python - Why is a function argument default value (an empty list) static? -

this question has answer here:

• “least astonishment” , mutable default argument 30 answers

code:

def func(a=[]):     a.append(1)     print(a)  func() func() func() 

output:

[1] [1, 1] [1, 1, 1] 

i thought default value, list, re-assigned every time func called, , answer be:

[1] [1] [1] 

you "thought default value, list, re-assigned everytime func called." thought wrong. if learning python should work through the official tutorial @ point. here what says default arguments:

the default value evaluated once. makes difference when default mutable object such list, dictionary, or instances of classes.

read tutorial details.