Why don't optional mutable objects show up in vars(func)?

[dannycolligan]

So I just got bitten by the "don't use a mutable object as an optional
argument" gotcha. I now realize that for this function:

>>def func(x, y=[]):

.... y.append(x)
.... print y
....

y is initialized when the function is imported, not when the function
is executed. However, if this is the case, then why is y not showing
up as an attribute of func?

>>vars(func)

{}

>>dir(func)

['__call__', '__class__', '__delattr__', '__dict__', '__doc__',
'__get__', '__getattribute__', '__hash__', '__init__', '__module__',
'__name__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
'__setattr__', '__str__', 'func_closure', 'func_code', 'func_defaults',
'func_dict', 'func_doc', 'func_globals', 'func_name']

I'm using Python 2.4.3, if that is at all relevant. Thanks in advance
for any help.

Danny

[Georg Brandl]

da***********@gmail.com wrote:

So I just got bitten by the "don't use a mutable object as an optional
argument" gotcha. I now realize that for this function:

>>>def func(x, y=[]):

... y.append(x)
... print y
...

y is initialized when the function is imported, not when the function
is executed. However, if this is the case, then why is y not showing
up as an attribute of func?

>>>vars(func)

{}

>>>dir(func)

['__call__', '__class__', '__delattr__', '__dict__', '__doc__',
'__get__', '__getattribute__', '__hash__', '__init__', '__module__',
'__name__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
'__setattr__', '__str__', 'func_closure', 'func_code', 'func_defaults',
'func_dict', 'func_doc', 'func_globals', 'func_name']

I'm using Python 2.4.3, if that is at all relevant. Thanks in advance
for any help.

y is not an attribute of func, it's a default parameter value and as such
stored in func_defaults:

>>def f(x=1):

.... pass
....

>>print f.func_defaults

(1,)

>>>

Georg

[Fredrik Lundh]

da***********@gmail.com wrote:

So I just got bitten by the "don't use a mutable object as an optional
argument" gotcha. I now realize that for this function:

>>>def func(x, y=[]):

... y.append(x)
... print y
...

y is initialized when the function is imported, not when the function
is executed. However, if this is the case, then why is y not showing
up as an attribute of func?

>>func.func_defaults

([],)

</F>

[Martin v. Löwis]

da***********@gmail.com schrieb:

So I just got bitten by the "don't use a mutable object as an optional
argument" gotcha. I now realize that for this function:

>>>def func(x, y=[]):

... y.append(x)
... print y
...

y is initialized when the function is imported, not when the function
is executed. However, if this is the case, then why is y not showing
up as an attribute of func?

Because it's not an attribute of the function object; it's a default
value of the function.

>>>vars(func)

{}

>>>dir(func)

['__call__', '__class__', '__delattr__', '__dict__', '__doc__',
'__get__', '__getattribute__', '__hash__', '__init__', '__module__',
'__name__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
'__setattr__', '__str__', 'func_closure', 'func_code', 'func_defaults',
'func_dict', 'func_doc', 'func_globals', 'func_name']

I'm using Python 2.4.3, if that is at all relevant. Thanks in advance
for any help.

Take a look at func_defaults.

Regards,
Martin

[Sean Hammond]

On Wed, 2006-09-27 at 09:32 -0700, da***********@gmail.com wrote:

So I just got bitten by the "don't use a mutable object as an optional
argument" gotcha. I now realize that for this function:

>def func(x, y=[]):

... y.append(x)
... print y
...

y is initialized when the function is imported, not when the function
is executed.

I thought it was initialised the first time the function gets called?

[Fredrik Lundh]

Sean Hammond wrote:

>>>>def func(x, y=[]):

... y.append(x)
... print y
...

y is initialized when the function is imported, not when the function
is executed.

I thought it was initialised the first time the function gets called?

it's initialized (in the surrounding context) when the "def" statement
is executed.

</F>