How to improve code clarity in nested try-except-else clauses?

Question

Sometimes I have a cascade of different things I can try to accomplish a task, e. g. If I need to get a record I can first try to find the record, and if this fails, I can c

Answer 1

You can use a for loop to successively try variants:

for task, error in ((find_record, NoSuchRecord), (create_record, CreateFailed)):
    try:
        result = task()
    except error:
        continue
    else:
        break
else:
    # for..else is only entered if there was no break
    result = tape

If you need an else clause, you can provide it as a separate function:

for task, error, success in (
    (find_record, NoSuchRecord, lambda: logger.info("Record found")),
    (create_record, CreateFailed, lambda: logger.info("Created a new record"))
):
    try:
        result = task()
    except error:
        continue
    else:
        success()
        break
else:
    result = tape
    logger.info("Using a tape now")

Take note that the default case tape is not part of the variants - this is because it has no failure condition. If it should execute with the variants, it can be added as (lambda: tape, (), lambda: None).

---

You can put this all into a function for reuse:

def try_all(*cases):
    for task, error, success in cases:
        try:
            result = task()
        except error:
            continue
        else:
            success()
            return result

try_all(
    (find_record, NoSuchRecord, lambda: logger.info("Record found")),
    (create_record, CreateFailed, lambda: logger.info("Created a new record")),
    (lambda: tape, (), lambda: logger.info("Using a tape now")),
)

In case the tuples seem difficult to read, a NamedTuple can be used to name the elements. This can be mixed with plain tuples:

from typing import NamedTuple, Callable, Union, Tuple
from functools import partial

class Case(NamedTuple):
    task: Callable
    error: Union[BaseException, Tuple[BaseException, ...]]
    success: Callable


try_all(
    Case(
        task=find_record,
        error=NoSuchRecord,
        success=partial(logger.info, "Record found")),
    (
        create_record, CreateFailed,
        partial(logger.info, "Created a new record")),
    Case(
        task=lambda: tape,
        error=(),
        success=partial(logger.info, "Using a tape now")),
)