[Python-ideas] fixing mutable default argument values (original) (raw)

Jan Kanis jan.kanis at phil.uu.nl
Thu Jan 25 02:03:38 CET 2007

I don't like new syntax for something like this, but I think the default
argument values can be fixed with semantic changes (which should not break
the most common current uses):

What I think should happen is compile a function like this

def popo(x=[]): x.append(666) print x

as if it had read

def popo(x=default_argument_marker): if x == default_argument_marker: x = [] x.append(666) print x

This way, every execution of popo gets its own list. Of course,
default_argument_marker is just a way to tell the python runtime that
no argument was provided, it should not be exposed to the language.

If a variable is used in the default argument, it becomes a closure
variable:

d = createMyListOfLists() n = getDefaultIndex()

def foo(x=d[n]): x.append(666) print x

this is compiled as if it had read

d = createMyListOfLists() n = getDefaultIndex()

def foo(x=default_argument_marker): if x == default_argument_marker: x = d[n] # d and n are closure variables x.append(666) print x

d and n are looked up in foo's parent scope, which in this example is the
global scope. Of course the bytecode compiler should make sure d and n
don't name-clash with any variables used in the body of foo.

When you use variables as default value instead of literals, I think most
of the time you intend to have the function do something to the same
object the variable is bound to, instead of the function creating it's own
copy every time it's called. This behaviour still works with these
semantics:

a = [] def foo(x=[[],a]): x[0].append(123) x[1].append(123) print x foo() [[123], [123]] foo() [[123], [123, 123]] foo() [[123], [123, 123, 123]]

foo is compiled as if it had read:

def foo(x=default_argument_marker): if x == default_argument_marker: x = [[],a] # a is a closure variable x[0].append(123) x[1].append(123) print x

An other difference between this proposal and the current situation is
that it would be possible to change the value of a default argument after
the function is defined. However I don't think that would really be a
problem, and this behaviour is just the same as that of other closure
variables. Besides, this (what I perceive as a) problem with closure
variables is fixable on its own.

Jan

On Mon, 22 Jan 2007 01:49:51 +0100, Josiah Carlson <jcarlson at uci.edu>
wrote:

Chris Rebert <cvrebert at gmail.com> wrote: Josiah Carlson wrote: > As provided by Calvin Spealman, the above can be fixed with: > > def popo(x=None): > x = x if x is not None else [] > x.append(666) > print x > > I would also mention that forcing users to learn about mutable arguments > and procedural programming is not a bad thing. Learning the "gotcha" > of mutable default arguments is a very useful lesson, and to remove that > lesson, I believe, wouldn't necessarily help new users to Python, or new > programmers in general. > > - Josiah Maybe you are taking me a bit too seriously, but hopefully this will add some levity; I'm a poo-poo head. Moving on...

First, your 'fix' misses the point: though the proposed feature isn't necessary, and code can be written without using it, it allows mutable default argument values to be expressed more clearly and succinctly than the idiom your 'fix' uses. As I stated, it wasn't my fix. And using previously existing syntax that adds 1 line to a function to support a particular desired result, I think, is perfectly reasonable. Had the conditional syntax been available for the last decade, those "gotchas" pages would have said "mutable default arguments are tricky, always use the following, and it will probably be the right thing" and moved on. Second, Python isn't (inherently) about teaching new programmers about programming, and what is good for newbies isn't necessarily good for experienced programmers. Indeed, and what may be good for certain experienced programmers, may not be good for other experienced programmers, or for the language in general. And personally, I am not sure that I could be convinced that a syntax to support what can be emulated by a single line is even worthy of addition. In the case of decorators, or even the py3k support for argument annotation, there are certain operations that can be made significantly easier. In this case, I'm not convinced that the extra syntax baggage is worthwhile. Nevermind that it would be one more incompatible syntax that would make it difficult to write for 2.5/2.6 and 3.x . - Josiah

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