Is there a difference between setting a variable to None or deleting it?

Question

Is there a noticeable difference in performance or memory if you clear variables by setting

x = 'something'
x = None

as opposed to

x = 'something'
del x

for many variables?

There is no difference in memory. Performance you can check yourself. — Idos, Mar 18 '16 at 14:56

score 14 · Accepted Answer · edited May 23 '17 at 12:22

Both times the memory for 'something' will be reclaimed if x was the last reference to that string object. In the first example, you use a tiny bit more memory because you keep the name x around.

I did some searching and it looks like the memory will be reclaimed immediately: further reading.

There is a conceptual difference, however, and your program will behave differently. When you del x, you unbind the name x, and trying to use that name will raise a NameError:

>>> x = 'something'
>>> del x
>>> x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined

When you set x = None you rebind x to be a name for the object None, and can still use the variable as expected. For example for truthy/falsy tests:

>>> x = None
>>> if not x:
...     print('hello cshin9!')
... 
hello cshin9!

score 5 · Answer 2 · answered Mar 18 '16 at 14:57

5

Setting a variable to None or deleting it both causes the reference to the object x to be released. The main difference is that with x = None the name x still exists, even though it's now pointing to a different object (None). When using del the name is deleted as well.

For the garbage collector, there is no difference. del does not force any garbage collection of the referenced object.

Edit: as @Viroide pointed out, using del causes an exception to be thrown when trying to call the variable x.

answered Mar 18 '16 at 14:57

Lars de Bruijn

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What would force garbage collection of the object? – cshin9 Mar 18 '16 at 15:02
3

According to https://docs.python.org/2/library/gc.html `gc.collect()` will force the garbage collector to release unreferenced memory. I do not know of a method to explicitly release a single variable and I doubt such a method exists. – Lars de Bruijn Mar 18 '16 at 15:05

Markon · Answer 3 · 2016-03-18T15:17:07.780

I was curious, so I decided to have a look at what the module dis says by running this code:

import dis

def f():
    v = 'x' 
    v = None
    del v 
    return None

print(dis.dis(f))

The result is

  4           0 LOAD_CONST               1 ('x')
              3 STORE_FAST               0 (v)

  5           6 LOAD_CONST               0 (None)
              9 STORE_FAST               0 (v)

  6          12 DELETE_FAST              0 (v)

  7          15 LOAD_CONST               0 (None)
             18 RETURN_VALUE

and the documentations says that DELETE_FAST

Deletes local co_varnames[var_num].

while LOAD_CONST

Pushes co_consts[consti] onto the stack.

So, in terms of operations/memory, LOAD_CONST performs two operations (where one is using memory), and I think that it becomes relevant only when you have a high amount of elements (not just one) - therefore you might want to use del.

This also explains why you cannot access the variable v anymore (as it was deleted from the namespace, while LOAD_CONST is just another assignment).

score 1 · Answer 4 · answered Mar 18 '16 at 14:56

The two statements differ in a way they operate on the current namespace: the former simply changes the value of the variable, while del completely removes the variable. The previous value pointed by x gets garbage collected in any case.

Performance differences, if any, are negligible.

score 1 · Answer 5 · edited Jun 15 '23 at 19:49

No, it's not the same, look at the following example:

x = "something"
x = None
type(x)
<type 'NoneType'>

z = "asd"
del z
type(z)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'z' is not defined

Setting to None makes the variable value be None and have NoneType. But deleting the variable makes the variable inaccessible by deleting from the context.

Is there a difference between setting a variable to None or deleting it?

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