Python: why dictionary type data can automatically exclude certain elements?
I created a dictionary containing six elements as shown below:
>>> dict1 = {
'A': ['A','A'],
'AB':['A','B'],
'A':['A','O'],
'B':['B','B'],
'B':['B','O'],
'O':['O','O']
}
But when I check the dictionary, I find that the elements "{'A': ['A', 'A'], 'B': ['B', 'B']}"
have been excluded.
>>> dict1
Out[19]: {'A': ['A', 'O'], 'AB': ['A', 'B'], 'B': ['B', 'O'], 'O': ['O', 'O']}
>>> len(dict1)
Out[17]: 4
However, if I create a new dictionary with excluded elements. It becomes normal.
>>> dict2 ={'A': ['A', 'A'], 'B': ['B', 'B']}
>>> dict2
Out[21]: {'A': ['A', 'A'], 'B': ['B', 'B']}
Can anyone explain to me why?
source to share
You cannot have duplicate keys, but you can have multiple values. In other words, each key is unique.
This way, every time you assign new values to the same key, you override the previous key values.
A way to assign 2 values (or lists) as in your example could be as follows:
dict1 = {'A': [['A','A'],['A','O']], 'B':[['B','B'],['B','O']], 'O':['O','O'], 'AB':['A','B']}
Result
{'A': [['A', 'A'], ['A', 'O']], 'B': [['B', 'B'], ['B', 'O']], 'AB': ['A', 'B'], 'O': ['O', 'O']}
Finally, you can access each key like this:
dict1['A']
Result
[['A', 'A'], ['A', 'O']]
This is similar to what you want to do.
Hope it helps.
source to share
The thing with Python dictionaries is that each key is unique . That is, when you add an existing entry, the previous saved value is overwritten with the new one.
When typing:
dict1 = {
'A': ['A','A'],
'AB':['A','B'],
'A':['A','O'], # Overrides ['A', 'A']
'B':['B','B'],
'B':['B','O'], # Overrides previous entry
'O':['O','O']
}
You have given the dictionary two meanings for the keys 'A'
and 'B'
. It is you asked the dict to change the previously saved value.
Hope my answer was clear enough :)
EDIT: format and language
source to share
As python dictionary
you can not be a duplicate keys. If any duplicate key is present in python dictionary
, python
automatically replaces the first values with new ones. python dictionary
behaves like unique
.
In your example:
dict1 = {
'A': ['A','A'],
'AB':['A','B'],
'A':['A','O'], # 'A': ['A','A'] and 'A': ['A','O'] override.
'B':['B','B'],
'B':['B','O'], # 'B': ['B','B'] and 'B': ['B','O'] override.
'O':['O','O']
}
Then yours dictionary
would be:
dict1 = {
'A': ['A','O'],
'AB':['A','B'],
'B':['B','O'],
'O':['O','O']
}
I think it will be helpful.
source to share
As the Python documentation says
It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys be unique (within the same dictionary). A pair of curly braces creates an empty dictionary: {}. Placing comma separated lists of keys: value pairs in curly braces adds initial key: values pairs to the dictionary; it is also a way to write dictionaries in the output.
Basic operations in a dictionary are storing a value using some key and retrieving the value given the key. You can also remove the key: value pair with del. If you store a key that is already in use, the old value associated with that key will be forgotten. It is an error to retrieve a value using a non-existent key.
Link: https://docs.python.org/3/tutorial/datastructures.html
source to share