|  | Should Python raise a warning for mutable default arguments? | | |
| | | Steven D'Aprano |  |
|  Posted: Fri Aug 22, 2008 12:42 pm Post subject: Should Python raise a warning for mutable default arguments? |  |
Sometimes it seems that barely a day goes by without some newbie, or not-
so-newbie, getting confused by the behaviour of functions with mutable
default arguments. No sooner does one thread finally, and painfully, fade
away than another one starts up.
I suggest that Python should raise warnings.RuntimeWarning (or similar?)
when a function is defined with a default argument consisting of a list,
dict or set. (This is not meant as an exhaustive list of all possible
mutable types, but as the most common ones that I expect will trip up
newbies.) The warning should refer to the relevant FAQ or section in the
docs.
What do people think?
--
Steven |
| |
| | | Diez B. Roggisch | |
| | Posted: Fri Aug 22, 2008 12:49 pm Post subject: Re: Should Python raise a warning for mutable default argume | |
Steven D'Aprano schrieb:
| Quote: | Sometimes it seems that barely a day goes by without some newbie, or not-
so-newbie, getting confused by the behaviour of functions with mutable
default arguments. No sooner does one thread finally, and painfully, fade
away than another one starts up.
I suggest that Python should raise warnings.RuntimeWarning (or similar?)
when a function is defined with a default argument consisting of a list,
dict or set. (This is not meant as an exhaustive list of all possible
mutable types, but as the most common ones that I expect will trip up
newbies.) The warning should refer to the relevant FAQ or section in the
docs.
|
I just suggested a documentation enhancement in one of the plethora of
threads... so I'm certainly +1 for any enhancement in this area.
Diez |
| |
| | | Christian Heimes | |
| | Posted: Fri Aug 22, 2008 1:09 pm Post subject: Re: Should Python raise a warning for mutable default argume | |
Steven D'Aprano wrote:
| Quote: | I suggest that Python should raise warnings.RuntimeWarning (or similar?)
when a function is defined with a default argument consisting of a list,
dict or set. (This is not meant as an exhaustive list of all possible
mutable types, but as the most common ones that I expect will trip up
newbies.) The warning should refer to the relevant FAQ or section in the
docs.
What do people think?
|
I don't see a chance for your proposal. How are you going to detect
mutable objects? Custom types can be mutable as well as immutable.
Christian |
| |
| | | Peter Otten | |
| | Posted: Fri Aug 22, 2008 1:32 pm Post subject: Re: Should Python raise a warning for mutable default argume | |
| |  | |
Christian Heimes wrote:
| Quote: | Steven D'Aprano wrote:
I suggest that Python should raise warnings.RuntimeWarning (or similar?)
when a function is defined with a default argument consisting of a list,
dict or set. (This is not meant as an exhaustive list of all possible
mutable types, but as the most common ones that I expect will trip up
newbies.) The warning should refer to the relevant FAQ or section in the
docs.
What do people think?
|
-0 from me. I'd rather feature it more prominently in the tutorial, a
section "The five most common pitfalls" or something like that.
| Quote: | I don't see a chance for your proposal. How are you going to detect
mutable objects? Custom types can be mutable as well as immutable.
|
A check at compilation time for list literals would catch 90 % of the cases.
The warning would be targeted at newbies after all. It might still be a
source of confusion when they try to import someone else's code that uses
mutable defaults intentionally.
Peter |
| |
| | | Peter Otten | |
| | Posted: Fri Aug 22, 2008 1:55 pm Post subject: Re: Should Python raise a warning for mutable default argume | |
bearophileHUGS@lycos.com wrote:
| Quote: | DrScheme is an implementation of Scheme that is very newbie-friendly.
It has several limited sub-languages, etc.
So maybe a command line option can be added to Python3 ( -
newbie ?  ) that just switches on similar warnings, to help newbies
(in schools, where there's a teacher that encourages to always use
that command line option) avoid some of the most common traps.
|
Or maybe bundle pychecker with idle?
$ cat tmp.py
def test(x, a=[]):
a.append(x)
return a
for i in range(5):
print test(i)
$ pychecker tmp.py
Processing tmp...
