Global module variables as default parameters

[Peter Otten]

Christoph Haas wrote:

> Hi, list...
> 
> I wondered if it's possible to use global (module) variables as default
> parameters. A simple working example:
> 
> ----------------------------------------
> #!/usr/bin/python
> 
> globalvar = 123
> 
> def test(foo=globalvar):
>   print foo
> 
> test()
> ----------------------------------------
> 
> Running this script prints "123". That's what I expected.
> 
> Now I'm trying the same thing in a module context. A non-working example:
> 
> test.py
> ----------------------------------------
> #!/usr/bin/python
> 
> import TestModule
> 
> TestModule.globalvar = 123
> TestModule.def1()
> TestModule.def2()
> ----------------------------------------
> 
> TestModule.py
> ----------------------------------------
> globalvar = 0
> 
> def def1():
>   print globalvar
> 
> def def2(foo=globalvar):
>   print foo
> ----------------------------------------
> 
> Running the test.py script prints "123" and "0". So accessing the
> globalvar in def1() works. But if I try to use the global variable as a
> default parameter in def2() it uses the default "0". What is the
> difference between these two? Are there contexts of default parameters?

Yes, and that context is the function definition which is executable code,
too. Whatever the variable 'right' in a statement like

def f(left=right): ...

is bound to when the def is executed will become the default for the 'left'
argument. This should become clear when you create more than one function
with the same

>>> fs = []
>>> for i in range(3):
...     def f(i=i): print i
...     fs.append(f)
...
>>> for f in fs: f()
...
0
1
2

Use a sentinel when you don't want that behaviour:

def f(foo=None):
    if foo is None: foo = globalvar
    # ...

Peter