* Mike Kent:
On Mar 4, 12:30 pm, Robert Kern <robert.k...@gmail.com> wrote:
He's ignorant of the use cases of the with: statement, true.
<humor> Ouch! Ignorant of the use cases of the with statement, am I?
Odd, I use it all the time. </humor>
Given only your
example of the with: statement, it is hard to fault him for thinking that try:
finally: wouldn't suffice.
<humor> Damn me with faint praise, will you? </humor>
I'm kinda amazed at the drama my innocent request for the use case
elicited. From what I've gotten so far from this thread, for the
actual example Mr. Steinbach used, the only disadvantage to my counter-
example using try/finally is that the chdir in the finally part will
always be executed, even if the chdir in the try part did not
succeed. I concede that, and was aware of it when I wrote it. For
the simple example given, I did not consider it compelling.
Uhm, well.
My example was:
with Cleanup as at_cleanup:
# blah blah
chdir( somewhere )
at_cleanup.call( lambda: chdir( original_dir ) )
# blah blah
It was not intended to compel, rather just to illustrate usage. :-)
And you asked about comparing that with ...
original_dir = os.getcwd()
try:
os.chdir(somewhere)
# Do other stuff
finally:
os.chdir(original_dir)
# Do other cleanup
.. which does something different, namely, always executing the
os.chdir(original_dir) or more generally the action-specific cleanup.
The action-specific cleanup might be much more costly than a chdir, and/or, in
the case where the action failed, incorrect.
In the same number of lines and with fewer keystrokes you could have written
code that was equivalent to the code I posted and that you wanted to compare
with, e.g. ...
original_dir = os.getcwd()
os.chdir(somewhere)
try:
# Do other stuff
finally:
os.chdir(original_dir)
# Do other cleanup
... so given how easy it is to write such an equivalent code snippet, I assumed
that the different behavior was /not/ intended, that instead, you'd attempted to
write equivalent code but made a slight error in the translation to lower level
construct -- but impossible to say exactly what.
Now you write that you were "aware of [the different behavior] when I wrote it",
and that just baffles me: why not then, as a basis of sound comparision, write
the equivalent code shown above, the same number of lines as what you wrote?
A more
complex example, that would have required multiple, nested try/finally
blocks, would show the advantages of Mr Steinbach's recipe more
clearly.
However, I fail to understand his response that I must have meant try/
else instead, as this, as Mr. Kern pointed out, is invalid syntax.
Perhaps Mr. Steinbach would like to give an example?
OK.
Assuming that you wanted the chdir to be within a try block (which it was in
your code), then to get code equivalent to my code, for the purpose of a
comparision of codes that do the same, you'd have to write something like ...
original_dir = os.getcwd()
try:
os.chdir(somewhere)
except Whatever:
# E.g. log it.
raise
else:
try:
# Do other stuff
finally:
os.chdir(original_dir)
# Do other cleanup
... which would be a more general case.
I've also given this example in response to Robert earlier in the thread.
Although I haven't tried it I believe it's syntactically valid. If not, then the
relevant typo should just be fixed. :-)
I have no idea which construct Robert thought was syntactically invalid. I think
that if he's written that, then it must have been something he thought of.
Cheers & hth.,
- Alf
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