Need help with Python class idioms

[Tad Marko]

Howdy!

I'm trying to get my head around Python and I've come to realize that
there are a lot of idioms in Python that are a bit different than in
other languages. I have a class similar to what I've included below.
The only difference is that in the real class, I have even more
mandatory attributes. Too many to specify at once on a constructor and
they don't always exist at construction time anyway, so the accessor
idiom must remain.

Can anyone suggest a more Pythony way to do the following?

Thanks,
Tad

#!/bin/env python

class A:
def __init__(self):
self.mandatoryV ar1 = None
self.mandatoryV ar2 = None

def setMV1(self, mv1):
self.mandatoryV ar1 = mv1

def setMV2(self, mv2):
self.mandatoryV ar2 = mv2

def saveToDB(self, db):
if self.mandatoryV ar1 == None:
raise Exception, "Mandatory Var 1 not set."
if self.mandatoryV ar2 == None:
raise Exception, "Mandatory Var 2 not set."

# Using db, do whatever saving stuff is needed.

[A.Schmolck]

Re: Need help with Python class idioms

tad@tadland.net (Tad Marko) writes:[color=blue]
> Can anyone suggest a more Pythony way to do the following?
>
> Thanks,
> Tad
>
> #!/bin/env python
>
> class A:
> def __init__(self):
> self.mandatoryV ar1 = None
> self.mandatoryV ar2 = None
>
> def setMV1(self, mv1):
> self.mandatoryV ar1 = mv1
>
> def setMV2(self, mv2):
> self.mandatoryV ar2 = mv2
>
> def saveToDB(self, db):
> if self.mandatoryV ar1 == None:
> raise Exception, "Mandatory Var 1 not set."
> if self.mandatoryV ar2 == None:
> raise Exception, "Mandatory Var 2 not set."
>
> # Using db, do whatever saving stuff is needed.[/color]

class A(object):
def saveToDB(self, db):
# Using db, do whatever saving stuff is needed.

'as

[James Henderson]

Re: Need help with Python class idioms

On Wednesday 21 January 2004 8:43 pm, Tad Marko wrote:[color=blue]
> Howdy!
>
> I'm trying to get my head around Python and I've come to realize that
> there are a lot of idioms in Python that are a bit different than in
> other languages. I have a class similar to what I've included below.
> The only difference is that in the real class, I have even more
> mandatory attributes. Too many to specify at once on a constructor and
> they don't always exist at construction time anyway, so the accessor
> idiom must remain.
>
> Can anyone suggest a more Pythony way to do the following?
>
> Thanks,
> Tad
>
> #!/bin/env python
>
> class A:
> def __init__(self):
> self.mandatoryV ar1 = None
> self.mandatoryV ar2 = None
>
> def setMV1(self, mv1):
> self.mandatoryV ar1 = mv1
>
> def setMV2(self, mv2):
> self.mandatoryV ar2 = mv2
>
> def saveToDB(self, db):
> if self.mandatoryV ar1 == None:
> raise Exception, "Mandatory Var 1 not set."
> if self.mandatoryV ar2 == None:
> raise Exception, "Mandatory Var 2 not set."
>
> # Using db, do whatever saving stuff is needed.[/color]

What about this:

class A:
def __init__(self, mandatoryVar1, mandatoryVar2):
self.mandatoryV ar1 = mandatoryVar1
self.mandatoryV ar2 = mandatoryVar2

--
James Henderson, Logical Progression Ltd.

http://www.logicalprogression.net/

http://sourceforge.net/projects/mailmanager/

[Josiah Carlson]

