This is a tongue-in-cheek follow-up to the thread on Python 4 incompatible
code, but it did happen to me, and may help someone else.
I execute a lot of database SELECT commands. In many cases I expect to get
only one row returned, but it could be zero or more than one.
Using LBYL, one would retrieve the row(s) and check the length. I found a
way to use EAFP, as follows -
cur.execute('SELECT ...')
(row,) = cur
This uses tuple unpacking, and only works if exactly one row is returned. If
it fails it raises ValueError, but I need to know whether it failed because
no rows were returned, or more than one row was returned. The only way I
could figure out how to achieve this was to parse the error message.
If 0 rows were returned, the message is 'need more than 0 values to unpack'.
If > 1 rows were returned, the message is 'too many values to unpack'.
So my test was -
except ValueError as e:
if str(e).startswith('need'):
# 0 rows returned
else:
# > 1 rows returned
This has worked for years, but with 3.5 it stopped working. It took me a
while to figure out why. Lo and behold, the error message has changed! Now,
if 0 rows are returned, the message is 'not enough values to unpack'.
Luckily the other message has not changed, so now my test is -
except ValueError as e:
if str(e).startswith('too many'):
# > 1 rows returned
else:
# 0 rows returned
Now it works with both versions.
Frank Millman
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