Python function unexpectedly memorizes the length of list and adds it to every iteration

Question

I'm trying to check number of "stopping times" of Collatz sequence. The script appends result of every function "step" in a list which length I use to figure out overall number of "steps". the script is ok, but! Whenever I use several times it returns wrong length of a list. I don't know wht but it keeps previous length of a list and adds it to the next function result.

def stopping_time(n, l=[]):
    if n == 1:
        return
    elif n % 2 == 0:
        n = n // 2
        l.append(n)
        stopping_time(n)
    else:
        n = n * 3 + 1
        l.append(n)
        stopping_time(n)

    return len(l)


a = stopping_time(4)
b = stopping_time(13)
c = stopping_time(19)
d = stopping_time(27)

print(a, b, c, d)

# 'a' shoulb be 2, 'b' should be 9, 'c' should be 20 and 'd' should be 111
# instead its 2, 11,  31,  142

Any thoughts?

See the linked duplicates for a lot of discussion; tl;dr you want to use `l=None` in the function definition and add `if l is None: l = []` inside the body. — tzaman, May 26 '20 at 10:33

score 1 · Accepted Answer · answered May 26 '20 at 10:31

It's not a bug, but a feature. Python indeed memorizes mutable values passed as default arguments between calls (cf. https://docs.quantifiedcode.com/python-anti-patterns/correctness/mutable_default_value_as_argument.html).

The solution may be passing the sentinel value as an argument, something like:

def foo(bar=None):
    if bar is None:
        bar=[]
     ...

In general, you set your list argument to None by default and then set it inside the function to [] if no other value was passed by the caller.

Python function unexpectedly memorizes the length of list and adds it to every iteration

1 Answers1