Re: Need help with Python class idioms


On 21 Jan 2004 12:43:28 -0800
tad@tadland.net (Tad Marko) wrote:
[color=blue]
> Howdy!
>
> I'm trying to get my head around Python and I've come to realize that
> there are a lot of idioms in Python that are a bit different than in
> other languages. I have a class similar to what I've included below.
> The only difference is that in the real class, I have even more
> mandatory attributes. Too many to specify at once on a constructor and
> they don't always exist at construction time anyway, so the accessor
> idiom must remain.
>
> Can anyone suggest a more Pythony way to do the following?
>
> Thanks,
> Tad
>
> #!/bin/env python
>
> class A:
> def __init__(self):
> self.mandatoryV ar1 = None
> self.mandatoryV ar2 = None
>
> def setMV1(self, mv1):
> self.mandatoryV ar1 = mv1
>
> def setMV2(self, mv2):
> self.mandatoryV ar2 = mv2
>
> def saveToDB(self, db):
> if self.mandatoryV ar1 == None:
> raise Exception, "Mandatory Var 1 not set."
> if self.mandatoryV ar2 == None:
> raise Exception, "Mandatory Var 2 not set."
>
> # Using db, do whatever saving stuff is needed.[/color]


mandatoryvariab les = dict(mandatoryv ar1=mv1, mandatoryvar2=m v2...)
inst = A()
inst.__dict__.u pdate(mandatory variables)

That should work for you.
- Josiah

[James Henderson]

Re: Need help with Python class idioms

On Wednesday 21 January 2004 9:01 pm, Jonathan Daugherty wrote:[color=blue]
> # P.S. Since you're asking about idiom, it would be more Pythonic to write:
> #
> # if self.mandatoryV ar1 is None:
> #
> # using the identity test "is" rather than equality test "==", since there
> is # only one None.
>
> I'd write
>
> if not self.mandatoryV ar1:
>
> but the first method works, too.[/color]

Are you sure an empty sequence or zero aren't valid values? J.
--
James Henderson, Logical Progression Ltd.

http://www.logicalprogression.net/

http://sourceforge.net/projects/mailmanager/

[Jonathan Daugherty]

Re: Need help with Python class idioms

# P.S. Since you're asking about idiom, it would be more Pythonic to write:
#
# if self.mandatoryV ar1 is None:
#
# using the identity test "is" rather than equality test "==", since there is
# only one None.

I'd write

if not self.mandatoryV ar1:

but the first method works, too.

--

Jonathan Daugherty

http://www.cprogrammer.org

"It's a book about a Spanish guy called Manual, you should read it."
-- Dilbert

[Mike C. Fletcher]

Re: Need help with Python class idioms

Tad Marko wrote:
[color=blue]
>Howdy!
>
>I'm trying to get my head around Python and I've come to realize that
>there are a lot of idioms in Python that are a bit different than in
>other languages. I have a class similar to what I've included below.
>The only difference is that in the real class, I have even more
>mandatory attributes. Too many to specify at once on a constructor and
>they don't always exist at construction time anyway, so the accessor
>idiom must remain.
>
>Can anyone suggest a more Pythony way to do the following?
>
>[/color]
Note: shameless plug follows...

With basicproperty, it would look something like this:

class A( propertied.Prop ertied ):
mandatoryVar1 = common.IntegerP roperty(
"mandatoryVar1" , """Property that's mandatory 'cause we're cool""",
)
defaultVar1 = common.IntegerP roperty(
"defaultVar 1", """Property that's got a default value""",
defaultValue = 32,
)
defaultVar2 = common.StringPr operty(
"defaultVar 2", """Property that's got a default function instead""",
defaultFunction = lambda prop, client: client.__class_ _.__name__,
)

a = A( mandatoryVar1 = 3 )
a.defaultVar2
a.defaultVar1

however, you continue on to show you're using a database, so you'd
probably prefer something like pytable (which is built on top of
basicproperty, but uses database schemas to control the creation and
operation of the properties):

from pytable.schemab uilder import *
schema = table(
"my_table",
comment = """Table for storing whatevers""",
fields = (
field(
'mandatoryVar1' , 'integer',0, # name, sql data-type, size
constraints = (
notNull(),
),
),
field(
'defaultVal1', 'integer',0, # name, sql data-type, size
defaultValue = 32, # sql default value
constraints = (
notNull(),
),
),
field(
'Var2', 'timestamptz',0 , # name, sql data-type, size
defaultValue = '"timestamp"(\' now\'::text)', # sql default value
constraints = (
notNull(),
),
),
),
indices = (
index( primary=1, fields=('mandat oryVar1', 'defaultVal1')) ,
),
)

print schema.itemClas s
help( schema.itemClas s )

row = schema.itemClas s( mandatoryVar1 = 33 )
row.insertQuery ( my_db_connectio n ) # get one from pytable
row.mandatoryVa r1 = 34
row.updateQuery ( my_db_connectio n )
row.deleteQuery ( my_db_connectio n )