[0]
[0, 1]
[0, 1, 2]
[0, 1, 2, 3]
[0, 1, 2, 3, 4]
Warnings...
tmp.py:2: Modifying parameter (a) with a default value may have unexpected
consequences
Though it might be interesting to ask a newbie what he expects when warned
of "unexpected consequences" ;)
Peter |
| |
| | | Guest | |
| | Posted: Fri Aug 22, 2008 3:43 pm Post subject: Re: Should Python raise a warning for mutable default argume | |
DrScheme is an implementation of Scheme that is very newbie-friendly.
It has several limited sub-languages, etc.
So maybe a command line option can be added to Python3 ( -
newbie ? ) that just switches on similar warnings, to help newbies
(in schools, where there's a teacher that encourages to always use
that command line option) avoid some of the most common traps.
Bye,
bearophile |
| |
| | | Emile van Sebille | |
| | Posted: Fri Aug 22, 2008 4:14 pm Post subject: Re: Should Python raise a warning for mutable default argume | |
Steven D'Aprano wrote:
| Quote: | Sometimes it seems that barely a day goes by without some newbie, or not-
so-newbie, getting confused by the behaviour of functions with mutable
default arguments. No sooner does one thread finally, and painfully, fade
away than another one starts up.
I suggest that Python should raise warnings.RuntimeWarning (or similar?)
when a function is defined with a default argument consisting of a list,
dict or set. (This is not meant as an exhaustive list of all possible
mutable types, but as the most common ones that I expect will trip up
newbies.) The warning should refer to the relevant FAQ or section in the
docs.
What do people think?
|
-1
People that have worked through the tutorial, something everyone should
do when they're starting out, will find this explicitly discussed. See
LINK
People that just skim the surface get stung -- sorry.
Emile |
| |
| | | Christian Heimes | |
| | Posted: Fri Aug 22, 2008 4:41 pm Post subject: Re: Should Python raise a warning for mutable default argume | |
MRAB wrote:
| Quote: | Could there be a new special method __mutable__?
|
Why should we add a new method?
Seriously, why should Python be cluttered and slowed down to warn about
mutable arguments. It's neither a design flaw nor a surprising feature
*ONCE* you have understood how functions work. I agree that the behavior
may be surprising for a newbie but it's logical once you got the big
picture.
Christian |
| |
| | | castironpi | |
| | Posted: Fri Aug 22, 2008 4:53 pm Post subject: Re: Should Python raise a warning for mutable default argume | |
| | | |
On Aug 22, 9:42 am, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com.au> wrote:
| Quote: | Sometimes it seems that barely a day goes by without some newbie, or not-
so-newbie, getting confused by the behaviour of functions with mutable
default arguments. No sooner does one thread finally, and painfully, fade
away than another one starts up.
I suggest that Python should raise warnings.RuntimeWarning (or similar?)
when a function is defined with a default argument consisting of a list,
dict or set. (This is not meant as an exhaustive list of all possible
mutable types, but as the most common ones that I expect will trip up
newbies.) The warning should refer to the relevant FAQ or section in the
docs.
What do people think?
--
Steven
|
I like the idea of calling it to your attention. +1.
If a warning, you should be able to silence it with an annotation,
decorator, or a module-level flag.
Or perhaps, require it be declared explicit, and make it an error.
def test(x, a=defmut([])):
Python raises an actual error unless default arguments are known
immutable or instances of 'defmut'. |
| |
| | | Diez B. Roggisch | |
| | Posted: Fri Aug 22, 2008 5:14 pm Post subject: Re: Should Python raise a warning for mutable default argume | |
| | | |
Emile van Sebille schrieb:
| Quote: | Steven D'Aprano wrote:
Sometimes it seems that barely a day goes by without some newbie, or not-
so-newbie, getting confused by the behaviour of functions with mutable
default arguments. No sooner does one thread finally, and painfully,
fade away than another one starts up.
I suggest that Python should raise warnings.RuntimeWarning (or
similar?) when a function is defined with a default argument
consisting of a list, dict or set. (This is not meant as an exhaustive
list of all possible mutable types, but as the most common ones that I
expect will trip up newbies.) The warning should refer to the relevant
FAQ or section in the docs.
What do people think?
-1
People that have worked through the tutorial, something everyone should
do when they're starting out, will find this explicitly discussed. See
LINK
People that just skim the surface get stung -- sorry.
|
But obviously enough, it's not emphazized enough. Even if the
interpreter isn't touched, at least the docs should be.
Diez |
| |
| Page 1 of 3 .:. Goto page 1, 2, 3 Next | |
|
|
