First Ever FlaskCon Tickets Sales

[Abdur-Rahmaan Janhangeer]

Greetings list,

I count my events-related posts to the mailing list.

But i'd like to make an exception for FlaskCon this year.

As the first ever tickets sales is ongoing:
https://ti.to/flaskcon/flaskcon-2021

Python fans, please forward / tweet the message.

Any questions, ping me!

Kind Regards,

Abdur-Rahmaan Janhangeer
about  | blog

github 
Mauritius
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Re: Recursion on list

[Pieter van Oostrum]

ast  writes:

>> li = []
>> li.append(li)
>> li
> [[...]]
>
>>li[0][0][0][0]
> [[...]]
>
> That's funny
>

You made a list whose only element is itself.

In [1]: li = []

In [3]: li.append(li)

In [4]: li[0] is li
Out[4]: True

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Recursion on list

[ast]

> li = []
> li.append(li)
> li
[[...]]

>li[0][0][0][0]
[[...]]

That's funny
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Syntax not understood

[ast]

Hello

In this function

def get4(srcpages):
scale = 0.5
srcpages = PageMerge() + srcpages
x_increment, y_increment = (scale * i for i in srcpages.xobj_box[2:])
for i, page in enumerate(srcpages):
page.scale(scale)
page.x = x_increment if i & 1 else 0
page.y = 0 if i & 2 else y_increment
return srcpages.render()

found here

https://www.blog.pythonlibrary.org/2018/06/06/creating-and-manipulating-pdfs-with-pdfrw/

I do not understand this line:

x_increment, y_increment = (scale * i for i in srcpages.xobj_box[2:])

(scale * i for i in srcpages.xobj_box[2:]) is a generator, a single
object, it should not be possible to unpack it into 2 variables.

x, y = 1 generates an error
x, y = (i for i in range(10)) too

but not

x_increment, y_increment = (scale * i for i in srcpages.xobj_box[2:])

why ?
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Re: Recursion on list

[Peter Pearson]

On Thu, 4 Nov 2021 08:57:14 +0100, ast  wrote:
> > li = []
> > li.append(li)
> > li
> [[...]]
>
> >li[0][0][0][0]
> [[...]]
>
> That's funny

After the coming AI upheaval, such cruelty to machines will
be considered punishable and not funny.


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Re: Syntax not understood

[ast]

Le 04/11/2021 à 16:41, Stefan Ram a écrit :

ast  writes:

(scale * i for i in srcpages.xobj_box[2:]) is a generator, a single
object, it should not be possible to unpack it into 2 variables.


   But the value of the right-hand side /always/ is a single object!

   A syntax of an assignment statement that has been simplified
   by me but is sufficient for this post is:

target list = source expression

   . The evaluation of the source expression yields an object.

   If the target list is not a single target, that object must
   be an iterable with the same number of items as there are targets
   in the target list, and the items are assigned, from left to
   right, to the corresponding targets.

   A generator object /is/ an iterable, and, here, it apparently
   yields exactly two items.




understood

It is like:

x, y = (i for i in range(2))

thx
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Re: Syntax not understood

[David Lowry-Duda]

> x_increment, y_increment = (scale * i for i in srcpages.xobj_box[2:])
> 
> (scale * i for i in srcpages.xobj_box[2:]) is a generator, a single
> object, it should not be possible to unpack it into 2 variables.

If you know the exact number of values in the generator, you can do 
this. Here is an oversimplified example.

  l = [0, 1, 2, 3, 4, 5]
  a, b = (elem * 10 for elem in l[:4])
  print(a, b) # prints 40 50

This is very fragile code and I would recommend against using it.

- DLD
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Re: Syntax not understood

[2QdxY4RzWzUUiLuE]

On 2021-11-04 at 14:36:48 -0400,
David Lowry-Duda  wrote:

> > x_increment, y_increment = (scale * i for i in srcpages.xobj_box[2:])
> > 
> > (scale * i for i in srcpages.xobj_box[2:]) is a generator, a single
> > object, it should not be possible to unpack it into 2 variables.
> 
> If you know the exact number of values in the generator, you can do 
> this. Here is an oversimplified example.
> 
>   l = [0, 1, 2, 3, 4, 5]
>   a, b = (elem * 10 for elem in l[:4])
>   print(a, b) # prints 40 50
> 
> This is very fragile code and I would recommend against using it.

How is that any more fragile than any other operation that destructs (or
doesn't) a tuple?

Is the following fragile:

quotient, remainder = divmod(numerator, denominator)

Would you surround it with try/except, or always unpack it defensively?

result = divmod(numerator, denominator)
if len(result) == 2:
quotient, remainder = result
else:
raise SomeException("divmod didn't work")

If I know where a generator (or an iterator, or a tuple, or a list) came
from, and it's documented to contain/yield a known quantity of values,
then why is it fragile to depend on that?

