函数前和函数中<try>和<except>的区别

Question

当我在函数前使用 try 时，它会引发错误。但是如果我使用它 in 函数就可以了。为什么？（这些代码的评估顺序是什么？）。感谢任何帮助。

try:
    def chiz(dict, key):
        return dict[key]
except:
    print("somethig went wrong")
user_01 = {
    "name": "armin",
    "age": 20.51
}
chiz(user_01, "weight")

>>>Traceback (most recent call last):
  File "C:\Users\user\PycharmProjects\pythonWhile\test.py", line 11, in <module>
    chiz(user_01, "weight")
  File "C:\Users\user\PycharmProjects\pythonWhile\test.py", line 3, in chiz
    return dict[key]
KeyError: 'weight'

def chiz(dict, key):
    try:
        return dict[key]

    except :
        print("somethig went wrong")

user_01 = {
    "name": "armin",
    "age": 20.51
}

chiz(user_01, "weight")

Answer 1

第一个示例中的 try 不会在函数被调用时被求值，只有当函数被首次定义时才会被求值，此时不会引发异常。

直到您实际使用给定字典中不存在键的参数调用函数时，才会实际引发异常，然后才不会评估 try/except 子句。

你的第二个例子是做你想做的事情的正确方法：

In [4]: def chiz(dict, key): 
   ...:     try: 
   ...:         return dict[key] 
   ...:     except: 
   ...:         print("Something went wrong") 
   ...:                                                                         

In [5]: chiz(user_01, "weight")                                                 
Something went wrong

话虽这么说，请重新考虑那个简单的 except 子句。相当于说except BaseException:，这可能不是你想要的，是反对PEP8 recommendations的。在这种情况下，您可能应该使用 except KeyError: 或最广泛的 except Exception:.

When catching exceptions, mention specific exceptions whenever possible instead of using a bare except: clause. A bare except: clause will catch SystemExit and KeyboardInterrupt exceptions, making it harder to interrupt a program with Control-C, and can disguise other problems. If you want to catch all exceptions that signal program errors, use except Exception: (bare except is equivalent to except BaseException:). A good rule of thumb is to limit use of bare 'except' clauses to two cases:

If the exception handler will be printing out or logging the traceback; at least the user will be aware that an error has occurred.

If the code needs to do some cleanup work, but then lets the exception propagate upwards with raise. try...finally can be a better way to handle this case.

来自 PEP463：

any situation where you catch an exception you don't expect to catch is an unnecessary bug magnet.

函数前和函数中<try>和<except>的区别

difference between <try> and <except> before the function & in the function

python

try-except