Re: Puzzling difference between lists and tuples
On 20Sep2020 20:33, Avi Gross wrote:
>('M','R','A','B') is correct. I appreciate the correction. I did not look to
>see the content of what I created, just the type!
>
a = tuple("first")
a
>('f', 'i', 'r', 's', 't')
type(a)
>
>
>But I thought adding a comma would help and it does not!
Ah, the comma is for an expression. But you made a function call (well,
"tuple" is a type and types are callable to make instances).
In a function call the parmeter separating commas have higher precedence
than the commas with defines a tuple, you you've made a function call
with one argument "first", not a function call with one _tuple_ argument
("first",).
Consider:
foo(1, 2)
That supplies 2 arguments, not a single tuple.
As with other situations where the default precedence groups things in a
different way from your intent, brackets get required here if you want
to express a tuple.
This:
foo( (1,2) )
isn't notionally different from needing brakcets to express this:
(3+5) * 8
which means something else without its brackets.
Also, function calls allow you to include a trailing comma because it
helps with code. Consider the following bigger call:
x = foo(
1,
2,
fred=5,
)
laid out on separate lines for readability (a little redundant here, but
some complex function calls, or those exceeding the deired line length,
are often folded this way).
Bu allowing a trailing comma we get consistent formatting and nicer
diffs. If a trailing comma were forbidden, the dopping "fred=5" with
produce a diff removing not just that line but also the comma on the
preceeding line. Ugly and noisy.
Cheers,
Cameron Simpson
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RE: Puzzling difference between lists and tuples
('M','R','A','B') is correct. I appreciate the correction. I did not look to
see the content of what I created, just the type!
>>> a = tuple("first")
>>> a
('f', 'i', 'r', 's', 't')
>>> type(a)
But I thought adding a comma would help and it does not!
>>> b = tuple("first",)
>>> b
('f', 'i', 'r', 's', 't')
Yet something this simple without invoking tuple(), works!
>>> c = 'first',
>>> c
('first',)
So I read the manual page and tuple takes an iterable as an argument and
treats a string as an iterator on characters! It is not a simple
initializer.
I got around it, sort of, using n array with a single object of type string
in it so the iterator is iterating at a different level.
>>> d = ["first"]
>>> tuple(d)
('first',)
>>> tuple(["first"])
('first',)
I understand the design choice and can imagine there may be another function
that initializes a tuple more directly in some module.
Returning to lurking mode ...
-Original Message-
From: Python-list On
Behalf Of MRAB
Sent: Sunday, September 20, 2020 7:35 PM
To: python-list@python.org
Subject: Re: Puzzling difference between lists and tuples
On 2020-09-20 23:59, Avi Gross via Python-list wrote:
> There is a simple and obvious way to make sure you have a tuple by
invoking the keyword/function in making it:
>
a=('first')
type(a)
>
>
a=("first",)
type(a)
>
>
a=tuple("first")
type(a)
>
>
> That seems more explicit than adding a trailing comma. It also is a simple
way to make an empty tuple but is there any penalty for using the function
tuple()?
>
[snip]
>>> tuple("first")
('f', 'i', 'r', 's', 't')
Not the same as ("first",).
A simpler way to make an empty tuple is just ().
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Re: Puzzling difference between lists and tuples
On 21/09/20 10:59 am, Avi Gross wrote:
a=tuple("first")
type(a)
That seems more explicit than adding a trailing comma.
It doesn't do what you want, though:
>>> a = tuple("first")
>>> print(a)
('f', 'i', 'r', 's', 't')
If you really want to use tuple() to create a 1-tuple without
using a trailing comma, you can do it this way:
>>> a = tuple(["first"])
>>> print(a)
('first',)
But this costs you both a list creation and a tuple() call.
On my machine it seems to be about 17 times slower than using
a trailing comma:
>>> timeit.repeat("tuple(['first'])")
[0.1774688908953, 0.1768788059062, 0.1768771102082,
0.176763284033, 0.1768448921963]
>>> timeit.repeat("('first',)")
[0.0117392889055, 0.01156933400708, 0.01158800017473,
0.01156976132486, 0.01157938358281]
--
Greg
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Re: Puzzling difference between lists and tuples
On 2020-09-20 23:59, Avi Gross via Python-list wrote:
There is a simple and obvious way to make sure you have a tuple by invoking the
keyword/function in making it:
a=('first')
type(a)
a=("first",)
type(a)
a=tuple("first")
type(a)
That seems more explicit than adding a trailing comma. It also is a simple way
to make an empty tuple but is there any penalty for using the function tuple()?
[snip]
>>> tuple("first")
('f', 'i', 'r', 's', 't')
Not the same as ("first",).
A simpler way to make an empty tuple is just ().
--
https://mail.python.org/mailman/listinfo/python-list
RE: Puzzling difference between lists and tuples
There is a simple and obvious way to make sure you have a tuple by invoking the
keyword/function in making it:
>>> a=('first')
>>> type(a)
>>> a=("first",)
>>> type(a)
>>> a=tuple("first")
>>> type(a)
That seems more explicit than adding a trailing comma. It also is a simple way
to make an empty tuple but is there any penalty for using the function tuple()?
-Original Message-
From: Python-list On
Behalf Of "???"
Sent: Saturday, September 19, 2020 11:39 PM
To: python-list@python.org
Subject: Re: Puzzling difference between lists and tuples
William Pearson writes:
> ...
> for n in ('first'):
> print n
>
>
> ... but "f","i","r","s","t" in the second.
#+BEGIN_SRC: python
for n in ('first',):
print n
#+BEGIN_SRC
Then, that will print 'first'. And please use Python3...
Sincerely, Byung-Hee
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^고맙습니다 _救濟蒼生_ 감사합니다_^))//
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Re: importlib changes from py3.7 to py3.8
Hello Greg, thank you for your answer. I have checked it twice and the file is really empty. I even opened it with a hex editor - no content. BUT in order to create a bug ticket I tried to make a mcve and I didn't succeed. I copied the function (load_plugins) 1 to 1, but outside my program it works as it should. So it seems that something inside my application is causing this problem. Hmm ... Fortunately this is a pure python application so debugging should lead to enlightenment. :-) Thx Eren Am So., 20. Sept. 2020 um 02:06 Uhr schrieb Greg Ewing < greg.ew...@canterbury.ac.nz>: > On 19/09/20 7:51 am, Eko palypse wrote: > > ValueError: source code string cannot contain null bytes > > > > Any idea what happened here? > > > Seems I've missed that __init__.py's aren't allowed to be empty anymore. > > Adding a single # solved the issue. > > I just tried importing a package with an empty __init__.py in > Python 3.8 and there was no problem: > > % ls -l empty > total 0 > -rw-r--r-- 1 greg staff 0 20 Sep 12:00 __init__.py > % python3 > Python 3.8.2 (default, Mar 23 2020, 11:36:18) > [Clang 8.1.0 (clang-802.0.42)] on darwin > Type "help", "copyright", "credits" or "license" for more information. > >>> import empty > >>> > > It looks more like the file in question wasn't completely empty, > but had a null character in it for some reason. I expect that if > you re-saved it as truly empty it would work. If not, you should > probably report it as a bug. > > -- > Greg > -- > https://mail.python.org/mailman/listinfo/python-list > -- https://mail.python.org/mailman/listinfo/python-list
