learning python ...

[Stestagg]

On Sun, 23 May 2021 at 20:37, hw <hw at adminart.net> wrote:

> On 5/23/21 7:28 PM, Peter Otten wrote:
> > On 23/05/2021 06:37, hw wrote:
> >>
> >> Hi,
> >>
> >> I'm starting to learn python and have made a little example program
> >> following a tutorial[1] I'm attaching.
> >>
> >> Running it, I'm getting:
> >>
> >>
> >> Traceback (most recent call last):
> >>    File "[...]/hworld.py", line 18, in <module>
> >>      print(isinstance(int, float))
> >> TypeError: isinstance() arg 2 must be a type or tuple of types
> >>
> >>
> >> I would understand to get an error message in line 5 but not in 18.
> >> Is this a bug or a feature?
> >
> > It is a bug in your code (which you don't provide). Did you assign some
> > value to float, e. g.:
> >
> >  >>> float = 42.0
> >  >>> isinstance(int, float)
> > Traceback (most recent call last):
> >    File "<pyshell#313>", line 1, in <module>
> >      isinstance(int, float)
> > TypeError: isinstance() arg 2 must be a type or tuple of types
> >
> > If you do not shadow the built-in you should get
> >
> >  >>> isinstance(int, float)
> > False
> >
>
> Apparently the attachment was stripped from my message.  I'll put a
> smaller version directly into this message instead of an attachment:
>
>
> #!/usr/bin/python
>
> print("world!")
>
> int = 17
> print("world", int)
>
> float = 6.670
> print("world", float)
>
> foo = 0
> print(type(int))
> print(type(float))
> print(type(foo))
>
> print(isinstance(foo, str))
> print(isinstance(int, float))
> print(isinstance(float, float))
>
>
> I don't know about shadowing.


Shadowing is effectively saying “within this bit of code, (scope) I’m going
to use an already-used name for my own value”

If I have defeated a whole variable type
> by naming a variable like a variable type, I would think it is a bad
> idea for python to allow this without warning.


There are some reasons why allowing this is quite nice. And there’s
actually a ton of corner cases to consider when thinking about changing the
rules

Interestingly python 3 made this a little bit better by stopping you from
rebinding (shadowing) a number of built ins, such as True and False.

In your case, I agree that it is super confusing. One thing to learn to
look out for is if you assign to a name, then use that name on a different
context, expecting it to be different, then that’s not likely to work as
you expect.

It seems like a recipie
> for creating chaos.


Luckily almost every python code checker and/or linter will highlight this
for you.

If you’re learning python, I’d highly recommend doing so in an ide or
editor that has a code checker running.


> --



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