New submission from Denver Coneybeare:
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.
----------
components: Library (Lib)
files: ThreadingTimerInitDefaultArgsIssueDemo.01.patch
keywords: patch
messages: 184278
nosy: denversc
priority: normal
severity: normal
status: open
title: threading.Timer.__init__() should use immutable argument defaults for
args and kwargs
type: behavior
versions: Python 2.7, Python 3.1, Python 3.2, Python 3.3, Python 3.4
Added file:
http://bugs.python.org/file29422/ThreadingTimerInitDefaultArgsIssueDemo.01.patch
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Python tracker <[email protected]>
<http://bugs.python.org/issue17435>
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