如何在python中更新字典中的值？

Question

I have a dictionary like this:

my_dict = {key1:value1, key2:value2}

I want to first check if some key exists if yes I have to convert the value of existing key into a list and add other value to that like this:

my_dict = {key1:[value1,new_value], key2:value2}

I mean just I need to update one key in the dictionary to list not all of them. any idea about that?

Answer 1

This is a solution that works even if you add multiple values to a certain key. You can have a separate function for this:

#!/usr/bin/env python

def append_value(d, key, value):
    if not key in d:
        d[key] = value
    else:
        if type(d[key]) != list:
            d[key] = [d[key], value]
        else:
            d[key].append(value)



my_dict = {"key1": 1, "key2": 2}
print(my_dict)


append_value(my_dict, 'key1', 4)
print(my_dict)
append_value(my_dict, 'key1', 5)
print(my_dict)
append_value(my_dict, 'key3', 6) # adding a key that doesn't exist
print(my_dict)
append_value(my_dict, 'key3', 7)
print(my_dict)

The output would be:

{'key2': 2, 'key1': 1} # initial value
{'key2': 2, 'key1': [1, 4]} # adding 4 to key1
{'key2': 2, 'key1': [1, 4, 5]} # adding 5 to key1
{'key3': 6, 'key2': 2, 'key1': [1, 4, 5]} # adding new key: key3
{'key3': [6, 7], 'key2': 2, 'key1': [1, 4, 5]} # adding 7 to key3

Answer 2

If you may have one or more values for a given key, it's usually a better idea to always use a list for the value, rather than sometimes having a list and sometimes having the value directly. The list can have just one item in it, or it can have more. This way you can always use the same code to access and modify the values, regardless of how many of them there are.

There are several good ways to add values to such a dictionary. The setdefault method is an easy one that works with regular dictionaries:

d = {"key1": ["value1"], "key2": ["value2"]}  # values are always lists

d.setdefault("key1", []).append("new_value")      # this adds a value to an existing list
d.setdefault("new_key", []).append("new_value2")  # this time the same code creates a new list

You can also use a collections.defaultdict , which will create new values automatically whenever you look up a key that isn't in the dictionary yet.

from collections import defaultdict

d = defaultdict(list)  # list is the "factory" that gets called to create new values

d["key1"].append("value1")
d["key2"].append("value2")
d["key1"].append("new_value")