On the other hand, yes, if I build code that depends on a lot of
unstated relationships between values of questionable (or unknown)
origin, then I am definitely asking for trouble.
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Re: Syntax not understood

[Chris Angelico]

On Fri, Nov 5, 2021 at 6:23 AM <2qdxy4rzwzuui...@potatochowder.com> wrote:
>
> On 2021-11-04 at 14:36:48 -0400,
> David Lowry-Duda  wrote:
>
> > > x_increment, y_increment = (scale * i for i in srcpages.xobj_box[2:])
> > >
> > > (scale * i for i in srcpages.xobj_box[2:]) is a generator, a single
> > > object, it should not be possible to unpack it into 2 variables.
> >
> > If you know the exact number of values in the generator, you can do
> > this. Here is an oversimplified example.
> >
> >   l = [0, 1, 2, 3, 4, 5]
> >   a, b = (elem * 10 for elem in l[:4])
> >   print(a, b) # prints 40 50
> >
> > This is very fragile code and I would recommend against using it.
>
> How is that any more fragile than any other operation that destructs (or
> doesn't) a tuple?
>

The only part that's fragile, in my opinion, is (from the initial
post) that it's using removal syntax to slice off some, which is
fragile against the original input length:

x_increment, y_increment = (scale * i for i in srcpages.xobj_box[2:])

But since it says "box", I would start by assuming that it has four
elements. (They might be x1,y1,x2,y2 or x,y,w,h but it'll almost
always be four.) So it's not TOO fragile, when working with boxes, but
it is a little less clear that thing[2:] will always give exactly two
results.

ChrisA
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Re: Syntax not understood

[2QdxY4RzWzUUiLuE]

On 2021-11-05 at 06:28:34 +1100,
Chris Angelico  wrote:

> On Fri, Nov 5, 2021 at 6:23 AM <2qdxy4rzwzuui...@potatochowder.com> wrote:
> >
> > On 2021-11-04 at 14:36:48 -0400,
> > David Lowry-Duda  wrote:
> >
> > > > x_increment, y_increment = (scale * i for i in srcpages.xobj_box[2:])
> > > >
> > > > (scale * i for i in srcpages.xobj_box[2:]) is a generator, a single
> > > > object, it should not be possible to unpack it into 2 variables.
> > >
> > > If you know the exact number of values in the generator, you can do
> > > this. Here is an oversimplified example.
> > >
> > >   l = [0, 1, 2, 3, 4, 5]
> > >   a, b = (elem * 10 for elem in l[:4])
> > >   print(a, b) # prints 40 50
> > >
> > > This is very fragile code and I would recommend against using it.
> >
> > How is that any more fragile than any other operation that destructs (or
> > doesn't) a tuple?
> >
> 
> The only part that's fragile, in my opinion, is (from the initial
> post) that it's using removal syntax to slice off some, which is
> fragile against the original input length:
> 
> x_increment, y_increment = (scale * i for i in srcpages.xobj_box[2:])
> 
> But since it says "box", I would start by assuming that it has four
> elements. (They might be x1,y1,x2,y2 or x,y,w,h but it'll almost
> always be four.) So it's not TOO fragile, when working with boxes, but
> it is a little less clear that thing[2:] will always give exactly two
> results.

I won't disagree that most boxes are going to be described by four
elements, in which case slicing off the first two *should* leave two
more (presumably x_increment and y_increment).

So if you're saying that all that's missing is a description of
srcpages.xobj_box, then I agree.  :-)  But such is life on a mailing
list where posters don't always give their questions the context they
deserve.
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Re: Syntax not understood

[Chris Angelico]

On Fri, Nov 5, 2021 at 6:45 AM <2qdxy4rzwzuui...@potatochowder.com> wrote:
> > But since it says "box", I would start by assuming that it has four
> > elements. (They might be x1,y1,x2,y2 or x,y,w,h but it'll almost
> > always be four.) So it's not TOO fragile, when working with boxes, but
> > it is a little less clear that thing[2:] will always give exactly two
> > results.
>
> I won't disagree that most boxes are going to be described by four
> elements, in which case slicing off the first two *should* leave two
> more (presumably x_increment and y_increment).

Exactly. To an experienced Python programmer, it's obvious that that
line of code is scaling the dimensions and assigning them. But if you
aren't comfortable with that, then it isn't so obvious.

Perhaps it'd be better to spell it as [-2:], which will clearly give
two results regardless of the length of the input (unless it's too
short). But that's minor IMO.

> So if you're saying that all that's missing is a description of
> srcpages.xobj_box, then I agree.  :-)  But such is life on a mailing
> list where posters don't always give their questions the context they
> deserve.

Indeed :)

ChrisA
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