Python 字典一个易犯的错误

一个易犯的错误，关于 Python 的传值(对于不可变量) 和 传引用(对于可变量)，浅拷贝和深拷贝。废话不多说，看例子，

直接改变可变字典值，失败，

>>> dic = dict.fromkeys(range(5), [])
>>> dic
{0: [], 1: [], 2: [], 3: [], 4: []}
>>> dic[0]
[]
>>> dic[0].append(123)
>>> dic
{0: [123], 1: [123], 2: [123], 3: [123], 4: [123]}
>>> 

间接改变可变字典值，失败。

>>> dic = dict.fromkeys(range(5), [])
>>> dic
{0: [], 1: [], 2: [], 3: [], 4: []}
>>> v = dic[0]
>>> v
[]
>>> v.append(123)
>>> v
[123]
>>> dic
{0: [123], 1: [123], 2: [123], 3: [123], 4: [123]}

切片浅拷贝，对于非嵌套可变字典值，赋单值，成功，

>>> dic = dict.fromkeys(range(5), [])
>>> dic
{0: [], 1: [], 2: [], 3: [], 4: []}
>>> v = dic[0][:]
>>> v
[]
>>> v.append(123)
>>> v
[123]
>>> dic
{0: [], 1: [], 2: [], 3: [], 4: []}
>>> 
>>> dic[0] = v
>>> dic
{0: [123], 1: [], 2: [], 3: [], 4: []}

切片浅拷贝，对于非嵌套可变字典值，赋可变列表，成功，

>>> dic = dict.fromkeys(range(5), [])
>>> dic
{0: [], 1: [], 2: [], 3: [], 4: []}
>>> v = dic[0][:]
>>> v.append([1, 2, 3])
>>> v
[[1, 2, 3]]
>>> 
>>> dic[0] = v
>>> dic
{0: [[1, 2, 3]], 1: [], 2: [], 3: [], 4: []}

copy.copy 浅拷贝，成功，

>>> dic = dict.fromkeys(range(5), [])
>>> dic
{0: [], 1: [], 2: [], 3: [], 4: []}
>>> 
>>> v = copy.copy(dic[0])   # shallow copy
>>> v
[]
>>> v.append([1, 2, 3])
>>> v
[[1, 2, 3]]
>>> 
>>> dic[0] = v
>>> dic
{0: [[1, 2, 3]], 1: [], 2: [], 3: [], 4: []}
>>> 

copy.deepcopy 深拷贝，成功，这个最靠谱。

>>> dic = dict.fromkeys(range(5), [])
>>> dic
{0: [], 1: [], 2: [], 3: [], 4: []}
>>> 
>>> v = copy.deepcopy(dic[0])
>>> v
[]
>>> v.append([1, 2, 3])
>>> v
[[1, 2, 3]]
>>> 
>>> dic[0] = v
>>> dic
{0: [[1, 2, 3]], 1: [], 2: [], 3: [], 4: []}
>>> 

完。