Passing variables by value in Python
Let say you want to create a function that uses a variable from it’s outer scope. By default python creates a closure for each of these functions created and the variables are evaluated during the call to the function. And you need to be careful about these when you are coding!
f_list= for i in range(5): f_list.append(lambda a:print(a*i,end=',')) for f in f_list: f(5) #prints: 20,20,20,20,20,
In this example we are intended to create a list of functions that prints various multiple’s
of input values. However since the variable
i is passed by reference, it is not bounded until
we call the functions in the second loop. Since the
i is set to be for at the end of the first
loop all of the functions multiply the input with 4.
So how can we evaluate the value of i during the definition and bound the variable
its current value. One way to do it is to use named arguments with default values! A
f_list2= for i in range(5): f_list2.append(lambda a,i=i: print(a*i,end=',')) for f in f_list2: f(5) #prints: 0,5,10,15,20,
And it works like
a charm. Please use it on your
lambda or regular
def functions ;)