Write two-dimensional list to JSON file
json, python
Solution
I thought you could use my answer to another similar question to do what you want. While it works with `json.dumps()`, you pointed out that it didn't for some reason with `json.dump()`.
After looking into the matter, I discovered that the `encode()` method of the derived `json.JSONEncoder` class that was being overridden in the linked answer, is only called when `dumps()` is called, but not when `dump()` is called.
Fortunately, I was able to determine that the `iterencode()` method does get invoked in both cases — so was able to fix the problem by more-or-less simply moving the code from `encode()` to `iterencode()`.
The code immediately below is a revised version with this change in it:
Modified version of the code in my answer to other question:
from _ctypes import PyObj_FromPtr # see https://stackoverflow.com/a/15012814/355230
import json
import re
class NoIndent(object):
""" Value wrapper. """
def __init__(self, value):
if not isinstance(value, (list, tuple)):
raise TypeError('Only lists and tuples can be wrapped')
self.value = value
class MyEncoder(json.JSONEncoder):
FORMAT_SPEC = '@@{}@@' # Unique string pattern of NoIndent object ids.
regex = re.compile(FORMAT_SPEC.format(r'(\d+)')) # compile(r'@@(\d+)@@')
def __init__(self, **kwargs):
# Keyword arguments to ignore when encoding NoIndent wrapped values.
ignore = {'cls', 'indent'}
# Save copy of any keyword argument values needed for use here.
self._kwargs = {k: v for k, v in kwargs.items() if k not in ignore}
super(MyEncoder, self).__init__(**kwargs)
def default(self, obj):
return (self.FORMAT_SPEC.format(id(obj)) if isinstance(obj, NoIndent)
else super(MyEncoder, self).default(obj))
def iterencode(self, obj, **kwargs):
format_spec = self.FORMAT_SPEC # Local var to expedite access.
# Replace any marked-up NoIndent wrapped values in the JSON repr
# with the json.dumps() of the corresponding wrapped Python object.
for encoded in super(MyEncoder, self).iterencode(obj, **kwargs):
match = self.regex.search(encoded)
if match:
id = int(match.group(1))
no_indent = PyObj_FromPtr(id)
json_repr = json.dumps(no_indent.value, **self._kwargs)
# Replace the matched id string with json formatted representation
# of the corresponding Python object.
encoded = encoded.replace(
'"{}"'.format(format_spec.format(id)), json_repr)
yield encoded
Applying it to your question:
# Example of using it to do get the results you want.
alfa = [('a','b','c'), ('d','e','f'), ('g','h','i')]
data = [(1,2,3), (2,3,4), (4,5,6)]
data_struct = {
'data': [NoIndent(elem) for elem in data],
'alfa': [NoIndent(elem) for elem in alfa],
}
print(json.dumps(data_struct, cls=MyEncoder, sort_keys=True, indent=4))
# Test custom JSONEncoder with json.dump()
with open('data_struct.json', 'w') as fp:
json.dump(data_struct, fp, cls=MyEncoder, sort_keys=True, indent=4)
fp.write('\n') # Add a newline to very end (optional).
Resulting output:
{
"alfa": [
["a", "b", "c"],
["d", "e", "f"],
["g", "h", "i"]
],
"data": [
[1, 2, 3],
[2, 3, 4],
[4, 5, 6]
]
}
Problem
I have a two-dimensional list like: ``` data = [[1,2,3], [2,3,4], [4,5,6]] ``` I want to write it to JSON file like this: ``` { 'data':[ [1,2,3], [2,3,4], [4,5,6] ] } ``` I get this with: `json.dumps(data, indent=4, sort_keys=True)`: ``` { 'data':[ [ 1, 2, 3 ], [ 2, 3, 4 ], [ 4, 5, 6] ] } ``` Here is anther question How to implement custom indentation when pretty-printing with the JSON module?, but that's dictionaries.