Catching An Exception While Using A Python 'with' Statement


Answer :

from __future__ import with_statement  try:     with open( "a.txt" ) as f :         print f.readlines() except EnvironmentError: # parent of IOError, OSError *and* WindowsError where available     print 'oops'

If you want different handling for errors from the open call vs the working code you could do:

try:     f = open('foo.txt') except IOError:     print('error') else:     with f:         print f.readlines()

The best "Pythonic" way to do this, exploiting the with statement, is listed as Example #6 in PEP 343, which gives the background of the statement.

@contextmanager def opened_w_error(filename, mode="r"):     try:         f = open(filename, mode)     except IOError, err:         yield None, err     else:         try:             yield f, None         finally:             f.close()

Used as follows:

with opened_w_error("/etc/passwd", "a") as (f, err):     if err:         print "IOError:", err     else:         f.write("guido::0:0::/:/bin/sh\n")

Catching an exception while using a Python 'with' statement

The with statement has been available without the __future__ import since Python 2.6. You can get it as early as Python 2.5 (but at this point it's time to upgrade!) with:

from __future__ import with_statement

Here's the closest thing to correct that you have. You're almost there, but with doesn't have an except clause:

with open("a.txt") as f:      print(f.readlines()) except:                    # <- with doesn't have an except clause.     print('oops')

A context manager's __exit__ method, if it returns False will reraise the error when it finishes. If it returns True, it will suppress it. The open builtin's __exit__ doesn't return True, so you just need to nest it in a try, except block:

try:     with open("a.txt") as f:         print(f.readlines()) except Exception as error:      print('oops')

And standard boilerplate: don't use a bare except: which catches BaseException and every other possible exception and warning. Be at least as specific as Exception, and for this error, perhaps catch IOError. Only catch errors you're prepared to handle.

So in this case, you'd do:

>>> try: ...     with open("a.txt") as f: ...         print(f.readlines()) ... except IOError as error:  ...     print('oops') ...  oops

Comments

Post a Comment

Popular posts from this blog

Chemistry - Bond Angles In NH3 And NCl3

Answer : Solution 1: As Tan Yong Boon stated, it is Bent’s Rule with which we can explain the lower bond angle of \ce N F 3 \ce{NF3} \ce NF 3 when compared to \ce N H 3 \ce{NH3} \ce N H 3 . The rule as stated by Henry Bent: Atomic s character concentrates in orbitals directed towards electropositive substituents. Fluorine is more electronegative that hydrogen, and the \ce N − F \ce{N−F} \ce N − F bond would have greater p character than the \ce N − H \ce{N−H} \ce N − H bond. And more s character leads to large bond angles. Thus, the bond angle is greater in \ce N H 3 \ce{NH3} \ce N H 3 than in \ce N F 3 \ce{NF3} \ce NF 3 . Now, consider \ce N C l 3 \ce{NCl3} \ce NCl 3 . Clearly, \ce C l \ce{Cl} \ce Cl atom islarger in size than the central atom, nitrogen. Hence the higher bond angle here is due to the steric crowding caused by \ce C l \ce{Cl} \ce Cl atoms.(More pronounced than the electrongativity of \ce C l \ce{Cl} \ce Cl atom). They repel eachother and hence b...
Read more

Are Regular VACUUM ANALYZE Still Recommended Under 9.1?

Answer : VACUUM is only needed on updated or deleted rows in non-temporary tables. Obviously you're doing lots of INSERTs but it's not obvious from the description that you're also doing lots of UPDATEs or DELETEs. These operations can be tracked with the pg_stat_all_tables view, specifically the n_tup_upd and n_tup_del columns. Also, even more to the point, there is a n_dead_tup column that tells, per table, how much rows need to be vacuumed. (see Monitoring statistics in the doc for functions and views related to statistics gathering). A possible strategy in your case would be to suppress the scheduled VACUUM, keeping an eye on this view and checking on which tables the n_dead_tup is going up significantly. Then apply the aggressive VACUUM to these tables only. This will be a win if there are large tables whose rows never get deleted nor updated and the aggressive VACUUM is really necessary only on smaller tables. But keep running the ANALYZE for the optimiz...
Read more

Change The Font Size Of Visual Studio Solution Explorer

Answer : Go to Tools->Options->Environment->Fonts and Colors In "Show Settings for" chose "Environment Font" In "Font" replace Automatic to for example, Arial and change size. Change the font size of that setting, all Font size in the Explorers (Solution Explorer, Server Explorer and Toolbox) and menus will change. You can set that in Tools, Options.
Read more