Pass by reference or by value?

[Steve Holden]

Robert Dailey wrote:
> Hi,
> 
> I previously created a topic named "Pass by reference or by value" where 
> I inquired on how python's function parameters work. I received a lot of 
> nice responses, however I'm still confused on the topic. Note that I 
> come from a C++ background to Python, so any comparisons to C++ would be 
> very helpful.
> 
> I ran a few tests. There's two tests in particular I wanted to show you 
> guys:
> ------------------------------------------------------------------------------------------------
> myvar = []
> 
> def changeme( param ):
>     param.append( "blah" )
>     print param
> 
> changeme( myvar )
> 
> print myvar
> 
> The above code yields the following output:
> ['blah']
> ['blah']
> 
> This means that the list passed in was modified by the function.
> ------------------------------------------------------------------------------------------------
> Now test case 2:
> 
> myvar = 4
> 
> def changeme( param ):
>     param = 5
>     print param
> 
> changeme( myvar )
> 
> print myvar
> 
> The above code yields the following output:
> 5
> 4
> 
> This means that the integer passed in was NOT modified by the function.
> ------------------------------------------------------------------------------------------------
> 
> Between these two tests, both types passed in are mutable objects. I'm 
> having trouble figuring out what mandates an object to be changed from 
> within a function versus not. What is happening in test case 2 to cause 
> it to not be modified?
> 
> Thanks for reading guys. Hopefully one day I'll understand this lol.
> 
The first thin to realise is that all Python names are just bindings to 
values. In C++ terms you can think of them all as pointers. 
De-referencing is automatic when a value is to be retrieved.


 >>> def changeme( param ):
...     param = 3
...     print param
...
 >>> myvar = []
 >>> changeme(myvar)
3
 >>> myvar
[]
 >>>

In this case there is no attempt to mutate the argument, the argument 
name is simply bound to another value. Since the argument is a name 
local to the function, this does not result in any change outside the 
function.

In this case the argument is bound to a mutable value, so the call to 
append mutates the object (a list) referenced by the argument.

Unlike C++ and similar languages a variable does not hold a value, it 
holds a pointer to a value. When a list is passed as a function argument 
the reference to the list is copied into the argument. Does this help at 
all?

regards
  Steve
-- 
Steve Holden        +1 571 484 6266   +1 800 494 3119
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