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Functions in Python presentation
Here is my presentation part of the in company Python course.
The last slide – “Problem to solve” is something like a simple homework. Sample solutions will be uploaded later this week.
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Yeah, this one may be a bit confusing without extra explanation.
The simple explanation is that the mutable arguments keep their state between the calls of the function.
The more complex one is that when you set the default value the variable points to it. So when the default values is used it doesn’t create a new object but uses the existing one in the memory. In this case [] creates a list object and “a” points to it. So when you call a.append it actually modifies the object instead of creating a new one.
Some people see this as a feature, others like a bug. I am not planning to take a side but this is important part of the language implementation and users should be aware of it cause it may lead to unexpected results.
This behaviour is replicated if you initialise variables in the class definition but more about it in the next presentation ))) -
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this slide confuses me completely.
why does func() remembers the a from the previous run? the default should be an empty list and this should be the default for the each and new call of func ? where’s it the catch I miss?
I meant slide no. 12
def func(a=[]):
a.append(2)
print a