A quick note about something I've seen a few times in several autopilot tests
as I'm porting them to Python 3. The issue isn't Python 3 specific though.
Please do not use mutable objects in default arguments!
There is a subtle bug in this function. Without looking ahead can you guess
what it is?
def foo(spec={}):
print('spec:', spec)
if not spec:
spec['a'] = 7
Here's a hint: Python parses default arguments when the function is defined,
not when it's run.
The bug is that every invocation of foo() gets the same dictionary object, not
a new one. So subsequent runs of foo() see the mutated dict, e.g.
% python3 -i /tmp/foo.py
>>> foo()
spec: {}
>>> foo()
spec: {'a': 7}
Here's a better way that doesn't exhibit this bug:
def bar(spec=None):
if spec is None:
spec = {}
print('spec:', spec)
if not spec:
spec['a'] = 7
>>> bar()
spec: {}
>>> bar()
spec: {}
>>>
The same holds true for using lists as default argument values. Tuples and
other immutable types are okay of course.
That's it!
Your friendly neighborhood snake wrangler,
-Barry
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