operation on string items in python dict-CodePudding

I found some unexpected behavior while trying to edit strings in a dictionary (python 3.8.9.0), the new value does not make it into the string.

A dictionary of lists appends as expected

a = {'a':[]}
for k, v in a.items():
   v =['o']
   
a
Out[24]: {'a': ['o']}

A dictionary of strings does not update:

a = {'a':'a'}
for k, v in a.items():
   v ='o'
   
a
Out[27]: {'a': 'a'}

CodePudding user response：

Strings are immutable, adding another string even in place leads to a new string, but you can use dict setting for the same thing.

d = {}

d[1] = "a"
print(id(d[1]), d[1])  # 4302987632 a

s = d[1]
print(id(s), s)  # 4302987632 a

s  = "c"
print(id(s), s)  # 4672398256 ac

d[1]  = "d"
print(id(d[1]), d[1])  # 4646283568 ad but this time it is placed in the dict

CodePudding user response：

This is because lists are mutable, and strings are not. To get the more "expected" (by some definition) behavior, modify the original dictionary value. Example:

a = {'a':'a'}
for k, v in a.items():
   v  = 'o'
print(a)
# {'a': 'a'}

for k, v in a.items():
   a[k]  = 'o'
print(a)
# {'a': 'ao'}