575414 - don't use mutable arguments to functions

Status: RESOLVED FIXED

(Testing :: Mozbase, defect)

Description

[Jeff Hammel]

In general, mutable arguments (chiefly [] and {}) should not be used as default arguments to functions.  The defaults are stored on the function object and changing the object will change them for all instances:

{{{
> cat tmp.py 
class Foo(object):
  def __init__(self, env={}):
    print env
    self.env = env
    self.env[len(self.env)] = 'foo'

Foo()
Foo()
> python tmp.py 
{}
{0: 'foo'}
}}}

The places where mutable arguments live should be changed to None;  then, if the passed argument is None then you can assign the value as desired:

{{{
> cat tmp.py 
class Foo(object):
  def __init__(self, env=None):
    print env
    self.env = env or {}
    self.env[len(self.env)] = 'foo'

Foo()
Foo()
> python tmp.py 
{}
{0: 'foo'}
}}}

Comment 1

[Jeff Hammel]

I've been fixing this incrementally.  Everyone else should too.  Can we close this bug since its not really about a specific piece of code?  Additional cases where mutable arguments are used as function defaults should either be fixed in situ or ticketed separately.

Comment 2

[Jeff Hammel]

I'm going to close this bug.  I've corrected in various commits the associated errors I've found here.  IMHO, it is not worth a full audit to find all the places where this should be done, and instead, these sorts of things should be corrected in situ.  Feel free to reopen if you disagree.