Questions on exceptions

[Steven D'Aprano]

On Mon, 21 Aug 2006 00:40:14 -0700, sc_wizard29 wrote:

> Hi everyone,
> 
> I've just finished studying O'Reilly's "Learning python" and since I
> come from the Java world, there are some things that bother me
> concerning python's exception handling.
> 
> In Java, all methods must declare the exceptions throwed (I'm speaking
> of checked exceptions)... but this is not the case in Python. So my
> question is : how can I know which exceptions I must catch ? (am I
> supposed to believe what's written in the documentation ? am I supposed
> to read the source code to see which exceptions are throwed ?)

How do you know what objects a method will return? Are you supposed to
believe the documentation? Are you supposed to read the source code?

Exceptions are no different. If a method claims to raise ValueError, but
sometimes raises TypeError, I call that a bug, regardless of whether the
line which causes it is "int(x) + str(y)" or "raise TypeError('My
documentation is incomplete')".


> Also, can someone explain me why there is no try...except...finally
> statement ? 

Historical reasons. I believe that Python 2.5 will support it.

> For example, the following code snippet is not valid, but
> what would be the correct python way to do it ?
> 
> myFile = open('file.txt') # assume file exists
> try:
> 	for nextLine in file:
>         	nextLine = nextLine.rstrip('\n');print "line = " + nextLine
> except IOError:
> 	print "Error while reading from file"
> finally:
> 	myFile.close

Nested try...except blocks.


-- 
Steven D'Aprano