Replacing elements of a Python dictionary using regex

Question

I have been trying to replace integer components of a dictionary with string values given in another dictionary. However, I am getting the following error:

Traceback (most recent call last):


File "<string>", line 11, in <module>
  File "/usr/lib/python3.11/json/__init__.py", line 346, in loads
return _default_decoder.decode(s)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/json/decoder.py", line 337, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 14 (char 13)

The code has been given below. Not sure why I am getting an error.

import re 
from json import loads, dumps

movable = {"movable": [0, 3, 6, 9], "fixed": [1, 4, 7, 10], "mixed": [2, 5, 8, 11]}
int_mapping = {0: "Ar", 1: "Ta", 2: "Ge", 3: "Ca", 4: "Le", 5: "Vi", 6: "Li", 7: "Sc", 8: "Sa", 9: "Ca", 10: "Aq", 11: "Pi"}

movable = dumps(movable)
for key in int_mapping.keys():
    movable = re.sub('(?<![0-9])' + str(key) + '(?![0-9])', int_mapping[key], movable)
    
movable = loads(movable)

I understand that this code can easily be written in a different way to get the desired output. However, I am interested to understand what I am doing wrong.

Print movable after you transformed it and see how it lacks all quotes around the strings. Anyway, you should probably work on the dict directly... — Thierry Lathuille
– Thierry Lathuille, Commented Feb 8, 2023 at 17:46

Savir · Accepted Answer · 2023-02-08 17:48:49Z

If you print how movable looks like right before calling json.loads, you'll see what the problem is:

for key in int_mapping.keys():
    movable = re.sub('(?<![0-9])' + str(key) + '(?![0-9])', int_mapping[key], movable)
print(movable)

outputs:

{"movable": [Ar, Ca, Li, Ca], "fixed": [Ta, Le, Sc, Aq], "mixed": [Ge, Vi, Sa, Pi]}

Those strings (Ar, Ca...) are not quoted, therefore it is not valid JSON.

If you choose to continue the way you're going, you must add the ":

movable = re.sub(
    '(?<![0-9])' + str(key) + '(?![0-9])', 
    '"' + int_mapping[key] + '"', 
    movable)

(notice the '"' + int_mapping[key] + '"')

Which produces:

{"movable": ["Ar", "Ca", "Li", "Ca"], "fixed": ["Ta", "Le", "Sc", "Aq"], "mixed": ["Ge", "Vi", "Sa", "Pi"]}

This said... you are probably much better off by just walking the movable values and substituting them by the values in int_mapping. Something like:

mapped_movable = {}
for key, val in movable.items():
    mapped_movable[key] = [int_mapping[v] for v in val]
print(mapped_movable)

Maurice Meyer · Accepted Answer · 2023-02-08 17:52:36Z

1

You could use a dict comprehension and make the mapping replacements directly in Python:

...
movable = {
    k: [int_mapping[v] for v in values]
    for k, values in movable.items()
}
print(type(movable))
print(movable)

Out:

<type 'dict'>
{'mixed': ['Ge', 'Vi', 'Sa', 'Pi'], 'fixed': ['Ta', 'Le', 'Sc', 'Aq'], 'movable': ['Ar', 'Ca', 'Li', 'Ca']}

answered Feb 8, 2023 at 17:52

Maurice Meyer

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