Python try-except-except

Question

我将包含此代码应该执行的任务的描述，以防有人需要它来回答我。

#Write a function called "load_file" that accepts one 
#parameter: a filename. The function should open the
#file and return the contents.#
#
# - If the contents of the file can be interpreted as
#   an integer, return the contents as an integer.
# - Otherwise, if the contents of the file can be
#   interpreted as a float, return the contents as a
#   float.
# - Otherwise, return the contents of the file as a
#   string.
#
#You may assume that the file has only one line.
#
#Hints:
#
# - Don't forget to close the file when you're done!
# - Remember, anything you read from a file is
#   initially interpreted as a string.


#Write your function here!
def load_file(filename):
    file=open(filename, "r")
    try:
        return int(file.readline())
    except ValueError:
        return float(file.readline())
    except:
        return str(file.readline())
    finally:
        file.close()


#Below are some lines of code that will test your function.
#You can change the value of the variable(s) to test your
#function with different inputs.
#
#If your function works correctly, this will originally
#print 123, followed by <class 'int'>.
contents = load_file("LoadFromFileInput.txt")
print(contents)
print(type(contents))

当使用包含“123”的文件测试代码时，一切正常。当网站加载另一个文件来测试这段代码时，会出现以下错误：

[Executed at: Sat Feb 2 7:02:54 PST 2019]

    We found a few things wrong with your code. The first one is shown below, and the rest can be found in full_results.txt in the dropdown in the top left:

    We tested your code with filename = "AutomatedTest-uwixoW.txt". We expected load_file to return the float -97.88285. However, it instead encountered the following error:

    ValueError: could not convert string to float:

所以我猜测错误发生在第一个 except 语句中，但我不明白为什么。如果将文件中的值转换为浮点数时发生错误，代码不应该转到第二个except 语句吗？而在第二个except 中，它将被转换为字符串，这仍然可以工作吗？我猜我误解了try-except(specified error)-except(no specified error) 的工作原理。

抱歉，帖子太长了。

Answer 1

代码不应该直接转到第二个 except 语句吗？

不：这个“扁平”的 try/except 语句仅适用于 first try 块。如果那里发生异常，except 分支会捕获此异常并立即评估相应的块。如果这个块发生异常，它不会被任何东西捕获，因为那里没有try块。

因此，您必须执行大量嵌套的 try/except 语句：

try:
    do_this()
except ValueError:
    try:
        do_that()
    except ValueError:
        do_this()
    except:
        do_that_one()
except:
    # a whole bunch of try/except here as well

您可能需要添加额外的嵌套级别。

就您需要编写的代码量而言，这非常低效。更好的选择可能是：

data = file.readline()
for converter in (int, float, str):
    try:
        return converter(data)
    except:
        pass

请注意，如果您执行converter(file.readline())，则每次迭代都会读取一个新行（或者，在您的情况下，在任何新的 try/except 块中），这可能不是您需要的。

Answer 2

不，只有一个 except 块——第一个匹配异常的块——将被执行。您所描述的行为将对应于

except ValueError:
    try:
        return float(file.readline())
    except:
        return str(file.readline())

Answer 3

def load_file(filename):
    file=open(filename, "r")
    try:
        val = file.readline()
        return int(val)
    except ValueError:
        try:
            return float(val)
        except:
            return str(val)
    finally:
        file.close()