Different ways of deleting lists

Question

I want to understand why:

• a = [];
• del a; and
• del a[:];

behave so differently.

I

Answer 1

Test 1: rebinds a to a new object, b still holds a reference to the original object, a is just a name by rebinding a to a new object does not change the original object that b points to.

Test 2: you del the name a so it no longer exists but again you still have a reference to the object in memory with b.

Test 3 a[:] just like when you copy a list or want to change all the elements of a list refers to references to the objects stored in the list not the name a. b gets cleared also as again it is a reference to a so changes to the content of a will effect b.

The behaviour is documented:

There is a way to remove an item from a list given its index instead of its value: the del statement. This differs from the pop() method which returns a value. The del statement can also be used to remove slices from a list or clear the entire list (which we did earlier by assignment of an empty list to the slice). For example:

>>>
>>> a = [-1, 1, 66.25, 333, 333, 1234.5]
>>> del a[0]
>>> a
[1, 66.25, 333, 333, 1234.5]
>>> del a[2:4]
>>> a
[1, 66.25, 1234.5]
>>> del a[:]
>>> a
[]

del can also be used to delete entire variables:

>>>
>>> del a

Referencing the name a hereafter is an error (at least until another value is assigned to it). We'll find other uses for del later.

So only del a actually deletes a, a = [] rebinds a to a new object and del a[:] clears a. In your second test if b did not hold a reference to the object it would be garbage collected.