[Chicago] List comprehension question

[Adam Forsyth]

Sometimes an ORM is useful, but there are lots of cases where it can't do
what you want, or encourages tight coupling of your logic and data model --
which is bad when your data model has to change.

See https://github.com/kennethreitz/records for a nice wrapper around raw
SQL.

On Fri, May 27, 2016 at 11:32 AM, Carl Karsten <carl at personnelware.com>
wrote:

> 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
>>>>
>>>> _______________________________________________
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>>>> https://mail.python.org/mailman/listinfo/chicago
>>>>
>>>>
>>>
>>> _______________________________________________
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>>>
>>>
>>
>>
>> --
>> --------------------------
>> 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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> Chicago mailing list
> Chicago at python.org
> https://mail.python.org/mailman/listinfo/chicago
>
>
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