[Python-ideas] fixing mutable default argument values (original) (raw)

Joel Bender jjb5 at cornell.edu
Thu Jan 18 14:44:12 CET 2007

Calvin Spealman wrote:

I dont understand how that would be different than doing

c = c if c is not None else Bar([2,3,4])

Because that would be calling Bar(), perhaps creating a new Bar object, every time foo() is called with None for c, which is not what a default argument values are about. I'm proposing a way to create function local singleton objects, removing them from the parameter list.

 def foo(x):
     local history = []
     history.append(x)

Rather than:

 def foo(x, history=[]):
     history.append(x)

and then hoping that nobody calls foo() with a history parameter.

Joel

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