BasicProperty:

http://basicproperty.sourceforge.net/

PyTable RDBMS Wrapper:

http://pytable.sourceforge.net/

In a more general sense, for DB operations, use a database wrapper
module, there's lots of them around. And even more generally,
properties are *very* good at describing (common) property-based systems
of objects such as you're apparently working with. Properties are new
with Python 2.2, so underused sometimes, but they are very good at
certain types of domain modelling.

Enjoy yourself,
Mike

_______________ _______________ _________
Mike C. Fletcher
Designer, VR Plumber, Coder

http://members.rogers.com/mcfletch/

[James Henderson]

Re: Need help with Python class idioms

[Tad Marko][color=blue][color=green]
> > def saveToDB(self, db):
> > if self.mandatoryV ar1 == None:
> > raise Exception, "Mandatory Var 1 not set."
> > if self.mandatoryV ar2 == None:
> > raise Exception, "Mandatory Var 2 not set."[/color][/color]

P.S. Since you're asking about idiom, it would be more Pythonic to write:

if self.mandatoryV ar1 is None:

using the identity test "is" rather than equality test "==", since there is
only one None.

James
--
James Henderson, Logical Progression Ltd.

http://www.logicalprogression.net/

http://sourceforge.net/projects/mailmanager/

[Paul McGuire]

Re: Need help with Python class idioms

"Jonathan Daugherty" <cygnus@cprogra mmer.org> wrote in message
news:mailman.61 1.1074719029.12 720.python-list@python.org ...[color=blue]
> # P.S. Since you're asking about idiom, it would be more Pythonic to[/color]
write:[color=blue]
> #
> # if self.mandatoryV ar1 is None:
> #
> # using the identity test "is" rather than equality test "==", since there[/color]
is[color=blue]
> # only one None.
>
> I'd write
>
> if not self.mandatoryV ar1:
>
> but the first method works, too.
>
> --
>
> Jonathan Daugherty
> http://www.cprogrammer.org
>
> "It's a book about a Spanish guy called Manual, you should read it."
> -- Dilbert
>[/color]

.... what if mandatoryVar1 was set to False?

-- Paul

[Jonathan Daugherty]

Re: Need help with Python class idioms

# ... what if mandatoryVar1 was set to False?

To all who have responded to me as such: of course, how one
treats the variable in a boolean context depends on what its
semantically "false" value is. You do the math; I just
provided a syntax alternative. :)

--

Jonathan Daugherty

http://www.cprogrammer.org

"It's a book about a Spanish guy called Manual, you should read it."
-- Dilbert

[kbass]

Python and writing a CSV file

I am new to Python and would like to attempt to write to a CSV file. I have
reviewed the documentation at http://www.object-craft.com.au/projects/csv/
for the csv 1.0 module but I didn't see any information referring to writing
(oe creating) a CSV file with this module. Does this module have the
capability to write a CSV file? Thanks!

Kevin

[Skip Montanaro]

Re: Python and writing a CSV file


Kevin> I am new to Python and would like to attempt to write to a CSV
Kevin> file. I have reviewed the documentation at
Kevin> http://www.object-craft.com.au/projects/csv/ for the csv 1.0
Kevin> module but I didn't see any information referring to writing (oe
Kevin> creating) a CSV file with this module. Does this module have the
Kevin> capability to write a CSV file?

