On Fri, 7 Oct 2016 10:38 pm, Daiyue Weng wrote:
> Hi, I declare two parameters for a function with default values [],
>
> def one_function(arg, arg1=[], arg2=[]):
>
> PyCharm warns me:
>
> Default argument value is mutable,
>
> what does it mean? and how to fix it?
The usual way to avoid that is:
def one_function(arg, arg1=None, arg2=None):
if arg1 is None:
arg1 = [] # gets a new list each call
if arg2 is None:
arg2 = [] # gets a new list each call
Otherwise, arg1 and arg2 will get the same list each time, not a fresh empty
list. Here is a simple example:
def example(arg=[]):
print(id(arg), arg)
arg.append(1)
py> def example(arg=[]):
... print(id(arg), arg)
... arg.append(1)
...
py> x = []
py> example(x)
3081877196 []
But with the default argument, you get the same list each time:
py> example()
3081877100 []
py> example()
3081877100 [1]
py> example()
3081877100 [1, 1]
py> example()
3081877100 [1, 1, 1]
--
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.
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