why del keyword does not delete a list when passed to a function

Question

in the following code when i use del keyword to delete the formal parameter a, and then print it outside the function the list that is passed still exists.

lst = [1,2,3]
def func(a):
    print lst
    lst.append(4) #this is modified as it is by default passed by reference
    print lst
    print locals
    del a # this does not delete the lst! why?
    print locals
func(a)
print lst #still exists!

These will help https://stackoverflow.com/questions/6146963/when-is-del-useful-in-python/6146992#6146992 https://stackoverflow.com/questions/41461169/python-delete-remove-from-memory-a-variable-from-inside-a-function — planet260, Oct 24 '17 at 12:14
because you have _defined_ `func` but you have not _executed_ it ;—) moreover, `del a` remove `a` from the name space of `func`, the list object survives because it is still referenced in the name space of the main. — gboffi, Oct 24 '17 at 12:21
a short addition: if you want to delete the contents of the list, you can do `del a[:]` inside the function `func` — patzm, Oct 02 '18 at 11:25

score 2 · Accepted Answer · answered Oct 24 '17 at 14:42

2

It's a problem of an object and the names you use to reference said object, your problem is very similar (except that in your problem the names live in different namespaces) to the following one:

a = [1, 2]
b = a
b.append(3)
del b

do you think that you deleted the list object? No. Another analogy for another domain, the shell

$ echo 1 > a
$ ln a b
$ echo 2 >> b
$ rm b
$ cat a

answered Oct 24 '17 at 14:42

gboffi

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so, i deleted name, but not the actual object! Thanks. – Max Oct 25 '17 at 05:30

why del keyword does not delete a list when passed to a function

1 Answers1