><item1>value1</item1><item2>value2</item2></section></root></root>'

tree = etree.fromstring(xml_data)
tree.xpath('//item2/text()')
Out[44]: ['value2']

Answer 10

def Dict(var, *arg, **kwarg):
  """ Return the value of an (imbricated) dictionnary, if all fields exist else return "" unless "default=new_value" specified as end argument
      Avoid TypeError: argument of type 'NoneType' is not iterable
      Ex: Dict(variable_dict, 'field1', 'field2', default = 0)
  """
  for key in arg:
    if isinstance(var, dict) and key and key in var:  var = var[key]
    else:  return kwarg['default'] if kwarg and 'default' in kwarg else ""   # Allow Dict(var, tvdbid).isdigit() for example
  return kwarg['default'] if var in (None, '', 'N/A', 'null') and kwarg and 'default' in kwarg else "" if var in (None, '', 'N/A', 'null') else var

foo = {
  'spam':'eggs',
  'morefoo': {
               'bar':'soap',
               'morebar': {'bacon' : 'foobar'}
              }
   }
print Dict(foo, 'morefoo', 'morebar')
print Dict(foo, 'morefoo', 'morebar', default=None)

Have a SaveDict(value, var, *arg) function that can even append to lists in dict...