Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
245 views
in Technique[技术] by (71.8m points)

python - Nested dictionary that acts as defaultdict when setting items but not when getting items

I want to implement a dict-like data structure that has the following properties:

from collections import UserDict

class TestDict(UserDict):
    pass

test_dict = TestDict()

# Create empty dictionaries at 'level_1' and 'level_2' and insert 'Hello' at the 'level_3' key.
test_dict['level_1']['level_2']['level_3'] = 'Hello'

>>> test_dict
{
    'level_1': {
        'level_2': {
            'level_3': 'Hello'
        }
    }
}

# However, this should not return an empty dictionary but raise a KeyError.
>>> test_dict['unknown_key']
KeyError: 'unknown_key'

The problem, to my knowledge, is that python does not know whether __getitem__ is being called in the context of setting an item, i.e. the first example, or in the context of getting and item, the second example.

I have already seen Python `defaultdict`: Use default when setting, but not when getting, but I do not think that this question is a duplicate, or that it answers my question.

Please let me know if you have any ideas.

Thanks in advance.

EDIT:

It is possible to achieve something similar using:

def set_nested_item(dict_in: Union[dict, TestDict], value, keys):
    for i, key in enumerate(keys):
        is_last = i == (len(keys) - 1)
        if is_last:
            dict_in[key] = value
        else:
            if key not in dict_in:
                dict_in[key] = {}
            else:
                if not isinstance(dict_in[key], (dict, TestDict)):
                    dict_in[key] = {}

            dict_in[key] = set_nested_item(dict_in[key], value, keys[(i + 1):])
        return dict_in


class TestDict(UserDict):
    def __init__(self):
        super().__init__()

    def __setitem__(self, key, value):
        if isinstance(key, list):
            self.update(set_nested_item(self, value, key))
        else:
            super().__setitem__(key, value)

test_dict[['level_1', 'level_2', 'level_3']] = 'Hello'
>>> test_dict
{
    'level_1': {
        'level_2': {
            'level_3': 'Hello'
        }
    }
}



See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Reply

0 votes
by (71.8m points)

It's impossible.

test_dict['level_1']['level_2']['level_3'] = 'Hello'

is semantically equivalent to:

temp1 = test_dict['level_1'] # Should this line fail?
temp1['level_2']['level_3'] = 'Hello'

But... if determined to implement it anyway, you could inspect the Python stack to grab/parse the calling line of code, and then vary the behaviour depending on whether the calling line of code contains an assignment! Unfortunately, sometimes the calling code isn't available in the stack trace (e.g. when called interactively), in which case you need to work with Python bytecode.

import dis
import inspect
from collections import UserDict

def get_opcodes(code_object, lineno):
    """Utility function to extract Python VM opcodes for line of code"""
    line_ops = []
    instructions = dis.get_instructions(code_object).__iter__()
    for instruction in instructions:
        if instruction.starts_line == lineno:
            # found start of our line
            line_ops.append(instruction.opcode)
            break
    for instruction in instructions:
        if not instruction.starts_line:
            line_ops.append(instruction.opcode)
        else:
            # start of next line
            break
    return line_ops

class TestDict(UserDict):
    def __getitem__(self, key):
        try:
            return super().__getitem__(key)
        except KeyError:
            # inspect the stack to get calling line of code
            frame = inspect.stack()[1].frame
            opcodes = get_opcodes(frame.f_code, frame.f_lineno)
            # STORE_SUBSCR is Python opcode for TOS1[TOS] = TOS2
            if dis.opmap['STORE_SUBSCR'] in opcodes:
                # calling line of code contains a dict/array assignment
                default = TestDict()
                super().__setitem__(key, default)
                return default
            else:
                raise

test_dict = TestDict()
test_dict['level_1']['level_2']['level_3'] = 'Hello'
print(test_dict)
# {'level_1': {'level_2': {'level_3': 'Hello'}}}

test_dict['unknown_key']
# KeyError: 'unknown_key'

The above is just a partial solution. It can still be fooled if there are other dictionary/array assignments on the same line, e.g. other['key'] = test_dict['unknown_key']. A more complete solution would need to actually parse the line of code to figure out where the variable occurs in the assignment.


与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
OGeek|极客中国-欢迎来到极客的世界,一个免费开放的程序员编程交流平台!开放,进步,分享!让技术改变生活,让极客改变未来! Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

...