[Tutor] functions and default argument

[Prasad, Ramit]

>The same thing occurs when you use a mutable object like a list or a 
>dict. The default value is assigned once, and once only. But notice that 
>you can modify the default value, say by appending to it:

Not sure this will work exactly the same way in other IDEs, but in mine:

>>> a = []
>>> def foo(x=a):
...     x.append(1)
...     
>>> a.append( 2 )
>>> foo()
>>> print a
[2, 1]
>>> help(foo)
Help on function foo in module __pieshell__:

foo(x=[2, 1])

>>> foo()
>>> help(foo)
Help on function foo in module __pieshell__:

foo(x=[2, 1, 1])

>>> a.append( 3 )
>>> help(foo)
Help on function foo in module __pieshell__:

foo(x=[2, 1, 1, 3])
>>> b = []
>>> foo(b)
>>> b
[1]


Notice how it is always bound to the list a, but can be "overridden".

I know this has been discussed on this list or the main list before if you take a look through the archives.
Sorry I can't remember what the thread would be like or when it was :(
http://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument 


Ramit


Ramit Prasad | JPMorgan Chase Investment Bank | Currencies Technology
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