Python: names, values, assignment and mutability

Posted 2020-09-03

推荐先看视频（youtube） Ned Batchelder - Facts and Myths about Python names and values - PyCon 2015

Change variable
# rebinding
x = x + 1
# mutating
nums.append(7)
# can also rebind lists:
nums = nums + [7]
# but you can‘t mutate immutable

 

make a new list, don‘t change the mutable param

def append_twice(a_list, val):
    a_list.append(val)
    a_list.append(val)

def append_twice_bad(a_list, val):
    """This function is useless"""
    a_list = a_list + [val, val]

def append_twice_good(a_list, val):
    a_list = a_list + [val, val]
    return a_list

shadowcopy and deepcopy

The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.
A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

通过图片了解 compound objects

# objects that contain other objects, like lists or class instances
items = [{‘id‘: 1, ‘name‘: ‘laptop‘, ‘value‘: 1000}, {‘id‘: 2, ‘name‘: ‘chair‘, ‘value‘: 300},]

　　

# 但是含有的是 int 这样的就不属于 compound objects  int   疑问：难道不属于 object?

items = [1, 2, 3]

　　

看看 shadow copy 与 deepcopy 对 compound object 的不同

from copy import deepcopy, copy

items = [{‘id‘: 1, ‘name‘: ‘laptop‘, ‘value‘: 1000}, {‘id‘: 2, ‘name‘: ‘chair‘, ‘value‘: 300},]
items_deep_copy = deepcopy(items)
items_deep_copy[1][‘id‘] = ‘b‘
items_deep_copy == items

items_copy = copy(items)
items_copy[1][‘id‘] = ‘b‘
items_copy == items

可以看出，deep_copy 是全部重新复制，而 shadow copy 只 deep copy复制了第一层，嵌套的只是复制了引用

疑问：

Python 3.6.0 (default, Dec 24 2016, 08:01:42) [GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin
items = [1, 2, 3, 4, 5, 6]
items_copy = items[:]
items_copy[0] = ‘a‘
items_copy == items
False
# I think this is a shallow copy, and items_copy == items should return True but it‘s False.

# But another example return True
items = [{‘id‘: 1, ‘name‘: ‘laptop‘, ‘value‘: 1000}, {‘id‘: 2, ‘name‘: ‘chair‘, ‘value‘: 300},]
items_copy = items[:]
items_copy[0][‘id‘] = ‘a‘
items_copy == items
True

疑问：Do all mutable objects are composed of immutable objects　　

[1, 2, 3] from 1, 2, 3

Like language are composed of words