Mutable Default Argument Returns None

Question

I have a simple code that finds paths using a graph stored in a dictionary. The code is exactly:

def find_path(dct, init, depth, path=[]):
   if depth == 0:
      return path
   next_ = dct[init]
   depth-=1
   find_path(dct, next_, depth)

If I print the path right before return path it prints to screen the correct path (after an initial depth of 5). However, the value returned is None. I don't know what's going on!

Why would the value of path right above the return is correct, yet the returned path is not what I want?

Answer 1

Shouldn't this

find_path(dct, next_, depth)

be

return find_path(dct, next_, depth)
# ^^^^
# Return

In Python (unlike in say, Ruby) you have to explicitly return a value. Otherwise None is returned.

Answer 2

Because you're calling it with depth greater than 0, which is causing it to fall off the end and return None.