list = [] 与 list.clear() 之间的区别

Question

What is the difference between list = [] and list.clear() ?

Base on the behavior of my code and my own observation, list.clear() removes its entries and also the entries I used to append its data.

Example:

container.append(list)
list.clear()

container will also be []

Answer 1

Calling clear removes all the element from the list. Assigning [] just replaces that variable with another empty list. This becomes evident when you have two variables pointing to the same list.

Consider the following snippet:

>>> l1 = [1, 2, 3]
>>> l2 = l1
>>> l1.clear()
>>> l1 # l1 is obviously empty
[]
>>> l2 # But so is l2, since it's the same object
[]

As compared to this one:

>>> l1 = [1, 2, 3]
>>> l2 = l1
>>> l1 = []
>>> l1 # l1 is obviously empty
[]
>>> l2 # But l2 still points to the previous value, and is not affected
[1, 2, 3]

Answer 2

You can also see this if you take a look at the bytecode that is generated. Here the part with x = []

import dis

print("Example with x = []")

s1 = """
x = [1,2,3]
x = []
"""

dis.dis(s1)

which outputs

Exmaple with x = []
  2           0 LOAD_CONST               0 (1)
              2 LOAD_CONST               1 (2)
              4 LOAD_CONST               2 (3)
              6 BUILD_LIST               3
              8 STORE_NAME               0 (x)

  3          10 BUILD_LIST               0
             12 STORE_NAME               0 (x)
             14 LOAD_CONST               3 (None)
             16 RETURN_VALUE

we can see that two lists are build since we have two BUILD_LIST . Now if we take a look at x.clear()

print("Exmaple with x.clear()")

s2 = """
x = [1,2,3]
x.clear()
"""

dis.dis(s2)

we get the following output

Exmaple with x.clear()
  2           0 LOAD_CONST               0 (1)
              2 LOAD_CONST               1 (2)
              4 LOAD_CONST               2 (3)
              6 BUILD_LIST               3
              8 STORE_NAME               0 (x)

  3          10 LOAD_NAME                0 (x)
             12 LOAD_ATTR                1 (clear)
             14 CALL_FUNCTION            0
             16 POP_TOP
             18 LOAD_CONST               3 (None)
             20 RETURN_VALUE

and here only one list is build and clear is called and LOAD_CONST is used to place None onto the stack as with the initial values 1,2,3 .