python - 为什么我的 `else` block 的 `try` 部分的代码没有运行？

Question

我将 Python 2.7.3 与 Flask 0.10.1 和 SQL-Alchemy 0.9.1 一起使用。

在我的 View 代码中，我正在为数据库输入构建一个对象，我的代码取决于两个 try/except/else 的正确执行 block 。对于第一个 block ，无论发生异常还是不发生异常，它都按预期工作。当出现异常时，我会得到一组错误。如果没有异常，数据会添加到数据库中并且样本计数器会增加。这是该片段:

        try:
            new_sample = Sample()
            new_sample.build(sample)
        except SampleBuildingError as e:
            result_response['errors'].append('{0}: {1}'.format(sample['sample_name'], e.value) )
        else:
            db.session.add(new_sample)
            num_added += 1

在 View 函数的更下方，我有一个 try/except/else/finally block 。数据被提交到数据库，所以 try 部分显然有效，finally block 也是如此。但是，else block 似乎没有被执行:

try:
    db.session.commit()
except Exception as e:
    result_response['error'] = "Failed on the database input: {0}".format( str(e) )
else:
    result_response['success'] = "The samples were input to the database successfully. {0} samples entered into the database, with {1} errors".format( num_added, len(errors) )
finally:
    return jsonify(result_response)

当出现异常时，我按预期返回带有 error 键和数据库错误的 json。但是，当数据库提交成功时，我得到一个 json 对象，其中包含每个预期的键 success 键。

else block 似乎被跳过了，但我不明白为什么。我试过将单词 bogus 放在 else block 的第二行以尝试强制出错，但 Python 没有提示!

完整 View 函数如下:

@app.route("/kapasubmit", methods=['POST'])
def kapasubmit():
    num_added = 0
    result_response = {'errors': []}
    samples = request.json['data']
    for sample in samples:
        sample_check = Sample.query.filter_by( sample_name = sample['sample_name'] ).first()
        if sample_check is None:
            try:
                new_sample = Sample()
                new_sample.build(sample)
            except SampleBuildingError as e:
                result_response['errors'].append('{0}: {1}'.format(sample['sample_name'], e.value) )
            else:
                db.session.add(new_sample)
                num_added += 1
        else:
            result_response['errors'].append('{0}: This is a duplicate sample'.format(sample['sample_name']) )

    if num_added > 0:
        try:
            db.session.commit()
        except Exception as e:
            result_response['error'] = "Failed on the database input: {0}".format( str(e) )
        else:
            result_response['success'] = "The samples were input to the database successfully. {0} samples entered into the database, with {1} errors".format( num_added, len(errors) )
            bogus
        finally:
            return jsonify(result_response)
    else:
        result_response['error'] = "No valid samples submitted for input"
        return jsonify(result_response)

Answer 1

如果 else block 引发异常，它会被忽略，因为解释器必须从函数执行 return。演示:

>>> def divide(x, y):
...     try:
...         result = x / y
...     except ZeroDivisionError:
...         print "division by zero!"
...     else:
...         print "result is", result
...         print 1/0
...     finally:
...         return 1
... 
>>> divide(2,1)
result is 2
1

1/0 不会导致任何回溯。