Python generic try/except function for setting dictionary values

Question

I find myself doing a lot of data transformation like this:

mydf = pd.DataFrame()
for i in my_array:
    data = {}
    try: 
        data["foo"] = i["foo"] ## more simple
    except:
        data["foo"] = ""
    ...
    try:
        data["baz"] = item["foo"]["baz"] ## more complex
    except:
        data["baz"] = ""

    mydf = mydf.append(data, ignore_index = True)

It seems repetitive do many try/except statements this way. How would I write a function to manage this scenario or is what I'm doing the best practise?

Have you tried the [`dict.get()`](https://stackoverflow.com/questions/11041405/why-dict-getkey-instead-of-dictkey/11041421) method? — Vladimir Fokow, Aug 19 '22 at 21:07

Amish Gupta · Accepted Answer · 2022-08-19T22:28:42.217

4

I don't see a reason why data.get() wouldn't do the job.

data[key] = i.get(key, val) #if key is there in dictionary i then data[key]=i[key], else sets val as the value for data[key]

example:

data["foo"] = i.get("foo", "")

edited Aug 19 '22 at 22:28

answered Aug 19 '22 at 21:07

Amish Gupta

139
8

1

this is what I was after. thanks! – Benjamin Sep 13 '22 at 15:29

Piotr Grędowski · Answer 2 · 2022-08-19T21:17:33.133

First of all - in such scenario just do source.get("key", ""). But If you want to do some more complicated assignments - it can be done as in below code snippet.

Probably there will appear question about used lambda. It is used to evaluate source[key] inside of function which is responsible for safe assign. We can't evaluate it directly in for loop - because then we will have to catch any exceptions here. When using lambda - we are postponing moment when this code gonna be evaluated to a moment when lambda gonna be called.

Also there can appear question why destination_dict and destination_key are passed separately. Because if it's used at the left side of = sign - it has to be constructed this way :). We can't pass destination_dict[destination_key] to function because it will be evaluated outside of function and it will raise KeyError.

from typing import Callable


def assign_safely(
    destination_dict: dict,
    destination_key: str,
    source_func: Callable[..., None],
    exceptions=(KeyError,),
):
    try:
        destination_dict[destination_key] = source_func()
    except exceptions:
        destination_dict[destination_key] = ""


source = {"a": 1, "b": {}}
destination = {}

for key in ["a", "b", "c"]:
    assign_safely(
        destination_dict=destination,
        destination_key=key,
        source_func=lambda: source[key],
    )

assert destination == {"a": 1, "b": {}, "c": ""}

Python generic try/except function for setting dictionary values

2 Answers2