Python Default Arguments Evaluation

Question

I was reading the python tutorial from Python Documentation Release 2.7.10 and I came across something like this.

Code

def fun1(a,L=[]):
    L.append(a)
    return L

print fun1(1)
print fun1(2)
print fun1(3)

def fun2(a,L = None):
    if L is None:
        L=[]
    L.append(a)
    return L

print fun2(1)
print fun2(2)
print fun2(3)

Output

[1]
[1, 2]
[1, 2, 3]
[1]
[2]
[3]

Process finished with exit code 0

If the L=[] in the first function fun1() is getting called only once , the output of fun1()is fine. But then why L=None is getting called every time in fun2().

Answer 1

When you define a function the values of the default arguments get evaluated, but the body of the function only get compiled. You can examine the result of the function definition via attributes. Theres a __defaults__ attribute containing the defaults and __code__ attribute containing the body (so these are created when the function is defined).

What's happening in the second example is that None do get evaluated at definition (it evaluates to None duh!), but the code that conditionally assigns [] to L only gets compiled and is run each time (the condition passes).