For example, in the code:
a = [1, 2, 3, 4] # huge list
y = sum(a)
print( do_stuff(y) )
Will the memory for the list a ever get freed up before the program ends? Will the do_stuff function call have to do all its stuff with a constantly taking up memory, even though a's never going to be used again?
And if a doesn't get garbage collected, is the solution to manually set a=None once I'm done using it?
Imagine do_stuff did this:
def do_stuff(y):
return globals()[input()]
And the user enters a, so that the list is used there after all. Python can't know that that won't happen, it would have to read the user's mind.
Consider a trivial case like this:
def do_stuff(y):
return y
Now it's clear that a doesn't get used anymore, so Python could figure that out right? Well... print isn't a keyword/statement anymore. Python would have to figure out that you didn't overwrite print with something that does use a. And even if you didn't, it would need to know that its own print doesn't use a.
You'll have to delete it yourself. I'd use del a. (Unless you want a to still exist and have the value None).
a will never be freed unless it goes out of scope (ie. it was in a function to begin with), or you manually set it to None.
Python's garbage collector uses a system called reference counting. In short, all variables have a reference counter that is incremented when a reference to the variable is created, and decremented when an variable is set to None or when it goes out of scope.
Example:
a = [999999] # 1 reference, the value [999999] is stored in memory
b = a # 2 references
def foo(x):
print(x)
foo(a) # 3 references during the function call
# back to 2 references
a = None # 1 reference
b = None # 0 references, the value [999999] is deleted from memory