Nested dictionary comprehension Python

Don’t do Python Nested dictionary comprehension for the sake of readability, don’t nest dictionary comprehensions and list comprehensions too much. Comprehension is good for simple one-line code and is easy to read.

Structure Nested dictionary comprehension Python.

data = {outer_k: {inner_k: myfunc(inner_v) for inner_k, inner_v in outer_v.items()} for outer_k, outer_v in outer_dict.items()}

Nested dictionary comprehension Python

Simple example code 2 level nested dictionary comprehension.

data = [
    {
        'id': 1,
        'parent_id': 101,
        'name': 'A'
    },
    {
        'id': 2,
        'parent_id': 101,
        'name': 'B'
    },
    {
        'id': 3,
        'parent_id': 102,
        'name': 'C'
    },
    {
        'id': 4,
        'parent_id': 102,
        'name': 'D'
    }
]
data = {parent_id: {d["name"]: d["id"] for d in [i for i in data if i["parent_id"] == parent_id]} for parent_id in
        set((j["parent_id"] for j in data))}

print(data)

Output:

Python nested dict comprehension with if else

{fk: None
 for k, v in my_dict.items()
 for fk in ([k] if v is None else (k + fv for fv in v))}

• If the value is None, you just want the key.
• If the value is not None, you want a list of each value concatenated with the key.
• Homogenize that to always return a list, either of one key or multiple:

[k] if v is None else [k + fv for fv in v]

Then you’re looking at a “simple” nested comprehension:

{k: None for k in [['a'], ['b'], ['c1', 'c2', 'c3']] for fk in k}

Do comment if you have any doubts or suggestions on this Python comprehension topic.

Note: IDE: PyCharm 2021.3.3 (Community Edition)

Windows 10

Python 3.10.1

All Python Examples are in Python 3, so Maybe its different from python 2 or upgraded versions.