Different ways of clearing lists
Question:
Is there any reason to do anything more complicated than one of these two lines when you want to clear a list in Python?
old_list = []
old_list = list()
The reason I ask is that I just saw this in some running code:
del old_list[ 0:len(old_list) ]
Answers:
Clearing a list in place will affect all other references of the same list.
For example, this method doesn’t affect other references:
>>> a = [1, 2, 3]
>>> b = a
>>> a = []
>>> print(a)
[]
>>> print(b)
[1, 2, 3]
But this one does:
>>> a = [1, 2, 3]
>>> b = a
>>> del a[:] # equivalent to del a[0:len(a)]
>>> print(a)
[]
>>> print(b)
[]
>>> a is b
True
You could also do:
>>> a[:] = []
It appears to me that del will give you the memory back, while assigning a new list will make the old one be deleted only when the gc runs.matter.
This may be useful for large lists, but for small list it should be negligible.
Edit: As Algorias, it doesn’t matter.
Note that
del old_list[ 0:len(old_list) ]
is equivalent to
del old_list[:]
There are two cases in which you might want to clear a list:
- You want to use the name
old_list further in your code;
- You want the old list to be garbage collected as soon as possible to free some memory;
In case 1 you just go on with the assigment:
old_list = [] # or whatever you want it to be equal to
In case 2 the del statement would reduce the reference count to the list object the name old list points at. If the list object is only pointed by the name old_list at, the reference count would be 0, and the object would be freed for garbage collection.
del old_list
There is a very simple way to clear a python list. Use del list_name[:].
For example:
>>> a = [1, 2, 3]
>>> b = a
>>> del a[:]
>>> print a, b
[] []
del list[:]
Will delete the values of that list variable
del list
Will delete the variable itself from memory
If you’re clearing the list, you, obviously, don’t need the list anymore. If so, you can just delete the entire list by simple del method.
a = [1, 3, 5, 6]
del a # This will entirely delete a(the list).
But in case, you need it again, you can reinitialize it. Or just simply clear its elements by
del a[:]
another solution that works fine is to create empty list as a reference empty list.
empt_list = []
for example you have a list as a_list = [1,2,3]. To clear it just make the following:
a_list = list(empt_list)
this will make a_list an empty list just like the empt_list.
Doing alist = [] does not clear the list, just creates an empty list and binds it to the variable alist. The old list will still exist if it had other variable bindings.
To actually clear a list in-place, you can use any of these ways:
alist.clear() # Python 3.3+, most obviousdel alist[:]alist[:] = []alist *= 0 # fastest
See the Mutable Sequence Types documentation page for more details.
Is there any reason to do anything more complicated than one of these two lines when you want to clear a list in Python?
old_list = []
old_list = list()
The reason I ask is that I just saw this in some running code:
del old_list[ 0:len(old_list) ]
Clearing a list in place will affect all other references of the same list.
For example, this method doesn’t affect other references:
>>> a = [1, 2, 3]
>>> b = a
>>> a = []
>>> print(a)
[]
>>> print(b)
[1, 2, 3]
But this one does:
>>> a = [1, 2, 3]
>>> b = a
>>> del a[:] # equivalent to del a[0:len(a)]
>>> print(a)
[]
>>> print(b)
[]
>>> a is b
True
You could also do:
>>> a[:] = []
It appears to me that del will give you the memory back, while assigning a new list will make the old one be deleted only when the gc runs.matter.
This may be useful for large lists, but for small list it should be negligible.
Edit: As Algorias, it doesn’t matter.
Note that
del old_list[ 0:len(old_list) ]
is equivalent to
del old_list[:]
There are two cases in which you might want to clear a list:
- You want to use the name
old_listfurther in your code; - You want the old list to be garbage collected as soon as possible to free some memory;
In case 1 you just go on with the assigment:
old_list = [] # or whatever you want it to be equal to
In case 2 the del statement would reduce the reference count to the list object the name old list points at. If the list object is only pointed by the name old_list at, the reference count would be 0, and the object would be freed for garbage collection.
del old_list
There is a very simple way to clear a python list. Use del list_name[:].
For example:
>>> a = [1, 2, 3]
>>> b = a
>>> del a[:]
>>> print a, b
[] []
del list[:]
Will delete the values of that list variable
del list
Will delete the variable itself from memory
If you’re clearing the list, you, obviously, don’t need the list anymore. If so, you can just delete the entire list by simple del method.
a = [1, 3, 5, 6]
del a # This will entirely delete a(the list).
But in case, you need it again, you can reinitialize it. Or just simply clear its elements by
del a[:]
another solution that works fine is to create empty list as a reference empty list.
empt_list = []
for example you have a list as a_list = [1,2,3]. To clear it just make the following:
a_list = list(empt_list)
this will make a_list an empty list just like the empt_list.
Doing alist = [] does not clear the list, just creates an empty list and binds it to the variable alist. The old list will still exist if it had other variable bindings.
To actually clear a list in-place, you can use any of these ways:
alist.clear() # Python 3.3+, most obviousdel alist[:]alist[:] = []alist *= 0 # fastest
See the Mutable Sequence Types documentation page for more details.