Re: [Tutor] How to create an object in database only if the object is not already there?

[Albert-Jan Roskam]

From: Tutor  on behalf of GMX 

Sent: Monday, September 11, 2017 9:38 AM
To: tutor@python.org
Subject: [Tutor] How to create an object in database only if the object is not 
already there?



Now when I create a Course object like this:

    >>> Course(title=‘A new course’)

An object is create in the database. I don’t want to have this behaviour, but 
what I want to be doing is create a Course object
and then only commit in the database if the course is not already available in 
the database. 


===> I have no experience with Pony, but with Sqlalchemy one could do something 
like:

if not session.query(Course.title).filter_by(title=‘A new course’).first():
course = Course(title=‘A new course’)

Or perhaps you could set a primary key constraint on course title.
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Re: [Tutor] question about calendar module in standard libriary

[Peter Otten]

Айнур Зулькарнаев wrote:

> Hello all!
> 
> 
> There is a class Calendar in calendar.py in standard libriary.
> 
> 
> class Calendar(object):
> """
> Base calendar class. This class doesn't do any formatting. It
> simply provides data to subclasses.
> """
> 
> def __init__(self, firstweekday=0):
> self.firstweekday = firstweekday # 0 = Monday, 6 = Sunday
> 
> def getfirstweekday(self):
> return self._firstweekday % 7
> 
> def setfirstweekday(self, firstweekday):
> self._firstweekday = firstweekday
> 
> firstweekday = property(getfirstweekday, setfirstweekday)
> 
> 
> As far as I understand, even if user enters inappropriate firstweekday
> parameter (bigger than 6) during instansiation of the Calendar, the
> Calendar swallows it (and latter returns correct firstweekday value due to
> %7 in getfirstweekday method).
> 
> 
> So, the question is why not explicitly raise ValueError if user enters the
> firstweekday parameter bigger that 6 (with accordance with the Zen). Am I
> missing something?

It does no harm to those who use the class properly while it allows those 
unfamiliar with the idea of a 0th day to specify 7 instead 0. The behaviour 
thus may be interpreted as an example of

"""Be strict when sending and tolerant when receiving."""

See also

https://tools.ietf.org/html/rfc1958
https://en.wikipedia.org/wiki/Robustness_principle

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Re: [Tutor] question about calendar module in standard libriary

[Steven D'Aprano]

On Mon, Sep 11, 2017 at 10:58:51AM +, Айнур Зулькарнаев wrote:

> class Calendar(object):
> def __init__(self, firstweekday=0):
> self.firstweekday = firstweekday # 0 = Monday, 6 = Sunday
> 
> def getfirstweekday(self):
> return self._firstweekday % 7
> 
> def setfirstweekday(self, firstweekday):
> self._firstweekday = firstweekday
> 
> firstweekday = property(getfirstweekday, setfirstweekday)
>
> As far as I understand, even if user enters inappropriate firstweekday 
> parameter (bigger than 6) during instansiation of the Calendar, the 
> Calendar swallows it (and latter returns correct firstweekday value 
> due to %7 in getfirstweekday method).

That looks right to me.

> So, the question is why not explicitly raise ValueError if user enters 
> the firstweekday parameter bigger that 6 (with accordance with the 
> Zen). Am I missing something?

I don't think there is any specific reason. Probably just the personal 
choice of the person who wrote the code. The decision doesn't appear to 
be documented anywhere, so I don't think its official behaviour.


-- 
Steve
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Re: [Tutor] question about calendar module in standard libriary

[Alan Gauld via Tutor]

On 11/09/17 11:58, Айнур Зулькарнаев wrote:

> So, the question is why not explicitly raise ValueError if 
> user enters the firstweekday parameter bigger that 6 

Its a valid question but you probably need to find the original PEP
document to find the answer and that module has been around for
a *very* long time!

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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Re: [Tutor] How to create an object in database only if the object is not already there?

[Alan Gauld via Tutor]

On 11/09/17 10:38, GMX wrote:
> I am using `pony` orm to write a simple class as my model.

pony is not a standard module (indeed I only heard about it
from your email) so you are probably best asking on the
pony support forum if such exists or via the author.


