Catalog

The derived type (comprehensions)

List derivation （list comprehensions）

Dictionary derivation （dict comprehensions）

Set derivation （set comprehensions）

The derived type (comprehensions)

The derived type （ Also known as analytic ） yes Python A unique characteristic of , If I'm forced to leave it , I will miss you very much .

The derivation is that the structure of another new data sequence can be constructed from one data sequence . There are three kinds of derivation , stay Python2 and 3 There is support in ：

1. list (list) The derived type
2. Dictionaries (dict) The derived type
3. aggregate (set) The derived type

We will discuss them one by one . Once you know the trick to using list derivation , You can easily use any derivation .

List derivation （list comprehensions）

List derivation （ Also known as list parsing ） Provides a concise way to create lists .

Its structure is to include an expression in a square bracket , And then a for sentence , And then there was 0 One or more for perhaps if sentence . That expression can be arbitrary , It means you can put any type of object in the list . The result will be a new list , In this case, with if and for Statement is the expression of the context, which is generated after running .

standard

variable = [out_exp for out_exp in input_list if out_exp == 2]

Here is another concise example :

multiples = [i for i in range(30) if i % 3 is 0]
print(multiples)

 Output: [0, 3, 6, 9, 12, 15, 18, 21, 24, 27]

This will be useful for quickly generating lists .

Some people even prefer to use it instead of filter function .

List derivation is great in some cases , Especially when you need to use for Loop to generate a new list . for instance , You usually do ：

squared = []
for x in range(10):
    squared.append(x**2)

You can use list derivation to simplify it , Just like this. ：

squared = [x**2 for x in range(10)]

Dictionary derivation （dict comprehensions）

Dictionary derivation and list derivation are used in a similar way . Here's an example I found recently ：

mcase = {'a': 10, 'b': 34, 'A': 7, 'Z': 3}
mcase_frequency = {
    k.lower(): mcase.get(k.lower(), 0) + mcase.get(k.upper(), 0)
    for k in mcase.keys()
}

result ： mcase_frequency == {'a': 17, 'z': 3, 'b': 34}

In the above example, we combined the values of the same letter but different case .

Personally, I don't use a lot of dictionary derivation .

You can also quickly change the keys and values of a dictionary ：

{v: k for k, v in some_dict.items()}

Set derivation （set comprehensions）

They are similar to list derivation . The only difference is that they use braces {}. for instance ：

squared = {x**2 for x in [1, 1, 2]}
print(squared)

result : {1, 4}