The __init__() method of threading.Timer uses *mutable* default values for the "args" and "kwargs" arguments.  Since the default argument objects are created once and re-used for each instance, this means that changing the args list or kwargs dict of a Timer object that used the argument defaults will specify those arguments to all future Timer objects that use the defaults too.

def __init__(self, interval, function, args=[], kwargs={}):

A fully backwards-compatible way to fix this is to instead use None as the default value for args and kwargs and just create a new list and/or dict inside __init__() if they are None.  That way each new instance of Timer will get its very own args list and kwargs dict object.

def __init__(self, interval, function, args=None, kwargs=None):
    ...
    self.args = args if args is not None else []
    self.kwargs = kwargs if kwargs is not None else {}

Here is a sample script that reproduces the issue:

    import threading

    event = threading.Event()
    def func(*args, **kwargs):
        print("args={!r} kwargs={!r}".format(args, kwargs))
        event.set()

    timer1 = threading.Timer(1, func)
    timer1.args.append("blah")
    timer1.kwargs["foo"] = "bar"

    timer2 = threading.Timer(1, func)
    timer2.start()
    event.wait()

Here is the example output when run before the fix:

c:\dev\cpython>PCbuild\python_d.exe ThreadingTimerInitDefaultArgsIssueDemo.py
args=('blah',) kwargs={'foo': 'bar'}
[44758 refs, 17198 blocks]

And after the fix:

c:\dev\cpython>PCbuild\python_d.exe ThreadingTimerInitDefaultArgsIssueDemo.py
args=() kwargs={}
[47189 refs, 18460 blocks]

As you can see, in the version without the fix, the elements added to timer1's args and kwargs were also given to timer2, which is almost certainly not what a user would expect.

A proposed patch, ThreadingTimerInitDefaultArgsIssueDemo.01.patch, is attached.  This fixes the issue, updates the docs, and adds a unit test.