> from pony.orm import Database
> from pony.orm import Required, Optional
> from pony.orm import db_session
> from pony.orm import select, commit
> 
> DB_PATH = ‘db.sqlite’
> 
> db = Database()
> 
> class Course(db.Entity):
> """
> A class to represent a course
> """
> title = Required(str, unique=True)
> url = Optional(str, unique=True)
> thumbnail = Optional(str)
> processed = Optional(bool, default=False)
> 
> db.bind(provider='sqlite', filename=DB_PATH, create_db=True)
> db.generate_mapping(create_tables=True)
> ```
> 
> Now when I create a Course object like this:
> 
> >>> Course(title=‘A new course’)
> 
> An object is create in the database. I don’t want to have this behaviour, 

But that's kind of what an ORM does. The whole point of using
an ORM is that it translates your objects into database
transactions.

> but what I want to be doing is create a Course object
> and then only commit in the database if the course is > not already available 
> in the database.

Thats very specific to your ORM. There probably are options
to do that, but you will need to ask the pony users.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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[Tutor] question about calendar module in standard libriary

[Айнур Зулькарнаев]

Hello all!


There is a class Calendar in calendar.py in standard libriary.


class Calendar(object):
"""
Base calendar class. This class doesn't do any formatting. It simply
provides data to subclasses.
"""

def __init__(self, firstweekday=0):
self.firstweekday = firstweekday # 0 = Monday, 6 = Sunday

def getfirstweekday(self):
return self._firstweekday % 7

def setfirstweekday(self, firstweekday):
self._firstweekday = firstweekday

firstweekday = property(getfirstweekday, setfirstweekday)


As far as I understand, even if user enters inappropriate firstweekday 
parameter (bigger than 6) during instansiation of the Calendar, the Calendar 
swallows it (and latter returns correct firstweekday value due to %7 in 
getfirstweekday method).


So, the question is why not explicitly raise ValueError if user enters the 
firstweekday parameter bigger that 6 (with accordance with the Zen). Am I 
missing something?


Thank you in advance


Best regards, Aynur



С уважением, А.И. Зулькарнаев

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[Tutor] How to create an object in database only if the object is not already there?

[GMX]

Hi, 

I am using `pony` orm to write a simple class as my model. Here is the class. 

```
from pony.orm import Database
from pony.orm import Required, Optional
from pony.orm import db_session
from pony.orm import select, commit

DB_PATH = ‘db.sqlite’

db = Database()

class Course(db.Entity):
    """
    A class to represent a course
    """
    title = Required(str, unique=True)
    url = Optional(str, unique=True)
    thumbnail = Optional(str)
    processed = Optional(bool, default=False)

db.bind(provider='sqlite', filename=DB_PATH, create_db=True)
db.generate_mapping(create_tables=True)
```

Now when I create a Course object like this:

    >>> Course(title=‘A new course’)

An object is create in the database. I don’t want to have this behaviour, but 
what I want to be doing is create a Course object
and then only commit in the database if the course is not already available in 
the database. If the course is already in the database, 
the orm should just return the same object. I have implemented the requirement 
as follows:

```
class Course(db.Entity):
    """
    A class to represent a course
    """
    title = Required(str, unique=True)
    url = Optional(str, unique=True)
    thumbnail = Optional(str)
    processed = Optional(bool, default=False)

    @staticmethod
    @db_session
    def new(title, url=None, thumbnail=None, processed=False):
        """Return a Course either new or from database"""
        course = select(c for c in Course if c.title == title)[:]
        if course:
            return course[0]
        return Course(title=title, url=url, thumbnail=thumbnail, 
processed=processed)

db.bind(provider='sqlite', filename=DB_PATH, create_db=True)
db.generate_mapping(create_tables=True)
```

Can there be a better method of doing that? Please let me know. 

Thanks. 

“ You are not born knowing everything.
You go on learning”  

- Anubhav Yadav
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