Yes, but you should upgrade to 2.3 (if you haven't already) and just use the
csv module which comes with it:

http://www.python.org/doc/current/lib/module-csv.html

Skip

[Mel Wilson]

Re: Need help with Python class idioms

In article <mailman.613.10 74720629.12720. python-list@python.org >,
James Henderson <james@logicalp rogression.net> wrote:[color=blue]
>On Wednesday 21 January 2004 9:01 pm, Jonathan Daugherty wrote:[color=green]
>> # P.S. Since you're asking about idiom, it would be more Pythonic to write:
>> #
>> # if self.mandatoryV ar1 is None:
>> #
>> # using the identity test "is" rather than equality test "==", since there
>> is # only one None.
>>
>> I'd write
>>
>> if not self.mandatoryV ar1:
>>
>> but the first method works, too.[/color]
>
>Are you sure an empty sequence or zero aren't valid values? J.[/color]

It can get weird if you have to allow your caller to
optionally specify the value None:


class _MissingParam:
"An object that no-one else will have a valid use for."
....
def a_func (opt1=_MissingP aram):
if opt1 is _MissingParam:
opt1 = default_calcula tion () # for instance
....


Regards. Mel.

[James Henderson]

Re: Need help with Python class idioms

On Thursday 22 January 2004 6:05 pm, Mel Wilson wrote:[color=blue]
> In article <mailman.613.10 74720629.12720. python-list@python.org >,
>
> James Henderson <james@logicalp rogression.net> wrote:[color=green]
> >On Wednesday 21 January 2004 9:01 pm, Jonathan Daugherty wrote:[color=darkred]
> >> # P.S. Since you're asking about idiom, it would be more Pythonic to
> >> write: #
> >> # if self.mandatoryV ar1 is None:
> >> #
> >> # using the identity test "is" rather than equality test "==", since
> >> there is # only one None.
> >>
> >> I'd write
> >>
> >> if not self.mandatoryV ar1:
> >>
> >> but the first method works, too.[/color]
> >
> >Are you sure an empty sequence or zero aren't valid values? J.[/color]
>
> It can get weird if you have to allow your caller to
> optionally specify the value None:
>
>
> class _MissingParam:
> "An object that no-one else will have a valid use for."
> ...
> def a_func (opt1=_MissingP aram):
> if opt1 is _MissingParam:
> opt1 = default_calcula tion () # for instance
> ...
>
>
> Regards. Mel.[/color]

In the original example (not by me) it was None that was being used as the
marker that the value had not been set - that's all.

The topic was Python idiom and None is the most often used marker that a value
has not been passed explicitly and the default value should be used. (Of
course, if the default value is immutable it can be used directly.) See:

http://www.python.org/doc/current/ref/function.html

You're talking about the case where None may be passed explicitly and should
not trigger the default. An even terser solution to this than yours is to
use an empty list, though any mutable type will do. You see this all over
the Zope code:

_marker = []

J.
--
James Henderson, Logical Progression Ltd.

http://www.logicalprogression.net/

http://sourceforge.net/projects/mailmanager/

[kbass]

Re: Python and writing a CSV file


----- Original Message -----
From: "Skip Montanaro" <skip@pobox.com >
To: "kbass" <kbass@midsouth .rr.com>
Cc: "Python List" <python-list@python.org >
Sent: Wednesday, January 21, 2004 6:39 PM
Subject: Re: Python and writing a CSV file

[color=blue]
>
> Kevin> I am new to Python and would like to attempt to write to a CSV
> Kevin> file. I have reviewed the documentation at
> Kevin> http://www.object-craft.com.au/projects/csv/ for the csv 1.0
> Kevin> module but I didn't see any information referring to writing[/color]
(oe[color=blue]
> Kevin> creating) a CSV file with this module. Does this module have[/color]
the[color=blue]
> Kevin> capability to write a CSV file?
>
> Yes, but you should upgrade to 2.3 (if you haven't already) and just use[/color]
the[color=blue]
> csv module which comes with it:
>
> http://www.python.org/doc/current/lib/module-csv.html
>
> Skip[/color]

Thanks for responding. Currently we do not have Python 2.3 installed on our
server, is there another way to perform this task with another module or
without a module? Thanks!

Kevin