[Python-ideas] Mutable default function parameter warning

M.-A. Lemburg mal at egenix.com
Mon Aug 23 20:32:46 CEST 2010

Terry Reedy wrote:
> On 8/23/2010 10:55 AM, Sérgio Surkamp wrote:
>> Hello list,
>> The documentation states that the default value of function parameter,
>> if mutable, can change it's default value at runtime due to be
>> evaluated only once on function object creation.
>> I would like to suggest the inclusion of an default language warning
>> when this kind of construction is used, as it's Python specific
>> behavior and can lead to "strange behavior" or misuse by programmers
>> that are migrating from other languages to Python.
> I am opposed to this for multiple reasons.
> 1. Static checking is for checking programs, not the compiler. IDLE
> currently has a Run syntax check option. I believe there have be
> proposals to extend that to running external checking programs. I would
> be in favor of that.
> 2. Dynamic warning are already done -- by eventually raising an
> exception. If no exception is raised, then code is legal and the result
> *might* be correct.
> 3. There are numerous Python features that *might* be incorrect. For
> instance, reuse of builtin names. This is a long, slippery, slope.
> 4. Defining 'ismutable' is even harder than defining 'iscallable'.

Same here... there's a common idiom used in Python code for
function localized globals which the above would break:

# Cache used for func()
_func_cache = {}

# Note: func_cache may be overridden with a caller private
# cache, default is to use the module global cache.
def func(a, b, c, func_cache=_func_cache):


# Speed up func() by localizing len and str globals/builtins
def func(a, b, c, len=len, str=str):
   ... code using len() and str() ...


The first is a perfectly legitimate use case.

The latter is not nice, but often needed to shortcut lookups
for globals and builtins when used in tight loops. There have
been numerous proposals on how to solve this, but none have made
it into the core.

>> This proposal was first open as a suggestion issue in bug track, but,
>> as a request from Mr. Peterson, I'm rewriting it to this list.
>> http://bugs.python.org/issue9646
> Please close the issue when the tracker is working again.

Marc-Andre Lemburg

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