The exception handling mechanism in Python is provided through the keywords try, except, finally and raise.
While the try keyword marks a block of Python code which may generate one or more exceptions during its run time, the except keyword marks a block of Python code as one of the handlers of those exceptions. These except handlers are searched from the inner scope to the outer scope. When a matching exception handler is found, it is executed and the program execution continues after the try block. If a matching exception handler is not found, then the program exits after printing the traceback.
Example:
The following Python code accesses an invalid index from a Python list which raises an exception IndexError. Since the code block is not enclosed by a try block and there are no except handlers, the program exits after printing the traceback.
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tokens = ["try", "catch", "except", "finally", "from"]; print(tokens[5]) |
Output:
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Traceback (most recent call last): File "ex_ex.py", line 3, in <module> print(tokens[5]) IndexError: list index out of range |
Example:
The Python code example below acesses a tuple of tokens with an invalid index, which raises an exception - IndexError:tuple index out of range. Since the code is enclosed by try block and an associated except block, the program handles the exception and continues after the try block.
Example:
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tokens = ("and", "or", "not");
def getToken(token): try: return tokens[token] except Exception as Ex: print("Exception:%s"%Ex)
tokenIndex = 4 token = getToken(tokenIndex) print("Token: %s"%(token))
tokenIndex = 2 token = getToken(tokenIndex) print("Token: %s"%(token)) |
Output:
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Exception:tuple index out of range Token: None Token: not |
Note the token value None in the second line of output which corresponds to the invalid tuple index.