Maybe this is a very easy question to someone, I can't understand what this happened.
I try to solve problems using python generator. However, sometime generator seems not to work that I expected.
The code below is the very simple python generator.
It takes integer n and set visited as its arguments.
visited should be initialized set() when the function is called.
def simple_gen(n, visited=set()):
for i in range(0, n):
if i not in visited:
yield i
visited.update([i])
a = simple_gen(10)
b = simple_gen(10)
print(f"a: {list(a)}")
print(f"b: {list(b)}")
But, when the code is executed, the output is like below.
generator a generator value like that I expected, but b doesn't. b doesn't generator any value, it is just empty value.
arguement visited always initialized to empty set as default value.
but it doesn't look like initializing.
a: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
b: []
I don't understand why this happened. Are there anyone know this, please let me know.
I always thanks for your help.
Python's default parameters are stored in the function object. This means they're only created once when the function is defined, they are not initialised every time the function is called.
As a result, if you modify a mutable parameter, that modification will remain between calls.
And that's exactly what you're doing here: you have a mutable default parameter, you add stuff to it on the first call, and on the second call visited starts as {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}. That's why you don't normally use mutable default parameters in Python, it's very error prone and somewhat tricky to debug.
Create your visited as a normal local variable, I don't see any reason why it'd even be a parameter.