How can I raise an EOF error?

Question

I am a newbie learning python. Please take a look at the below code (From Data Structures and Algorithms in Python by Goodrich, et al).

age = -1   # an initially invalid choice
while age <= 0:
    try:
        age = int(input('Enter your age in years: '))
        if age <= 0:
            print('Your age must be positive.')
    except ValueError:
        print('That is an invalid age specification.')
    except EOFError:
        print('There was an unexpected error reading input.')
        raise  # let's re-raise this exception.

I know what ValueError is. For example the ValueError occurs if the input is given as characters instead of an integer.

On the other hand, I have no idea when EOFError raises.

I can't get what 're-raise this exception' means
The book says, 'the call to input will raise an EOFError if the console input fails.' Again, I have no idea what console input is and when console input fails.

I have tried several ways of raising EOFError, but every time I tried there was only ValueError. Can someone give me some idea?

Thanks in advance.

score 3 · Accepted Answer · answered Jun 29 '18 at 15:32

3

From input's documentation:

When EOF is read, EOFError is raised.

EOF is sent when the input stream reaches the end, or if it's from the console, it means the user presses ctrl-D on *NIX, or ctrl-Z on Windows.

You can catch the EOFError exception and break your while loop as a way to end the program gracefully, so change your exception block to:

except EOFError:
    print('Done.')
    break

answered Jun 29 '18 at 15:32

blhsing

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Can you please show me an example of how to catch the EOFError? What input should be? – Andy Junghyun Kim Jun 29 '18 at 15:41
The `except EOFError` block catches `EOFError`. If you want to test it, simply press ctrl-D on *NIX, or ctrl-Z on Windows, and then enter. – blhsing Jun 29 '18 at 15:43
Um... Im using mac and command-D or Z did not work as wanted. – Andy Junghyun Kim Jun 29 '18 at 15:45
Result of command-D: Enter your age in years: That is an invalid age specification. Enter your age in years: – Andy Junghyun Kim Jun 29 '18 at 15:46
result of command-Z: Enter your age in years: That is an invalid age specification. – Andy Junghyun Kim Jun 29 '18 at 15:46
Command-D should work on OS X. Not sure why it doesn't work for you. See: [Signal EOF in mac osx terminal](https://stackoverflow.com/questions/21364313/signal-eof-in-mac-osx-terminal) – blhsing Jun 29 '18 at 15:50
I test with pycharm under win10 and `ctrl + d` will cause `EOFError`. – Travis Mar 31 '20 at 08:39

chepner · Answer 2 · 2018-06-29T16:06:29.280

2

You caught the exception, meaning Python will continue with the loop instead of letting the exception percolate up the stack, ultimately ending the program if it remains uncaught. Calling raise without an argument in an except clause simply raises the same exception again, equivalent to

except EOFError as exc:
    print("Unexpected error")
    raise exc

Re-raising the exception is necessary if you don't actually handle the exception, instead simply adding additional logging before (possibly) someone else handles it.

Triggering the EOFError can be done by running

$ python -c 'input()' < /dev/null
Traceback (most recent call last):
  File "<string>", line 1, in <module>
EOFError: EOF when reading a line

as all attempts to read from /dev/null look like an attempt to read from the end of a file.

edited Jun 29 '18 at 16:06

answered Jun 29 '18 at 15:29

chepner

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I'm sorry but when I enter $ python -c 'input()' < /dev/null as an input, it still says it is ValueError, maybe because it is a string? – Andy Junghyun Kim Jun 29 '18 at 15:35
I get an `EOFError` in both Python 2 and Python 3; not sure how you are getting a `ValueError`. – chepner Jun 29 '18 at 15:38
Have you copied and pasted the code and entered the input as $ python -c 'input()' < /dev/null? – Andy Junghyun Kim Jun 29 '18 at 15:40
In addition, if I run $ python -c 'input()' < /dev/null in the blank editor, the result would be Syntax Error: invalid syntax: , line 1, pos 1 – Andy Junghyun Kim Jun 29 '18 at 15:43
The command (minus the `$`) is meant to be run from the shell, not from inside the interpreter. The `$` is just a convention for indicating a shell prompt. – chepner Jun 29 '18 at 16:05

score 0 · Answer 3 · edited Jul 15 '23 at 09:18

0

If you're looking to raise a The following code will produce a Syntax Error, EOF in Python:

print("Where's EOF now?")
#### this is a test to see where the EOF occurs... suspect is that it will be EOF line 4
print("Hello world!"

I'm not sure if this is what you mean by "raise an error", though. Creates a new post in StackOverflow

edited Jul 15 '23 at 09:18

moken

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answered Dec 03 '18 at 18:55

Alex Kuligowski

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How can I raise an EOF error?

3 Answers3