[Chicago] List comprehension question

[Carl Karsten]

Now that we have answered your question....

Stop doing all this and use an ORM.

manual way:
sql = "SELECT tail FROM aircraft_state WHERE airline='WN'"
c.execute(sql)
comm = [x[0] for x in c.fetchall()]
for c in comm:
    print( c )

ORMy way:

class Aircraft_state(models.Model):
    airline = models.CharField(max_length=2)
    tail  = models.CharField(max_length=20)

states = Aircraft_state.objects.filter( airline='WN'" )
for state in states:
    print( state.tail )


All of the SQL and stuff has been done in the models.Model class. This
includes generating and executing the CREATE TABLE commands to build a new
db, parametrizing the parameters so no SQL injection exploits or mistakes,
lazy reads so it ie less likely to read a bunch of data that never gets
used, etc.

And you write less code.  less code is easier to read and debug.






On Fri, May 27, 2016 at 10:46 AM, Dan Mahoney <catdude at gmail.com> wrote:

> Very cool, thanks to both of you. That makes sense - I guess I'm just
> suffering from temporary (I hope) brain fade.
>
> On Fri, May 27, 2016 at 10:40 AM, Dale <dale at codefu.org> wrote:
>
>> I believe you want something like
>>
>> comm = [row[0] for row in c.fetchall()]
>>
>> Keep in mind that the list comprehension is going to evaluate that part
>> before the "for" for each row it gets from the cursor, so you need the [0]
>> inside there, not outside the list comprehension, which would be asking for
>> the first element from the list comprehension—not what you want.
>>
>> Note that c.fetchall() fetches all rows, which might be a waste of memory
>> for large data sets.  Psycopg (PostgreSQL DB API library) lets you iterate
>> over a cursor directly:
>>
>> comm = [row[0] for row in c]
>>
>> *In theory* this doesn't have to load all the rows into memory at once.
>>  (In practice I bet it does unless you do something special with
>> Psycopg/PostgreSQL.)  I'm not sure if you can just iterate over a cursor
>> with your MySQL driver, but there's no harm in trying.
>>
>> Dale
>>
>>
>> On Wed, May 25, 2016 at 10:38 AM, Dan Mahoney <catdude at gmail.com> wrote:
>>
>>> I've got a question about using list comprehensions.
>>>
>>> I've got a piece of code that is making a MySQL query:
>>> sql = "SELECT tail FROM aircraft_state WHERE airline='WN'"
>>> c.execute(sql)
>>>
>>> I'm currently using a loop to get the values into a list:
>>> rows = c.fetchall()
>>> for row in rows:
>>>     comm = row[0]
>>>
>>> since the return value from MySQL is delivered as tuples. I'd like to
>>> use a list comprehension to build my final list (no important reason, I
>>> just want to learn how). I tried:
>>>
>>> comm = [x for x in c.fetchall()][0]
>>>
>>> but that just gives me the first tuple. If I use:
>>>
>>> comm = [s for x in c.fetchall()]
>>>
>>> I end up with a list of tuples. Any suggestions as to how I could do
>>> this without using a for loop?
>>>
>>>
>>> --
>>> --------------------------
>>> Dan Mahoney
>>> catdude at gmail.com
>>> Skype: catdude60440
>>>
>>> "How you behave towards cats here below determines your status in
>>> Heaven."
>>> Robert Heinlein
>>>
>>> "There are two means of refuge from the miseries of
>>> life - music and cats" - Albert Schweitzer
>>>
>>> _______________________________________________
>>> Chicago mailing list
>>> Chicago at python.org
>>> https://mail.python.org/mailman/listinfo/chicago
>>>
>>>
>>
>> _______________________________________________
>> Chicago mailing list
>> Chicago at python.org
>> https://mail.python.org/mailman/listinfo/chicago
>>
>>
>
>
> --
> --------------------------
> Dan Mahoney
> catdude at gmail.com
> Skype: catdude60440
>
> "How you behave towards cats here below determines your status in Heaven."
> Robert Heinlein
>
> "There are two means of refuge from the miseries of
> life - music and cats" - Albert Schweitzer
>
> _______________________________________________
> Chicago mailing list
> Chicago at python.org
> https://mail.python.org/mailman/listinfo/chicago
>
>


-- 
Carl K
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