Python3 Dictionary

A dictionary is another kind of mutable container model that can store objects of any type.

Each key-value pair of a dictionarykey=>valuepair is separated by a colon:separated, and each pair is separated by a comma (,) separated, the entire dictionary is enclosed in curly braces{}, the format is as follows:

d = {key1 : value1, key2 : value2, key3 : value3 }

Note:dictAs Python keywords and built-in functions, variable names are not recommended to be nameddict。

Keys must be unique, but values need not be.

Values can take any data type, but keys must be immutable, such as strings or numbers.

A simple dictionary example:

tinydict = {'name': 'example', 'likes': 123, 'url': 'www.example.com'}

You can also create a dictionary like this:

tinydict1 = { 'abc': 456 }
tinydict2 = { 'abc': 123, 98.6: 37 }

Create an empty dictionary

Use curly braces{ }to create an empty dictionary:

The above example outputs:

{}
Length: 0
<class 'dict'>

Use the built-in functiondict()to create a dictionary:

The above example outputs:

{}
Length: 0
<class 'dict'>

Access values in a dictionary

Put the corresponding key inside square brackets, as in the example below:

The above example outputs:

tinydict['Name']:  Example
tinydict['Age']:  7

If you access data with a key that does not exist in the dictionary, an error will be output as follows:

The above example outputs:

Traceback (most recent call last):
  File "test.py", line 5, in <module>
    print ("tinydict['Alice']: ", tinydict['Alice'])
KeyError: 'Alice'

Modify dictionary

The way to add new content to a dictionary is to add new key/value pairs, or modify or delete existing key/value pairs, as in the following example:

tinydict['Age']:  8
tinydict['School']:  Example

Delete dictionary elements

You can delete a single element or clear the dictionary; clearing only requires one operation.

Explicitly delete a dictionary using the del command, as in the following example:

But this will raise an exception, because the dictionary no longer exists after the del operation:

Traceback (most recent call last):
  File "/example-test/test.py", line 9, in <module>
    print ("tinydict['Age']: ", tinydict['Age'])
NameError: name 'tinydict' is not defined

Note:The del() method will also be discussed later.

Characteristics of dictionary keys

Dictionary values can be any Python object, either standard objects or user-defined objects, but keys cannot.

Two important points need to be remembered:

1) The same key is not allowed to appear twice. When creating, if the same key is assigned twice, the later value will be remembered, as in the following example:

The above example outputs:

tinydict['Name']:  小Example

2) Keys must be immutable, so numbers, strings, or tuples can be used, but lists cannot, as in the following example:

The above example outputs:

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    tinydict = {['Name']: 'Example', 'Age': 7}
TypeError: unhashable type: 'list'

Dictionary built-in functions & methods

Python dictionaries contain the following built-in functions:

>>> tinydict = {'Name': 'Example', 'Age': 7, 'Class': 'First'}
>>> len(tinydict)
3

>>> tinydict = {'Name': 'Example', 'Age': 7, 'Class': 'First'}
>>> str(tinydict)
"{'Name': 'Example', 'Class': 'First', 'Age': 7}"

>>> tinydict = {'Name': 'Example', 'Age': 7, 'Class': 'First'}
>>> type(tinydict)
<class 'dict'>

Python dictionaries contain the following built-in methods:

x = True
country_counter = {}

def addone(country):
    if country in country_counter:
        country_counter[country] += 1
    else:
        country_counter[country] = 1

addone('China')
addone('Japan')
addone('china')

print(len(country_counter))

confusion = {}
confusion[1] = 1
confusion['1'] = 2
confusion[1] += 1

sum = 0
for k in confusion:
    sum += confusion[k]

print(sum)