correct way to catch exception with Python 'with' statement

[Thomas 'PointedEars' Lahn]

Ganesh Pal wrote:

> I am using Python 2.7 and Linux

As a rule of thumb¹, use at least Python 3.3 for new programs.

> What will be the best  way to catch the exception in the above program  ?
> Can we replace both the  with statement in the above program with
> something like below
> 
> try:
>     for i in range(1000):
>                with open(os.path.join(QA_TEST_DIR,"filename%d" %i),'w') as
>                f:
>                    f.write("hello")
> except IOError as e:
>     raise

If you do it like this, you will have a hard time figuring out what caused 
the exception, and it becomes the harder the more you do in the loop. 
Because *several* statements could throw an exception of the *same* type.

Therefore, as a rule of thumb¹, if you catch exceptions, catch them closest 
to where they could occur:

for i in range(1000):
    try:
        with open(os.path.join(QA_TEST_DIR, "filename%d" % i), 'w') as f:
            try:
                f.write("hello")
            except … as e:
                raise e

    except … as e:
        raise e

This is just an example; you do not have to just re-raise the exception; 
“raise” is just where your custom exception handling, if any, should be.  
For example, if f.write() fails with an IOError, you could decide to ignore 
that and “continue” with the next file, whereas you may decide that failing 
to open a file is so grave a problem that you want to abort the program at 
this point.

Finally, use uniform indentation and spacing in your code; the former is 
even more important in Python than in other programming languages because 
indentation makes a block statement in Python.  Use a Python-aware editor to 
make it easy to create uniform indentation by use of the Tab key, and a 
reformatting feature for existing code.  I can recommend Atom [0] with the 
packages “autocomplete-python”, “linter-pylint”, “python-isort”, and 
“python-tools” (“language-python” is a built-in); and Vim [1] with

if has("syntax")
    …
    syntax on
    …
endif
…
filetype plugin indent on

in ~/.vimrc and a ~/.vim/after/ftplugin/python.vim containing

setlocal expandtab
setlocal shiftwidth=4
setlocal softtabstop=4

_______
¹  YMMV

[0] <https://atom.io/>
[1] <http://www.vim.org/>
-- 
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.