Python conditional fails to reach the else statement when the if statement is not satisfied

Question

try:
            symbol = input("Specify symbol: ").upper()
            dt_string = input("Specify datetime in isoformat (e.g.'2021-05-27T03:30:00+00:00'): ")
            try:
                dt = datetime.fromisoformat(dt_string)
            except Exception as e:
                print(e)
            ts = Decimal(input("Specify ticksize after: "))
        except Exception as e:
            print(e)
        else:
            with open(join(file_directory, "ticksize_changes.json"), "r") as f:
                ts_dict = json.load(f)
                print(ts_dict)
                if ts_dict[symbol]: # if symbol exists in json file
                    ts_dict[symbol].append({'datetime': dt_string, 'ts_after': ts})
                else:
                    ts_dict[symbol] = [{'datetime': dt_string, 'ts_after': ts}]
            with open(join(file_directory, "ticksize_changes.json"), "w") as f:
                json.dump(ts_dict, f, default=str)

I have a try-except-else structure, and in the else, I have a with statement. In that with statement, I have a simple if-else. I tried a case where the if statement executed successfully, but as soon as I tried a case where it should reach the else statement I get the following error:

The console message is correct - indeed no such key exists. But why does the else statement fail to execute?

It doesn’t go into the else, because the condition is raising an error. You need to check if the key exists in a way that *doesn't* raise an error, but just produces a True/False. — deceze, Aug 24 '23 at 01:24
`# if symbol exists in json file` that is not what that code does. That code tries to *retrieve* `symbol` from the `ts_dict`. If that key doesn't exist, it raises a `KeyError`. An unhandled exception will terminate the program — juanpa.arrivillaga, Aug 24 '23 at 01:25

score 1 · Accepted Answer · answered Aug 24 '23 at 01:25

1

you should use this statement, first make sure the dictionary keys containing it.

if symbol in ts_dict.keys(): # if symbol exists in json file
    ts_dict[symbol].append({'datetime': dt_string, 'ts_after': ts})
else:
    ts_dict[symbol] = [{'datetime': dt_string, 'ts_after': ts}]

answered Aug 24 '23 at 01:25

Jiu_Zou

463
1
4

1

why are you using `ts_dict.keys()`? – juanpa.arrivillaga Aug 24 '23 at 01:26

score 0 · Answer 2 · answered Aug 24 '23 at 01:42

0

You could use dict.setdefault

ts_dict.setdefault(symbol, []).append({'datetime': dt_string, 'ts_after': ts})

answered Aug 24 '23 at 01:42

Jab

26,853
21
75
114

Python conditional fails to reach the else statement when the if statement is not satisfied

2 Answers2