On Sun, Oct 15, 2017 at 3:09 AM, Peter Otten <__pete...@web.de> wrote:
> boB Stepp wrote:
>
>> I have not used a "finally" block before. I just had the thought that
>> maybe it would run even if an uncaught exception might occur. I tried
>> to test this thought by generating a deliberate NameError in the "try"
>> block and added a print to the "finally" clause. I got the intended
>> NameError with no evidence of the added print printing. But I thought
>> I would ask just to be sure: If an uncaught exception occurs, will
>> the "finally" clause execute?
>
>
> Yes.
>
>>>> try:
> ... 1/0
> ... except ValueError:
> ... print("not triggered")
> ... finally:
> ... print("ALWAYS TRIGGERED")
> ...
> ALWAYS TRIGGERED
> Traceback (most recent call last):
> File "<stdin>", line 2, in <module>
> ZeroDivisionError: division by zero
That is what I thought after reading about "finally". But look what
happens if I modify my actual code to generate a NameError:
============================================================================================
def ensure_db(filename):
"""Open the database, "filename", if it exists; otherwise, create a
database named "filename"."""
db = sqlite3.connect(filename)
cur = db.cursor()
try:
sql_cmd = "SELECT VersionNumber FROM CurrentDBVersion"
a # This should generate a NameError
# First element of returned tuple will be the db version number:
current_db_version = int(cur.execute(sql_cmd).fetchone()[0])
except sqlite3.OperationalError:
# This means that the database and the table, "CurrentDBVersion", has
# not yet been created, implying "version 0".
current_db_version = 0
finally:
sql_scripts = ["../database/create_sqlite3_db.sql"]
for sql_scriptname in sql_scripts[current_db_version:]:
with open(sql_scriptname) as f:
cur.executescript(f.read())
print("THIS IS THE FINALLY BLOCK!!!") # And this *should* print
return db
============================================================================================
This results in the following Traceback:
============================================================================================
> py main.py
Traceback (most recent call last):
File "main.py", line 16, in ensure_db
a
NameError: name 'a' is not defined
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "main.py", line 36, in <module>
ensure_db(db_filename)
File "main.py", line 27, in ensure_db
for sql_scriptname in sql_scripts[current_db_version:]:
UnboundLocalError: local variable 'current_db_version' referenced
before assignment
============================================================================================
So what is going on here? Why does "finally" not have its print
executed? Does the "...another exception occurred:..." interrupt the
normal flow of the "try/except/finally" structure and prevent the
"finally" block from executing?
--
boB
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