删除列表的不同方法 [英] Different ways of deleting lists

问题描述

我想了解原因:

• a = [];
• del a;和
• del a[:];

• a = [];
• del a; and
• del a[:];

表现各异.

我对每一个进行测试以说明我目睹的差异:

I ran a test for each to illustrate the differences I witnessed:

>>> # Test 1: Reset with a = []
... 
>>> a = [1,2,3]
>>> b = a
>>> a = []
>>> a
[]
>>> b
[1, 2, 3]
>>> 
>>> # Test 2: Reset with del a
... 
>>> a = [1,2,3]
>>> b = a
>>> del a
>>> a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined
>>> b
[1, 2, 3]
>>> 
>>> # Test 3: Reset with del a[:]
... 
>>> a = [1,2,3]
>>> b = a
>>> del a[:]
>>> a
[]
>>> b
[]

我确实找到了清除列表的不同方式，但是我没有找到关于行为上的差异.有人可以澄清吗?

I did find Different ways of clearing lists, but I didn't find an explanation for the differences in behaviour. Can anyone clarify this?

推荐答案

测试1

>>> a = [1,2,3] # set a to point to a list [1, 2, 3]
>>> b = a # set b to what a is currently pointing at
>>> a = [] # now you set a to point to an empty list

# Step 1: A --> [1 2 3]

# Step 2: A --> [1 2 3] <-- B

# Step 3: A --> [     ] [1 2 3] <-- B

# at this point a points to a new empty list
# whereas b points to the original list of a

测试2

>>> a = [1,2,3] # set a to point to a list [1, 2, 3]
>>> b = a # set b to what a is currently pointing at
>>> del a # delete the reference from a to the list

# Step 1: A --> [1 2 3]

# Step 2: A --> [1 2 3] <-- B

# Step 3:       [1 2 3] <-- B

# so a no longer exists because the reference
# was destroyed but b is not affected because
# b still points to the original list

测试3

>>> a = [1,2,3] # set a to point to a list [1, 2, 3]
>>> b = a # set b to what a is currently pointing at
>>> del a[:] # delete the contents of the original

# Step 1: A --> [1 2 3]

# Step 2: A --> [1 2 3] <-- B

# Step 2: A --> [     ] <-- B

# both a and b are empty because they were pointing 
# to the same list whose elements were just removed  

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