variable in try clause not accessible in finally clause - python

Question

I am new to python, so sorry if this question is dumb, but can someone tell me what's going on here.

When I run the following code with no errors in the mdb.connect() call, the code runs fine.

But when I purposely insert an error (for example, put in 'localhostblahblah'), I get a 'NameError: name 'con' is not defined' error when I execute.

I thought that variables defined in the try clause should be accessible in the finally clause. What's going on?

#!/usr/bin/python

import MySQLdb as mdb
import sys

try:
    con = mdb.connect('localhost','jmtoung','','ptb_genetics')

except mdb.Error, e:
    print "Error"
    sys.exit(1)

finally:
    if con:
        con.close()

What would you expect to happen?

user395760
– user395760

2014-01-23 19:02:33 +00:00
Commented Jan 23, 2014 at 19:02 — user395760
– user395760, Commented Jan 23, 2014 at 19:02

mhlester · Accepted Answer · 2014-01-23 19:19:42Z

10

If mdb.connect errors, there's nothing to assign to con, so it doesn't get defined.

Instead of finally, try using else, which is run only when there was no exception. Docs

try:
    con = mdb.connect('localhost','jmtoung','','ptb_genetics')

except mdb.Error as e:
    print "Error"
    sys.exit(1)

else:  # else instead of finally
    con.close()

edited Jan 23, 2014 at 19:19

answered Jan 23, 2014 at 19:02

mhlester

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4 Comments

JAB Over a year ago

I always forget that you can use else like that in Python. It also works on while and for loops, as I remember.

mhlester Over a year ago

They just love throwing else into everything, don't they?

Kirk Strauser Over a year ago

Would you write that as except mdb.Error as e? It's a much clearer syntax and works in both Python 2.6+ and Python 3.

mhlester Over a year ago

@KirkStrauser, yes I personally would. hadn't inspected the less pertinent code. updated

Adam Smith · Accepted Answer · 2014-01-23 19:02:52Z

1

Do it EAFP style:

try: con = mdb.connect('localhost','jmtoung','','ptb_genetics')
except mdb.Error, e: #handle it
finally:
    try: con.close()
    except NameError: pass # it failed to connect
    except: raise # otherwise, raise that exception because it failed to close

answered Jan 23, 2014 at 19:02

Adam Smith

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Comments

JAB · Accepted Answer · 2014-01-23 19:04:54Z

0

If an error occurs in the process of variable assignment, the variable will not be assigned to.

>>> x = 3
>>> try:
...     x = open(r'C:\xxxxxxxxxxxxxxx')
... finally:
...     print(x)
...     
3
Traceback (most recent call last):
  File "<interactive input>", line 2, in <module>
IOError: [Errno 2] No such file or directory: 'C:\\xxxxxxxxxxxxxxx'

answered Jan 23, 2014 at 19:04

JAB

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