Forward References
I am attempting to use forward references in my program and I am failing. This also does not work with the older way of putting the name of a class as a string. Here is some sample code: from __future__ import annotations from dataclasses import dataclass from typing import TypeAlias ColorDef: TypeAlias = RGB | int | str @dataclass(frozen=True, slots=True) class RGB(object): Can anyone suggest how I should fix this without reversing the statement order? pass -- Jonathan Gossage -- https://mail.python.org/mailman/listinfo/python-list
Using generator expressions
I am having a problem using generator expressions to supply the arguments
for a class instance initialization. The following example shows the
problem:
class test1(object):
def __init__(self, a, b):
> self.name = a
self.value = b
st = 'Programming Renaissance, Any'.split(', ')
y = test1(a for a in st)
print(f'Object values are: {y._a}, {y._b}')
I would expect to get the values from the list generated by splitting the
string passed in as arguments to the new instance of test1, but instead
I get the generator expression by itself as a generator object. The
generator
expression is treated like a passive object instead of being run. If I had
wanted to pass the generator expression itself, I would have expected to
have
to use parentheses around the generator expression. Any suggestions on how
to
get the generator expression to run?
If I change the definition of the input arguments to *args I can capture the
arguments within __init__ but it is verbose and ugly. Also, I could accept
the
arguments from a Sequence and extract the Sequence members into the class
values. I would prefer my solution if I could get it to work.
Note that I tried generator expressions both inside parentheses and not,
without success.
--
Jonathan Gossage
--
https://mail.python.org/mailman/listinfo/python-list
Re: Using generator expressions
Many thanks, all. It turned out that my problem was not fully understanding
the use and power of the unpack operator *. Using it to activate my
generator made things start to work. I changed the line where I invoked the
generator to:
y = test1(*(a for a in st))
adding the unpack operator.
On Mon, Sep 25, 2023 at 11:15 AM Thomas Passin via Python-list <
python-list@python.org> wrote:
> On 9/25/2023 10:15 AM, Jonathan Gossage via Python-list wrote:
> > I am having a problem using generator expressions to supply the arguments
> > for a class instance initialization. The following example shows the
> > problem:
> >
> > class test1(object):
> > def __init__(self, a, b):
> >
> >> self.name = a
> >
> > self.value = b
> > st = 'Programming Renaissance, Any'.split(', ')
> > y = test1(a for a in st)
> > print(f'Object values are: {y._a}, {y._b}')
> >
> > I would expect to get the values from the list generated by splitting the
> > string passed in as arguments to the new instance of test1, but instead
> > I get the generator expression by itself as a generator object. The
> > generator
> > expression is treated like a passive object instead of being run. If I
> had
> > wanted to pass the generator expression itself, I would have expected to
> > have
> > to use parentheses around the generator expression. Any suggestions on
> how
> > to
> > get the generator expression to run?
> > If I change the definition of the input arguments to *args I can capture
> the
> > arguments within __init__ but it is verbose and ugly. Also, I could
> accept
> > the
> > arguments from a Sequence and extract the Sequence members into the class
> > values. I would prefer my solution if I could get it to work.
> > Note that I tried generator expressions both inside parentheses and not,
> > without success.
> >
>
> You should get an error at the y assignment. The argument of test1() is
> a generator, which would get assigned to the "a" argument, and there
> would be no "b" argument, which is an error.
>
> In any event, even if this were to work as you want, it would only work
> for strings that contain one comma. And you ask for values like y._a,
> but y._a is never created, only y.a. If you did convert the generator
> to a list, and if you fix the underscored variable names, it still
> wouldn't work because the arguments don't expect a list.
>
> Time to step back and figure out exactly what you actually want to do.
>
> --
> https://mail.python.org/mailman/listinfo/python-list
>
--
Jonathan Gossage
--
https://mail.python.org/mailman/listinfo/python-list
Using __new__
I am attempting to use the __new__ method in the following code:
class SingletonExample(object):
_instance = None
def __new__(cls, **kwargs):
if cls._instance is None:
cls._instance = super().__new__(cls, **kwargs)
return cls._instance
def __init__(self, **kwargs) -> None:
our_attributes = ('h', 'x')
if kwargs is not None:
for k, v in kwargs.items():
if k in our_attributes:
setattr(self, k, v)
a = SingletonExample(h=1)
and I get the following result:
(PRV) jonathan@jfgdev:/PR$ python -m Library.Testing.test2
Traceback (most recent call last):
File "", line 198, in _run_module_as_main
File "", line 88, in _run_code
File "/mnt/ProgrammingRenaissance/Library/Testing/test2.py", line 16, in
a = SingletonExample(h=1)
^
File "/mnt/ProgrammingRenaissance/Library/Testing/test2.py", line 6, in
__new__
cls._instance = super().__new__(cls, **kwargs)
^^
TypeError: object.__new__() takes exactly one argument (the type to
instantiate)
I am quite puzzled as it looks as if this code will not work if the
super-class is 'object'. Any suggestions on how to proceed?
--
Jonathan Gossage
--
https://mail.python.org/mailman/listinfo/python-list
Pip installs to unexpected place
I am using *Python 3.13* in a virtual environment under *Ubuntu Linux 24.04* . The version of Python was compiled from source code and installed with make altinstall. I attempted to use *pip* to install the *Sphinx* package into the virtual environment using the command *pip install sphinx* in the virtual environment*.* I expected that *sphinx* would be installed in the *site-packages* directory in the virtual environment. Instead, it was installed into the site-packages directory in */home/jonathan/.locals/lib/python3.13/site-packages* even though I did not specify *--user* to the *pip install* command. Is this expected behavior? I wanted Sphinx to be installed in the virtual environment so that it would be accessible to all users of the virtual environment. -- Jonathan Gossage -- https://mail.python.org/mailman/listinfo/python-list
Dynamic classes
I have created a dynamic class using the type() function:
x = type('MyFlags', (), {'Flag1': 1, 'Flag2': 2, 'Flag3: 4, ' '__init__' :
__init__})
The new class is there, and the class variables, Flag1, Flag2, and Flag3,
are present correctly. However, when I try to create an instance of this
class with the following code:
y = x('Flag1', 'Flag2')
it fails with a TypeError stating that 'MyFlags' does not accept arguments.
What do I have to do to make this happen?. BTW __init__(self, *args) is
defined as the instance initializer.
--
Jonathan Gossage
--
https://mail.python.org/mailman/listinfo/python-list
